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Controlled grafting of polymer brush layers from porous cellulosic membranes
Journal Pre-proof Controlled grafting of polymer brush layers from porous cellulosic membranes Cassandra J. Porter, Jay R. Werber, Cody L. Ritt, Yan-Fang Guan, Mingjiang Zhong, Menachem Elimelech PII:
S0376-7388(19)33112-6
DOI:
https://doi.org/10.1016/j.memsci.2019.117719
Reference:
MEMSCI 117719
To appear in:
Journal of Membrane Science
Received Date: 7 October 2019 Revised Date:
30 November 2019
Accepted Date: 3 December 2019
Please cite this article as: C.J. Porter, J.R. Werber, C.L. Ritt, Y.-F. Guan, M. Zhong, M. Elimelech, Controlled grafting of polymer brush layers from porous cellulosic membranes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117719. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Statement
Menachem Elimelech research Cassandra Porter Mingjiang Zhong supervise the ATRP work Jay Werber Yan-fang Guan Cody Ritt
Adviser of C Porter, supervised First author, carried bulk of work Secondary adviser to C Porter; Analyzes data with C Porter Helped with ATRP work carried out all TEM work
Controlled Grafting of Polymer Brush Layers from Porous Cellulosic Membranes
Journal of Membrane Science Revised: November 30 , 2019 Cassandra J. Portera, Jay R. Werbera,b, Cody L. Ritta, Yan-Fang Guana,c, Mingjiang Zhonga, Menachem Elimelecha*
Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, United States b. Department of Chemistry, University of Minnesota, Minneapolis, MN 55455-0431, United States c. CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science & Technology of China, Hefei 230026, China a.
* Corresponding author: Menachem Elimelech, Email: [email protected], Phone: (203) 432-2789
Declarations of interest: none.
1
Graphical Abstract 1
Abstract
2
Dense polymer brushes on porous supports present powerful routes toward fouling-resistant
3
coatings and ultra-thin, mechanically-stable selective layers. However, characterization of
4
polymer chains grafted from nonideal, planar substrates often relies on indirect methods to
5
determine the length, density, and location of polymeric brushes. In this study, we explore new
6
methods to control and characterize the properties of polymer brush layers. Starting from hand-
7
cast cellulose films and commercial cellulose membranes, we use surface-initiated atom transfer
8
radical polymerization to graft polymeric chains. We tailor brush density and simultaneously
9
stabilize the support by varying the proportions of initiator and crosslinker esterified to the
10
cellulose substrate. On films with grafted polyacrylic acid (PAA) chains, we use the silver-
11
binding method to determine brush density. We analyze brush growth behavior by directly
12
cleaving chains from the surface of films for analysis by size exclusion chromatography,
13
uniquely observing the divergence of two distinct populations of differing length beyond a
14
certain molecular weight. Such behavior would be important to consider if the alignment of
15
block copolymer layers is absolutely necessary in an application. By controlled contact of
16
polymerization solution with the top surface, we show that brush growth can be partially directed 2
17
to the top surface for asymmetric cellulose membranes with relatively low molecular weight
18
cutoffs. In a semi-quantitative method to locate brush growth, depleted uranium was bonded to
19
grafted PAA chains, and targeted emission x-ray spectroscopy was conducted for comparison of
20
uranium content in various regions. Finally, we used our developed methods and findings to
21
inform the synthesis of homopolymer and diblock copolymer brush layers, which we then used
22
as selective layers in pressure-driven filtration and diffusion cell experiments.
23
Keywords
24 25
Surface-initiated atom transfer radical polymerization; thin-film composite; selectivity; grafting density; polymer brush
26
1. Introduction
27
The inherent surface properties of membrane-forming materials can be disadvantageous in
28
certain applications. For instance, the surface chemistry of aromatic polyamide used in reverse
29
osmosis desalination membranes can promote fouling, causing flux decline and diminishing
30
membrane performance, while some membrane materials used in biomedical applications are not
31
biocompatible unless modified [1]. Meanwhile, insufficient hydrophobicity of membranes used
32
in membrane distillation can induce pore wetting, which reduces the thermal driving force and
33
can cause salt break-through [2]. Catalytic, reactive, and adsorptive membranes may not
34
inherently provide sufficient functional groups for binding the desired ligands and reactive
35
species. In all these situations, the addition of polymer brushes of carefully chosen composition
36
can alter the surface characteristics of the membrane material to improve performance [3, 4].
37
Additionally, densely-grafted brush layers grown on the surface of porous supports could
38
provide alternative routes to produce ultrathin selective layers with compositions tailored for
39
specific applications. All these applications of brush layers necessitate precise addition of the
40
polymer chains to beneficially modify the base membrane without compromising other
41
performance requirements.
42
Atom transfer radical polymerization (ATRP) is a particularly attractive method for brush
43
growth. The controlled monomer-by-monomer addition during the reaction can produce
44
relatively monodisperse polymers with control over molecular weight, ranging from ~0.5 to
45
~100 kDa [5]. Advantageously, the retention of the initiating halogen at the polymer end allows
46
for the synthesis of block copolymers through sequential reactions involving different monomers 3
47
[6, 7]. By tethering an initiator to a membrane substrate, polymers may be grafted from a surface
48
with highly controlled, consistent lengths in surface-initiated atom transfer radical
49
polymerization (SI-ATRP), which can produce a higher brush density than top down “grafting-
50
onto” methods in which pre-formed polymers are bound to a surface [8].
51
ATRP-produced polymers “grafted-onto” and “-from” surfaces have been used to reduce
52
biofouling by increasing hydrophilicity [9, 10], to produce stimuli-responsive layers [11], and to
53
increase surface energy in pervaporation [12]. In many cases, these applications do not require
54
relatively monodisperse polymers and/or full surface coverage and thus could have been
55
achieved through simpler synthesis schemes than using highly-controlled ATRP. The advantages
56
of organic-phase SI-ATRP were demonstrated in one study by producing uniform coverage of
57
gold surfaces with nanoscale-thick poly(2-vinylpyridine) films with controlled grafting density
58
and polymer size to optimize biomolecular adsorption [13]. SI-ATRP was used in another study
59
to modify (decrease) the pore size of cellulose ultrafiltration membranes, with hydrophilic
60
poly[poly(ethylene glycol methacrylate)] grafted throughout the cellulose pore structure [14].
61
Dextran rejection curves shifted uniformly to lower molecular weights, suggesting controlled
62
modification of pore sizes. Membrane active-layer preparation involving ATRP has focused
63
more on producing block copolymers in homogeneous reactions that are later self-assembled into
64
patterned membranes, especially for anion- and proton-exchange membranes for fuel cells [15,
65
16]. Although poly(ethylene glycol dimethacrylate) polymer brushes grown directly from porous
66
alumina have shown high selectivity of CO2 over CH4 [3], SI-ATRP-derived active layers
67
comprising dense polymeric brushes that are robustly supported to withstand applied pressures in
68
desalination and other processes beyond gas separations have yet to be fully explored. These
69
layers would wholly exploit the unique advantages of SI-ATRP on porous substrates, requiring
70
even layering and high grafting density.
71
There is a critical need for more control of SI-ATRP when synthesizing brush layers in
72
membrane applications. For example, in the synthesis of selective layers, brush growth would
73
need to be directed to the top surface of a porous support, as opposed to throughout the
74
membrane thickness. The support layer would also need a high density of binding sites for
75
initiator and a high degree of solvent stability in the organic solvents used for ATRP.
76
Additionally, improved characterization techniques are needed for assessing brush layer
77
properties. Previous characterization methods of SI-ATRP-produced brushes have relied heavily 4
78
on assumptions which may not be entirely accurate, especially when such polymers were grown
79
on nonideal, relatively rough surfaces such as porous membranes [17-31]. To date, a systematic
80
study of the methods to govern and measure SI-ATRP growth rates as well as verify and control
81
location and density of brush-layer growth on nonideal membranes and polymer films has not
82
been reported.
83
This study focuses on the development of such techniques. Through direct cleavage and
84
measurement of brush size, we show the unique growth characteristics of polymer chains grown
85
from the nonideal surface of a dense cellulose film, which contains both concave and convex
86
regions on a nanoscale. A more direct measurement of brush density is developed, whereby
87
silver ions are bonded to the negatively-charged sidechains of polyacrylic acid (PAA) brushes,
88
eluted, and measured in inductively-coupled mass spectrometry (ICP-MS). Using commercial,
89
asymmetric regenerated cellulose ultrafiltration membranes as substrates, growth location is
90
controlled by exploiting steric hindrance with a specially designed sealing wafer. Regions of
91
growth are directly verified and quantified through uranyl nitrate staining and scanning
92
transmission electron energy-dispersive x-ray microscopy (STEM-EDX). Finally, as a proof of
93
concept, we demonstrate that SI-ATRP-produced brushes can serve as effective selective layers
94
in pressure-driven filtration and diffusion experiments.
95
2. Materials and methods
96
2.1 Chemicals and materials
97
Cellulose acetate (CA) of ~50,000 Da, α-bromoisobutyryl bromide (BiBB), adipoyl chloride
98
(AC), triethylamine (TEA), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), copper(I)
99
and copper(II) bromides (Cu(I) and Cu(II)), ethyl α-bromoisobutyrate (EBiB), tert-butyl acrylate
100
(tBA), diethyl ether, phosphoric, glacial acetic, and trifluoroacetic acids, and silver nitrate were
101
purchased from Sigma-Aldrich (St. Louis, MO USA). Commercial regenerated cellulose
102
ultrafiltration disks (Ultracel) of 30 and 100 kDa molecular weight cut-off (MWCO) were
103
obtained from USA-based EMD Millipore (Jaffrey, NH USA) while benzyl chloride was a
104
German EMD Millipore product (Darmstadt, Germany). Acetone, dichloromethane (DCM), and
105
potassium hydroxide were of Macron Fine Chemicals brand while hexanes, 2-propanol,
106
methanol, nitric acid, and HPLC-grade acetonitrile were of J. T. Baker brand, both from Avantor
107
Performance Materials (Center Valley, PA USA). Magnesium perchlorate hexahydrate was from 5
108
Alfa Aesar (Word Hill, MA USA) while n-butyl acrylate (nBA), 2-hydroxy ethyl acrylate
109
(HEA), and dimethyl sulfoxide (DMSO) came from Alfa Aesar (Heysham, England). Ethanol
110
(EtOH) was purchased from Decon Labs (King of Prussia, PA USA). HPLC-grade
111
tetrahydrofuran (THF) and L(+)-ascorbic acid came from Fisher Scientific (Fair Lawn, NJ USA).
112
Dimethyl formamide (DMF) and 0.45 µm MWCO PTFE syringe filters were purchased from
113
VWR International (Radnor, PA USA). 1,1,3,3-tetramethylguanidine (TMG) was provided by
114
the Tokyo Chemical Industry (Portland, OR USA). Uranyl nitrate was purchased from SPI
115
Supplies, a division of Structure Probe (West Chester, PA USA). Alumina basic and neutral were
116
purchased from Sorbtech (Norcross, GA USA). LR White Resin came from the London Resin
117
Company (Reading, England).
118
Cellulose filter paper was purchased from Whatman International (Maidstone, England).
119
Chemical and organic-solvent resistant rubber and polytetrafluoroethylene sheets, blocks, and
120
other hardware for fabricating specially-designed membrane sealing wafers as well as a dead-end
121
cell were purchased from McMaster-Carr (Princeton, NJ USA). Deionized ultra-filtered (DI)
122
water was prepared through passage in a Milli-Q system with Elix Technology from EMD
123
Millipore and utilized throughout synthesis and experimentation.
Nitrogen was provided by Airgas East (Salem, NH USA).
124
Chemicals and materials were used as purchased, except monomers, Cu(I), and commercial
125
cellulose membranes. Monomers were passed through a basic alumina column to remove
126
inhibitor before usage. Cu(I) was prepared by stirring a suspension of 1 molar equivalent Cu(I)
127
with 2 molar equivalents L(+)-ascorbic acid in DI water for 15 min and filtering out the white
128
product. After thorough, consecutive washing with DI water, glacial acetic acid, and diethyl
129
ether, the product was fully vacuum-dried and stored in a glovebox under nitrogen and darkness
130
until use. Before any modification or testing, commercial membranes were soaked overnight in
131
1:1 2-propanol:DI water on a shake plate to remove glycerol and wet the membrane pores.
132 133
2.2 Production of dense cellulose films
134
Dense cellulose films were produced through long-established methods [32]. Specifically, a
135
dopant consisting of 15 wt% CA, 67 wt% acetone, and 18 wt% nonsolvent (10.5 wt%
136
magnesium perchlorate in water) was prepared by stirring at 70 °C until all polymer was
137
dissolved. The solution was cooled overnight in a refrigerator. The dopant was cast cold via a
138
doctor blade onto chilled glass plates to a thickness of 0.413 mm. Nonsolvent and solvent were 6
139
evaporated off for 1 hr, immersed in an ice bath for 15 minutes to precipitate the polymer, and
140
peeled from the glass. To regenerate cellulose by conversion of acetyl groups to hydroxyl
141
groups, films were set in 0.05 M sodium hydroxide in ethanol for 12 hours and rinsed several
142
times in DI water [33].
143
Cellulose films and commercial membranes were stored in DI water in a refrigerator to
144
prevent bacterial growth and pore or structural collapse. We observed that the pores of wet
145
cellulose membranes collapse when dried from water (Figure S1). Therefore, anytime a cellulose
146
sample was dried, it was first exchanged from water to 2-propanol to hexane.
147 148
2.3 Initiator and crosslinker binding
149
Cellulose films or commercial membranes were cut into circles 2 cm in diameter.
150
Approximately 20 of these samples per initiator and crosslinker binding reaction were patted
151
with lint-free wipes to remove excess water and immediately soaked in THF. Meanwhile, a
152
solution of 40 mL of THF with 1.10 mL TEA was bubbled with nitrogen in a cylindrical jar with
153
septum screw cap for 30 minutes over ice to mitigate evaporation of solvent. Membranes or films
154
were added, and this solution was bubbled an additional 30 minutes. AC as crosslinker and BiBB
155
as initiator were pre-mixed in a separate vial in the proportion of interest, and 1 mL of this
156
solution was added slowly dropwise to the still-bubbling solution submerged in ice water. The
157
formation of a white paste indicated that the reaction was proceeding, as TEA neutralizes the
158
solution by reacting with the released hydrogen ions and halogens to form a precipitant
159
triethylammonium chloride/bromide. Bubbling needles were removed, and the cap was covered
160
in Parafilm and tape to ensure no air seeped within the vessel. The reaction proceeded on a shake
161
plate for 12 hours in a 40 °C oven for full binding (Figure S2). Membranes were consecutively
162
rinsed in fresh baths of THF, acetone, 2-propanol, and DI water on a shake plate at room
163
temperature for 1 h in each solvent.
164 165
2.4 Growth of acrylate polymers through SI-ATRP
166
For the SI-ATRP reactions in this work, a homogeneous ATRP reaction with initiator EBiB
167
was conducted simultaneously in the same vessel as the SI-ATRP reaction. Proportions of
168
reagents were chosen to achieve targeted polymer molecular weights. Table S1 provides some
169
proportions of reagents and reaction conditions with resultant homogeneous polymer sizes and 7
170
dispersities, with tBA and HEA as the monomers. Figure S3 provides rates of butyl acrylate
171
growth when only proportions of Cu(I):Cu(II) are altered while other reagent quantities are held
172
constant.
173
In a typical SI-ATRP reaction, DMF was thoroughly mixed for an hour with ligand
174
PMDETA and Cu(II) within a cylindrical vessel with septum screw cap. Meanwhile, cellulose
175
membrane or film swatches with AC/BiBB bonded were patted with lint free wipes. Depending
176
on the particular studies with these samples, these swatches were either inserted within specially
177
designed sealing wafers to control brush growth location or left unsealed. The use of such a
178
sealing wafer will be further discussed in Section 2.7. Sealed or unsealed samples were set to
179
soak in DMF.
180
Meanwhile, monomer with inhibitor removed was added to the reaction solution before
181
bubbling with nitrogen for 10 min per 10 mL of reaction solution. EBiB initiator was added, and
182
membranes/films were rapidly added through the open cap to ensure minimal incorporation of
183
air. The solution was bubbled an additional 5 minutes per 10 mL of solution. In a small glass
184
vial, Cu(I) was suspended in 1 mL of DMF and rapidly injected into the reaction vessel through
185
the septum top. With nitrogen still purging, the reaction was set to proceed in an oil bath at 70 °C
186
for 45 min.
187
To quench the reaction, membranes were immediately set to soak in fresh DMF for 1h.
188
Meanwhile, to measure relative number average molecular weight (Mn), weight average
189
molecular weight (Mw), and dispersity index (Ð), a couple drops of the homogeneous reaction
190
solution were mixed with 2 mL of THF and filtered through neutral alumina. This filtered
191
solution was run through a SEC column (EcoSEC HLC-8320GPC, Tosoh Bioscience, Tokyo,
192
Japan) with THF mobile phase and polystyrene (PS) stationary phase using PS standards for
193
calibration. Resultant peaks were analyzed with equipment-provided software (EcoSEC
194
Analysis). Membranes or films were further rinsed sequentially in fresh DMF, acetone, 2-
195
propanol, and finally DI water baths on a shake plate for a half hour each and stored in DI water
196
in a refrigerator.
197 198
2.5 Polymer growth rate determination
199
Heterogeneous polymer growth rate was compared to homogeneous through direct cleavage
200
of poly(tert-butyl acrylate) (PtBA) brushes. Larger quantities of samples were used to have 8
201
enough polymer sample to be detectable in the SEC column. In glass vials, ~9 cm2 of film with
202
PtBA brushes were submerged overnight in 2 mL of 5 vol% potassium hydroxide in water to
203
hydrolyze ester bonds and cleave off the brush layers. This produced cleaved polyacrylic acid
204
(PAA) chains, which were incompatible with the SEC column. Thus, PAA was esterified into
205
poly(benzyl acrylate) (PBzA) through a modified version of a previously-described method [34].
206
In this method, the basic solution with cleaved chains was neutralized with hydrochloric acid,
207
and ~1.5 mL of water were evaporated off to reduce the presence of water but ensure polymer
208
was not trapped in dry salt matrices. To this, 2 mL of DMSO, 126 µL of TMG, and 172 µL of
209
benzyl chloride were added. The reaction proceeded at least 10 h on a shake plate at room
210
temperature. Upon completion, the solution was neutralized with 114 µL acetic acid. The
211
polymer was precipitated in excess of 50:50 % MeOH:water solution and recovered by vacuum
212
filtration. The polymer was rinsed twice with MeOH and thoroughly dried at 40°C. In
213
preparation for SEC, the polymer was re-dissolved in 2 mL of THF and filtered through neutral
214
alumina.
215
polymer and converted polymer was prepared (Figure S4). For this calibration, homogeneously-
216
grown PtBA was measured in SEC, cleaved into PAA, converted into PBzA through the method
217
described above, and rerun through SEC.
Because PBzA was produced as opposed to PtBA, a calibration between grown
218 219
2.6 Brush density determination
220
A method was developed in our group whereby silver ions were used to quantify the density
221
of carboxyl functional groups in polyamide reverse-osmosis membranes, during which 1:1
222
coordination of carboxyl groups with silver ions and complete elution of silver were verified
223
[37]. Here, we applied this silver bind-and-elute technique to determine the density of polymer
224
chains on a surface. In general, we produced grafted chains with one carboxyl group per repeat
225
unit, i.e., PAA, and quantified the carboxyl density using the silver ion probe method. The brush
226
length was determined through the direct cleavage methods described in the previous section.
227
Subsequently, brush surface density σ was determined through
228
=
,
[1]
229
where Ns is the total number of eluted silver probes (CS,EVE, or the concentration of silver eluted
230
from brush-layer carboxyl groups multiplied by the solution volume), Mn is the measured chain 9
231
molecular weight, Mn,R is the repeat unit molecular weight, and Ap is the apparent area of the
232
dense film subjected to the polymerization reaction. CS,E was determined from the silver
233
concentration measured in the elution solution minus the average concentration of silver eluted
234
from three blanks. Blanks were samples with only AC/BiBB bonded to account for binding of
235
silver on the cellulose and any residual acyl chlorides that would convert to carboxyl groups in
236
water.
237
More specifically, AC:BiBB ratios were varied to control brush density on dense cellulose
238
films. Because negatively-charged polymers cannot be grown in traditional ATRP reactions due
239
to unwanted interactions with charged metal activators/deactivators, PtBA chains were grown
240
and selectively cleaved at their tert-butyl ester bonds to produce carboxyl sidechains through a 6
241
h reaction on shake plate in 12 vol% trifluoroacetic acid (TFA) in DCM, a commonly used
242
deprotection system [38]. The same reaction conditions resulted in ~99% hydrolysis of tert-butyl
243
ester bonds for PtBA in bulk solution after 6 h of reaction time, verified through proton nuclear
244
magnetic resonance (1H-NMR) analysis (Agilent DD2 400 MHz NMR Spectrometer) (Figure
245
S5). After cleavage, films were rinsed in fresh DCM, water, and finally acetone before fully
246
drying.
247
For silver binding and elution, the method described elsewhere was used [37], with silver
248
solutions set to pH 10. Because the silver elution solution of 1% nitric acid in water removes
249
bound ions but also potentially cleaves the ester bonds between chains and support, repeated
250
silver binding and elution could not be conducted multiple times on each sample. Elution
251
solutions were run through ICP-MS (ELAN DRC-e, Perkin Elmer, Waltham, MA) to determine
252
silver concentration. Serially-diluted solutions of 0-100 µg L-1 silver nitrate in 1% nitric acid in
253
water were used to produce a calibration curve, while 40 µg L-1 indium nitrate was used as an
254
internal standard. The full method is represented in Figure 1.
255
10
256 257
Figure 1. Methodology for determining brush density. After binding initiator, poly(tert-butyl acrylate)
258
chains are grown from the support layer via surface-initiated atom transfer radical polymerization (SI-
259
ATRP). Ester bonds connected to tert-butyl sidechains are selectively cleaved into carboxyl groups in
260
dichloromethane with 12% trifluoroacetic acid. Positively-charged silver ions electrostatically bind to
261
deprotonated carboxyl groups in basic conditions. In a set volume of 1% nitric acid in water, bound silver
262
is eluted, and its concentration is measured in inductively coupled mass spectrometry. After cleaving and
263
directly measuring brush molecular weight, brush density can be determined with Equation 1.
264
2.7 Brush location control and verification
265
A special sealing wafer was designed to fit within the opening of the septum-capped jar to
266
ensure a controlled, oxygen-free reaction environment. This frame was crafted from solvent-
267
stable, high-density materials and consisted of laser-cut rubber and polytetrafluoroethylene rings
268
and disks, sandwiched together with polytetrafluoroethylene screws and nuts (McMaster Carr)
269
(Figure 2).
270
Utilizing STEM-EDX, we developed a more direct, semi-quantitative method of determining
271
growth location than typically employed. However, the brush layers in our study are not
272
elementally distinctive enough to distinguish from the cellulosic support. Therefore, PtBA chains
273
grafted-from films and membranes, with and without the use of a sealing wafer, were selectively
274
hydrolyzed to PAA, as described in Section 2.6. To enhance elemental contrast, depleted 11
275
uranium from uranyl nitrate was bonded to the carboxyl groups of the PAA layer using a
276
previously described procedure [40].
277
Small wedges were embedded in the water-miscible acrylic resin LR White as previously
278
reported [41]. After curing at 65 °C for 24 hours, 90-nm-thick cross-sections were sliced with a
279
diamond knife (2.5 mm Ultra 45°, Diatome) using a Leica Ultramicrotome UC7 (Leica
280
Microsystems) and mounted onto carbon/formvar coated copper grids (Carbon Type-B, Ted
281
Pella). Dark-field images and STEM-EDX elemental mapping were obtained at 14000x
282
magnification using an FEI Tecnai Osiris 200kV S/TEM with 0.25 nm point resolution. Uranium
283
maps were each produced with 3 minutes of scan time. To identify elemental spectra and the
284
relative abundance of uranium, targeted EDX measurements were taken at sample points
285
throughout the thickness of the membranes and films using 1 minute of sample time.
286
287 288
Figure 2. Methodology to control location of brush growth. a) Components of a sealing wafer that fits
289
within a septum-capped vessel for controlled atmospheric conditions. The top (skin) surface of an
290
asymmetric membrane can be exposed to the reaction solution so chains grow predominantly on the top
291
surface. The relatively small pores of the porous membrane prevent easy access of reactants to the
292
membrane interior. b) Photographs of the sealing wafer, showing exposed membrane on the ring side and
293
the sealed-off side of the membrane on the disk side.
294
12
295
2.8 Film and membrane characterization
296
General characterization of bare and modified cellulose membranes included SEM imaging,
297
water contact angles, water permeability, and roughness. To ensure production of dense films
298
rather than membranes, top and bottom surface morphology of cast films and membranes for
299
comparison were sputter coated with 5 nm of iridium (BTT-IV Evaporator, Denton Vacuum,
300
LLC) and examined by scanning electron microscopy (SEM, Hitachi SU-70). For comparison,
301
films and hand-cast membranes were formed, although only films were used in experiments.
302
Membranes were formed by immersion precipitation without any solvent evaporation time.
303
Conveniently, cellulose samples with pores, confirmed via scanning electron microscopy,
304
appeared white to the naked eye once dried, while dense films appeared clear (Figure S1).
305
For static fluid contact angle testing, samples were taped to microscope slides with double-
306
sided tape. Contact angles for DI water were measured via sessile drop method with a contact
307
angle goniometer (OneAttension, Biolin) by photographing the sideview of a 1 µL droplet sitting
308
on the surface every ~0.16 s with digital camera for 10 s total post drop placement. Left and
309
right-side contact angles were measured in processing software (OneAttension software), and the
310
measurements of three drops per sample type were averaged together.
311
FTIR measurements were taken with an FTIR/Raman Thermo Nicolet 6700 with commercial
312
bare and modified membrane top surfaces pressed firmly against a diamond attenuated total
313
reflection cell. Before measurement, samples were exchanged through solvent and dried for 24 h
314
in a vacuum oven at 40 °C to remove any water and solvent. Blank readings comprised 4 scans
315
while 32 scans were taken for samples. Samples were measured twice on two different locations
316
to ensure consistent reading.
317
Film roughness was characterized in air with atomic force microscopy (AFM, Dimension
318
FastScan, Bruker) using PeakForce Tapping mode and a silicon-tipped nitride lever probe
319
(FASTSCAN-B, Bruker). Sections of 1 µm2 were scanned at a rate of 2.90 Hz with 496 samples
320
per line and 512 histogram bins. In data analysis software (NanoScope Analysis 1.9, Bruker),
321
root-mean-square roughness, Rq, and maximum roughness, Rmax, of each were determined. Four
322
scans at different locations of each surface type were averaged for the reported value.
323
Water permeability was measured with a specially-made, rubber-bodied dead-end cell for
324
higher pressures (maximum of 14 bar) and relatively small sample sizes, with a 7-mm diameter
325
exit orifice and a 30-mL chamber. Permeability testing was conducted at 2.07 bar (30 psi) for 13
326
membranes without brushes and 6.89 bar (100 psi) for membranes with brushes. For loose
327
membranes (i.e. permeability exceeded 100 Lm-2h-1bar-1), permeability was tracked continuously
328
and recorded once steady state was reached. For tight membranes, the system was allowed to
329
compress overnight before permeate was collected for 48 h. Water permeabilities of 3-4 samples
330
were averaged.
331 332
2.9 Solute transport rate and rejection
333
To demonstrate how brush layers with varying hydrophobicity affect solute transport rates, a
334
variety of solutes with varying octanol/water partition coefficients were allowed to diffuse
335
through bare and modified commercial 30 kDa MWCO membranes. Two aqueous solutions (pH
336
6.0 ± 0.2) were prepared, one containing the three organic solutes—L-asparagine, hydroquinone,
337
and thymol— at 0.13 mM each and another containing sodium chloride at 0.20 mM.
338
Concentrations were chosen to eliminate osmotic pressure effects during experimentation and
339
ensure the full dissolution of the most hydrophobic solute utilized, thymol.
340
A membrane sample was pinch-clamped at the flat-ground joint of an unjacketed Valia-
341
Chien diffusion cell with a 7 mm orifice and 10 mL chambers with stir bars (PermeGear). The
342
chamber facing the active membrane layer was loaded with solution containing organic solutes
343
while the opposite chamber contained the saline solution. To come to diffusive steady-state, the
344
system sat stirring for 2 h for samples without brushes and 4 h for those with brushes before
345
taking initial samples. Final solution samples were taken after ~10 h for brush-free swatches and
346
~20 h for brush-covered swatches. These time points were selected after preliminary studies with
347
each membrane type in which concentration changes were measured more continuously to
348
determine when a linear increase in concentration was reached and when concentration changes
349
of all solutes significantly overcame measurement error. Transport rate was calculated as the
350
change in total species (molar concentration multiplied by cell volume) per time per membrane
351
area. Four samples of each membrane type were tested.
352
Organic solute concentrations in the saline side of the chamber were determined through
353
high-performance liquid chromatography (HPLC, 1260 Infinity LC System, Agilent
354
Technologies) with mobile phase consisting of 40% acetonitrile and 60% of 0.1% phosphoric
355
acid in water. Calibration curves were developed prior to diffusion experiments, pinpointing
356
retention time and detection wavelength, and data was analyzed with curve integration software 14
357
(OpenLAB CDS EZChrom Edition, Agilent Technologies). A conductivity probe (Con 2700,
358
Oaklon) was used to measure conductivity of the solution on the organic side and compute
359
sodium chloride concentration from conductivity-derived calibration curves.
360
Rejection studies were conducted in the same custom dead-end cell used for water
361
permeability testing, with an added stir bar. Sodium chloride and lysozyme solutions, run
362
separately, were 20 mM and 35 µM, respectively. As in water permeability tests, hydraulic
363
driving pressures of 2.07 bar (30 psi) and 6.89 bar (100 psi) were used for membranes without
364
and with brush layers, respectively. At least 8 mL of permeate were collected for a more accurate
365
representation of observed rejection, calculated through = 1−
366
[2]
367
where Cp is the concentration in the permeate and CF is the concentration in the feed. In the case
368
of high rejection, as with rejection of lysozyme by brush-layered membranes, CF was
369
approximated as the average of the initial feed concentration and the final retentate
370
concentration. Lysozyme concentration was determined by measuring total organic carbon
371
concentration (Shimadzu TOC Analyzer) while a conductivity probe was again used for sodium
372
chloride concentration.
373
3. Results and Discussion
374
3.1 Crosslinker maintains support-layer stability while tailoring brush density
375
A cellulosic support was chosen for growing SI-ATRP-produced brush layers because it
376
provides a high density of hydroxyl groups for bonding initiator in a relatively straightforward
377
manner. However, these hydroxyl groups are also responsible for the hydrogen bonds that render
378
cellulose as highly solvent stable. Thus, we hypothesized that without the addition of a
379
crosslinker during initiator binding, cellulose would lose stability as hydroxyl groups would be
380
fully occupied by initiator BiBB (Figure 3a,c). For a more insightful comparison on how the
381
proportion of crosslinker versus initiator affects brush density, the mole percentage of acyl
382
halides attributed to AC versus BiBB was used during analyses instead of volumetric or molar
383
portions of the reagents. Acyl halides are the functional groups at which these molecules esterify
384
with cellulose hydroxyl groups. Every crosslinker provided two acyl halides while every initiator
385
provided one. Thus, for instance, 25:75 vol% AC:BiBB at room temperature would be defined as
386
36:64 mol% AC:BiBB acyl halides. Note that these percentages refer only to those in the binding 15
387
solution and not to the final bonded AC:BiBB on the membrane as differing steric hindrances
388
and reactivities are sure to influence final proportions.
389 390
Figure 3. Stabilization of cellulose support membrane and simultaneous control of brush density. a)
391
Cellulose structure containing an abundance of hydroxyl groups, which hydrogen-bond in the presence of
392
harsh organic solvents, stabilizing this polymer in typical ATRP environments. b) SI-ATRP initiator α-
393
bromoisobutyryl bromide (BiBB) and crosslinker adipoyl chloride (AC) to be bonded to the cellulose
394
support membrane through an esterification reaction. c) The occupation of all hydroxyl groups with
395
initiator α-bromoisobutyryl bromide. Hydrogen bonds are eliminated, decreasing stability in organic
396
solvents. d) Hypothetical configuration of cellulose when AC as crosslinker is incorporated. By changing
397
the ratio of AC:BiBB, the support membrane can be stabilized and the brush density can be
398
simultaneously controlled within a certain range. Note that this is just an illustration and does not
399
represent exact bonding patterns.
400
As predicted, crosslinker was necessary to stabilize cellulose, evident in both
401
permeability testing on membranes and brush density determination on films (Figure 4). One can
402
expect that water permeability will diminish as a hydrophobic PtBA brush is grown on the
403
surface of a porous support. Indeed, in cases where BiBB acyl halide content was less than or 16
404
equal to 55%, with the remaining acyl halides attributed to crosslinker, permeability with the
405
addition of PtBA brushes is below that of the bare membranes (Figures 4a-b). However, at 64
406
mol% BiBB acyl halides, permeabilities for several membrane samples far exceeded that of the
407
bare membrane for both 30 kDa MWCO (Figure 4a) and 100 kDa MWCO membranes (Figure
408
4b) with PtBA brush. This suggests that too many stabilizing hydroxyl groups were occupied by
409
initiator, leading to membrane instability. As the membrane is exposed to DMF during SI-ATRP,
410
cellulose with depleted hydroxyl content is more easily dissolved.
411
In many studies, brush density, or the number of chains per unit surface area, has been
412
estimated by measuring dry brush layer thickness via ellipsometry on a harder substrate [18, 42]
413
or degree of polymerization [24]. These methods typically necessitate the growth of a relatively
414
long brush to overcome detection limits. Furthermore, in order to calculate brush density, the
415
heterogeneous polymer length (i.e., on the surface) is assumed to equal homogeneous length
416
(i.e., in solution). For optimization of a brush-layered membrane technology, an alternative, more
417
sensitive method to measure brush density may be helpful.
418
We directly measured the brush density by first binding-and-eluting silver ions to PAA
419
brush layers grown from dense cellulose films, thereby quantifying the total number of
420
carboxylic acid groups on the film surface. Chain length (~4 kg mol-1) was then measured by
421
SEC for equivalent samples following cleavage of the polymer brush from the surface. The
422
combination of the two methods allowed for direct measurement of brush density, as described
423
in Section 2.6. Using this method, we found that the crosslinker used for stabilization
424
simultaneously controlled brush density. We observed a reasonable positive trend between brush
425
density and initiator content used during AC/BiBB bonding (Figure 4c). However, at 64 mol%
426
BiBB acyl halides, brush density drastically dropped, again suggesting instability at this initiator
427
density as both cellulose and attached brush dissolve in the ATRP solvent. The maximum BiBB
428
acyl halide content that maintained membrane stability was 55 mol%, with a maximum observed
429
brush density of 1.70 ± 0.34 chains nm-2.
430
To evaluate the reasonableness of these observed densities, we compared our measured
431
density to the theoretical maximum brush density. Using atomic bromine percentages read from
432
x-ray photo electron spectroscopy (XPS) and hypothetically assuming that all bonded initiator
433
actually produces a polymer chain without inducing membrane instability, we can expect a
434
maximum of 7.0 chain nm-2 (see Section S1 and Figures S2, S6). If we assume that acyl halides 17
435
of both crosslinker and initiator have an equal probability of bonding, the use of 16, 23, 37, and
436
55 mol% BiBB acyl halides would produce around 1.13, 1.62, 2.61, and 3.88 chains nm-2,
437
respectively.
438
For all densities, we observed fewer brushes than theoretical (Figure S6). When
439
considering individual acyl halide groups, the slightly higher reactivity of acyl bromides over
440
acyl chlorides, based on bromine’s higher propensity to leave due to a lower halide-carbon bond
441
energy of 285 versus chlorine’s 327 kJ mol-1 [43], would tend to skew theoretical maximum in
442
the opposite direction (i.e., observed density would exceed our calculated maximum that
443
assumed equal acyl halide reactivity). However, it is possible that the presence of two acyl
444
chloride groups per one adipoyl chloride molecule could further enhance acyl chloride reaction
445
rates. That is, following reaction of the first acyl chloride, the second acyl chloride reacts soon
446
after owing to the proximity of neighboring hydroxyl groups on the surface of cellulose film.
447
Steric hindrance is likely another limiting factor. We observed a relatively high discrepancy
448
between theoretical and observed brush density at higher proportions of initiator, as crowded,
449
growing brushes would reduce effective brush density. Due to a combination of the above
450
effects, the discrepancy between theoretical and observed brush densities was non-linear. The
451
lowest discrepancy occurred at an intermediate initiator density (~30 mol% BiBB acyl halides).
452
Several previous studies with poly(methyl methacrylate) chains of much larger molecular
453
weights (i.e. 10-100 kg mol-1), grown on silicon wafers, all reported lower maximum brush
454
densities of 0.3-0.8 chains nm-2 [18-23]. However, by using brush thickness determined from
455
ellipsometry and assuming equal lengths for chains produced in bulk and on the surface, these
456
studies did not take into account dispersity and the possibility of divergent polymer populations
457
when calculating density. In our study, the density was also possibly measured as higher than
458
actual due to a few effects, but none of them account for the magnitude of the difference. First,
459
the surface roughness of the film increases the surface area for binding. However, from AFM
460
images using NanoScope Analysis software, we found that the actual surface area was only 6%
461
greater than the projected area and thus cannot fully explain the discrepancy. Second, our use of
462
PS standards in the SEC column may introduce a systematic error in measuring molecular
463
weights of acrylates. However, based on intrinsic viscosities of the relevant polymers, the error is
464
expected to be minimal (see Section S3 and Figure S7). The final possible source of error is that
465
trace quantities of free PtBA adsorbed within the film polymer matrix, to which silver was 18
466
electrostatically attracted after PtBA hydrolysis into PAA (see Section 3.2). While this
467
adsorption was likely minimal due to extensive rinsing, trace internally-bonded silver could
468
increase total eluted silver given a thick enough film. In the absence of polymer brushes, silver
469
binding was minimal (typically 1-2 orders of magnitude less than for brush-containing samples),
470
as determined for unmodified cellulose membranes and initiator/crosslinker-modified cellulose
471
membranes.
472
19
473
Figure 4. Observed control of support layer stability and brush density through competitive binding of
474
crosslinker and initiator. Verification of membrane stability by testing pure water permeability for a) 30
475
kDa MWCO and b) 100 kDa MWCO commercial cellulose membranes, subjected to initiator/crosslinker
476
(AC/BiBB) binding solutions of various BiBB content (as mol% of acyl halide groups) and different
477
lengths of grafted poly(tert-butyl acrylate) (PtBA) chains. For (a-b), the surface polymer length was
478
assumed to match that of a homogeneous reaction occurring simultaneously in the same environment.
479
Each data point represents an individual membrane sample. Water permeabilities below that of the bare
480
membrane (gray dashed lines) suggest membrane chemical stability in harsh organic solvents while
481
increased permeability suggests instability. c) Brush density on dense cellulose films resulting after
482
reaction with AC/BiBB solutions with varying BiBB content. The average of four samples with standard
483
deviation are presented for each data point. Note that BiBB contributes one acyl halide per molecule
484
while the crosslinker AC contributes two. Acyl halides are the functional groups at which these molecules
485
esterify with cellulose hydroxyl groups.
486
3.2 SI-ATRP growth behavior from a nonideal substrate
487
Simultaneous homogeneous reactions of polymer conducted in the same environment as the
488
heterogenous, surface-initiated polymerizations have frequently been used as indicators for
489
relative dispersity and growth rates. Measuring polymers from the bulk reaction is preferential
490
since they may be run through a size-exclusion chromatography (SEC) column while the product
491
consisting of substrate with tethered brush is left intact. There have been claims that a
492
simultaneous homogenous polymerization results in similar polymer length with comparable
493
dispersity index as the heterogeneous reaction [27, 42, 44-46]. To investigate such claims,
494
spherical nanoparticles have previously been employed since they maximize surface to volume
495
ratio, producing sufficiently large polymer populations for cleavage and direct measurement in a
496
SEC column [47]. Growth on nanoparticles has been shown to be similar to the homogeneous
497
reaction. However, the positive curvature of spherical particles means intertwining chains and
498
radical-crowding reduce as growth progresses [18].
499
Most comparisons of SI-ATRP on planar surfaces using direct cleavage of brushes have been
500
performed on smooth gold substrates rather than nonideal, relatively rough surfaces [48-50].
501
However, it is evident that topology of a substrate influences growth rate and dispersity.
502
Simulations of polymer growth on concave topologies like channels (pores) and inside spheres
503
have revealed that grafted polymer chains under higher physical confinement grow slower than
504
polymers in homogeneous reactions, even when radical-radical termination was not considered 20
505
[31]. Several empirical studies have reported that SI-ATRP conducted on flat surfaces is slower
506
than in the bulk [24-27], while some others have reported faster surface growth [28-30]. Two
507
schools of thought have formed that may explain slower growth observations: (1) “School of
508
Propagation” posits that crowding of chains causes radical burying, inducing a premature glassy
509
state; and (2) “School of Termination” posits that radical ends easily terminate each other when
510
initiators are forced into alignment on a plane [47]. Because the theoretical proportion of
511
activated radical ends at any one time is so low compared to dormant ends, or [P•]/[PX] = 10−4 to
512
10−6, the distance between them renders the “School of Termination” implausible by many
513
scientists [51]. However, with mixed results of growth phenomena that cannot be fully explained
514
by either school of thought [52], it was proposed that there is an induced rapid “hopping” of
515
radicals across the plane of aligned initiator ends, which could increase the chances of radical-
516
radical interactions and termination [51, 53] or increase growth rate depending on brush density
517
and exactly how “hopping” and termination mechanisms work.
518
In summary, literature results suggest that chains on convex surfaces grow similarly to in the
519
bulk. In contrast, concave and flat surfaces present ample opportunity for entanglement and
520
radical-radical interactions to slow growth and increase dispersity. Because topology has such an
521
apparent effect on growth behavior, we characterized our cellulose films in AFM in order to
522
identify under what category of surface morphology they fell. Cellulose films were used instead
523
of membranes to eliminate the complicated and inconsistent effects of asymmetric, polydisperse
524
pores. Our dry cellulose films exhibited a root-mean-square roughness Rq of 8.5 ± 2.3 nm and a
525
maximum roughness Rmax of 73.1 ± 20.5 nm (Figure 5a). Thus, the film topological peaks and
526
valleys possess considerable height difference on the nanoscale, especially when considering that
527
grafted polymeric chains produced in this study are only from 4–30 kg mol-1. Free-floating
528
polymers of the same composition have a radius of gyration of only 5–15 nm in good solvent
529
(tetrahydrofuran) at room temperature [54], which is comparable to the scale of roughness of the
530
cellulose film, although we can expect some stretching of crowded, densely-grafted chains.
531
Roughness of bare cellulose is evident in the Peak Force Error images that produce a more
532
visually comprehensible contour map while Height Sensor readings layout exact depths (Figure
533
5a). With a PtBA brush size of Mn ~ 4 kg mol-1, roughness somewhat decreases, with a Rq of 7.6
534
± 0.7 nm and a Rmax of 64.2 ± 10.0 nm (Figure S8).
21
535
A final factor for growth rate concerns initiator density. Intuitively, it makes sense that for
536
any ATRP reaction, whether surface-initiated or homogeneous, the more initiator there is for a
537
given amount of monomer and set ratio of activating to deactivating metal/ligand complexes, the
538
slower the chain growth occurs, as a set amount of monomer must be distributed amongst more
539
growing polymer chains. We indeed observed that homogeneous reactions with more initiator
540
would grow slower and reach shorter final lengths than reactions with less EBiB (Table S1).
541
Several studies utilizing direct brush cleavage, including a couple performed on polymeric
542
supports, found that initiator density can affect heterogeneous growth rates [28, 30, 49, 50].
543
Previously in molecular dynamics simulations, it was shown that whether heterogeneous
544
reactions grow similarly to or slower than a simultaneous homogeneous reaction primarily
545
depends on surface initiator density, even without factoring in termination reactions and chain
546
entanglement. That is, regions of higher local initiator density decreased the probability of each
547
chain to collide with reagents as crowded halogens competed with one another for a local supply
548
of a set amount of reagents [55].
549
From direct cleavage of brushes at various stages of growth, we empirically demonstrate—
550
for the first time—that both faster and slower surface growth accurately describe behavior on a
551
planar but nonideal (i.e. relatively rough) surface depending on the time-point of sampling. Four
552
distinct phases of growth occurred (Figure 5b,c). Considering previous empirical and modelling
553
findings on SI-ATRP growth behavior, we propose the following explanations for our growth
554
observations, keeping in mind three main factors dictating growth rates: 1) hopping of radicals,
555
2) entanglement and burying of chain ends, and 3) local initiator/radical density.
22
556 557
Figure 5. Growth behavior of brushes on nonideal topology. a) Characterization of dense cellulose film
558
topology through atomic force microscopy (AFM). Root-mean-square (Rq), maximum roughness (Rmax),
559
and both height sensor and peak force error 2D images are presented. b) Growth of homogeneous PtBA
560
chains in bulk solution compared to simultaneously-grown brushes on dense cellulose films. Brushes
561
were cleaved in highly basic conditions, esterified into benzyl acrylate, and run directly through a size-
562
exclusion chromatography (SEC) column with polystyrene stationary phase and tetrahydrofuran mobile
563
phase. c) Illustration of four distinct phases of SI-ATRP growth on a nonideal surface, coordinated with 23
564
phase colors and number labels in (b). Chain-end spheres represent radicals while x’s are dormant,
565
halogen-capped ends. d) Homogeneous SEC traces and e) sample heterogeneous SEC traces with colors
566
corresponding to homogeneous population molecular weight, with homogeneous Mn in the legend. The
567
heterogeneous population at 15 kg mol-1 homogeneous molecular weight is a sample from right before
568
population divergence, while the double population at 16 kg mol-1 is right after divergence. The blank,
569
which is only represented in (e), consists of a film with just crosslinker and initiator bonded, subjected to
570
all cleavage and conversion reactions. Beyond a time of 8.75 min in the column, other molecules like
571
cleaved crosslinker and reagents involved in conversions are detected. Polymer populations could not be
572
detected below ~3.5 kg mol-1 due to the presence of these residual molecules. Notice that homogeneous
573
signal is orders of magnitude larger than the achieved traces of heterogeneous populations. To increase
574
heterogeneous signal, a much larger dense film surface area would be needed.
575 576
In phase 1, growth on the surface appears similar to the homogeneous rate in the bulk
577
solution, although this conclusion relies mostly on the initial data point. Direct measurements of
578
molar mass below ~3.5 kg mol-1 were not possible due to the limited amount of cleaved polymer
579
and the presence of other residual molecules (Figure 5e). However, if phase 1 truly represents
580
similar homogeneous and heterogeneous growth rate, there are possibly two competing factors
581
during this phase, whereby the alignment of initiator points induces rapid radical hopping along
582
this plane while the effectively denser initiator on the surface keeps growth slower per polymer
583
chain than in the bulk. In phase 2 at around 4 kg mol-1, the chains seem to reach a critical length
584
where intertwining can occur, diminishing radical alignment and increasing radical burying.
585
Reagents approaching these tangled chains are likely consumed by the bulk (homogeneous)
586
reaction. Therefore, growth slowed to a near standstill. In phase 3, some chains have had enough
587
time to escape entanglement, producing a longer population. We hypothesize that these chains
588
are located at the peaks of convex features, where less spatial crowding occurs, while chains
589
within concave features are more likely to stay entangled. The apparent rapid growth of the
590
longer population could be explained by the hopping of radical ends because the chains are long
591
enough and free enough to sway and contact one another. Additionally, at a now effectively
592
lower halogen density along this plane, hopping radicals in the longer population no longer
593
compete with rapid monomer depletion, inducing rapid growth. In phase 4, both long and short
594
populations are now at effectively lower initiator concentrations with diminished alignment due
595
to increased radii of gyration. Thus, these populations have the same monomer depletion rate and 24
596
probability of radical hopping as the bulk solution and grow at a similar rate. Because the lower
597
population begins to show observable growth, we believe little to no termination occurred during
598
the slowed growth in phase 2.
599
Neither stir rates nor brush density within the ranges considered affected growth (Figure S9).
600
These observations suggest that Brownian motion modulates reagent collision rate more than
601
convection and that effective brush density for various populations is self-regulated for a given
602
nonideal surface. Although longer chains are likely located on convex regions while shorter ones
603
are within concave regions, it is even possible that divergence of populations could occur on
604
ideal, ultra-smooth flat surfaces because it is currently unclear if the entrapment that produces a
605
shorter population is spatially coordinated with concave regions or just statistically random. As
606
mentioned in Section 3.1, the observed smaller, highly-consistent brush densities of much longer
607
brushes, calculated through ellipsometry with the assumption that brush lengths equal that of
608
homogenous chains, support the idea of a self-regulated, effective brush density, even on ultra-
609
smooth supports. Our observed growth behaviors help explain discrepancies in previous SI-
610
ATRP studies as well as possibly bolster some prior phenomena observed in both empirical and
611
model studies. In our proposed mechanisms, we have uniquely stressed the influence of local
612
initiator density coupled with hopping of radicals across aligned initiator points.
613 614
3.3 Control and determination of brush location
615
In order to produce a desired active layer, SI-ATRP must be directed to certain locations on
616
the membrane substrate, namely top-surface only or within pores. Prior to this study, such
617
control has not been well achieved and verified. Recently, a method of utilizing a viscous pore-
618
filling solvent during initiator binding was developed to putatively prevent SI-ATRP polymer
619
brushes from growing inside the pores of ultrafiltration cellulose membranes [17]. Pore filling
620
was performed before initiator binding so poly(2-hydroxyethyl methacrylate) chains would
621
ideally only grow on the top surface. Highly viscous glycerol worked most effectively. However,
622
the only means of confirming brush location was based on preserved permeability and MWCO,
623
which could have resulted from several other factors. When filling cellulose pores with an
624
especially viscous liquid that also contains hydroxyl groups, it is very difficult to control where
625
this liquid goes. We have seen in SEM that glycerol-filled membranes are covered in glycerol,
626
which would reduce overall growth of tethered polymers. Additionally, glycerol’s hydroxyl 25
627
groups would also bind to the initiator through esterification, reducing the effective
628
concentration of initiator and subsequently its bonding density. Finally, initiator binding was
629
only performed for a minute at room temperature to minimize the time for glycerol to diffuse
630
into the bulk solution and to be consumed by reaction with the initiator. We have seen through
631
XPS analysis that initiator binding is not instantaneous, with full binding not reached until
632
around 12 h of reaction at 35 °C (Figure S2).
633
We hypothesized that with asymmetric membranes, steric hindrance of reagents in ATRP
634
reactions can be exploited to control location of brush growth on membranes. The top surface of
635
a commercial asymmetric cellulose with much smaller pores than the bottom can be directly
636
contacted with the reaction solution while the bottom is sealed-off in a specially-designed sealing
637
wafer. Growth was predicted to occur throughout the pores without use of the sealing wafer.
638
Following PtBA brush synthesis on commercial membranes of 30 and 100 kDa MWCO, with
639
and without the use of our sealing wafer, we first examined permeability as an indirect indicator
640
for brush location. However, we observed that membranes subjected to growth without the use of
641
a sealing wafer simply showed more variable permeability, averaging to a slightly higher
642
permeability than membranes produced with a sealing wafer but with large error bars, resulting
643
in an insignificant difference (Figure S10). Next, we speculated that if we cleaved PtBA chains
644
into PAA and stained the membranes by electrostatic attraction of uranium ions with carboxyl
645
groups, we could use STEM-EDX to visually map locations where uranium and thus also
646
brushes were present (Figure 6b, d, f, and h). Through this more direct yet qualitative method, it
647
initially appeared that growth location was not controlled at all, because samples produced with
648
the usage of the sealing wafer (Figure 6b, d) and without (Figure 6f) had uranium detected across
649
the entire depth of the membrane. Controls of bare cellulose membranes and membranes with
650
crosslinker and initiator were soaked in the trifluoroacetic acid solution used for t-butyl ester
651
cleavage and uranium staining solution to ensure these procedures did not incidentally cleave
652
other areas of the membrane and produce carboxyl groups. These samples both read as having
653
effectively no uranium (Figure 6h). It became apparent that visual mapping typically indicates
654
areas where uranium is present but does not clearly depict the quantity of uranium.
655 656 657 26
658 659
Figure 6. Visual analysis of brush location control from use of a specially-designed sealing wafer. For
660
brush detection, ~4 kg mol PtBA brushes were hydrolyzed to PAA, followed by binding of uranyl ions.
661
(a-d) Cross-sectional images in STEM-mode of a 30 kDa MWCO commercial cellulose membrane with
662
PAA brush produced using a sealing wafer, showing a) dark field mode and b) EDX mapping of uranium
663
of the top-surface sideview as well as c) dark field mode and d) EDX mapping of uranium of the bottom-
664
surface sideview. (e-f) Cross-sectional images in STEM-mode of a 30 kDa MWCO commercial cellulose
-1
27
665
membrane with PAA brush produced without using a sealing wafer, showing e) dark field mode and f)
666
EDX mapping of uranium of the bottom-surface sideview. g) STEM-mode, dark field cross-sectional
667
image of the top surface sideview of a dense cellulose film with PAA brush. Images (a-g) are taken at 14
668
kx magnification. Note that at select positions of targeted EDX, a white spot typically appears from the
669
burning of the TEM beam. Uranium visual maps were produced after 3 minutes of scan time. h) Typical
670
uranium mapping for samples that read as not having any uranium, even in targeted EDX sampling. Here,
671
the reading is from a bare cellulose membrane after being subjected to the tert-butyl ester bond cleavage
672
solution and uranium staining procedure.
673
Targeted EDX was next used to measure relative counts of uranium throughout the thickness
674
of the membrane, producing further insight on location control (Figure 7a). Some EDX sample
675
points are apparent as bright white spots in the dark-field images, where the beam has etched the
676
material (Figure 6a, c, e, and g). For all reported interior points except film measurements, we
677
targeted surfaces of the membrane and pore walls as opposed to cut areas within the membrane
678
polymer for better readings of brush growth on surfaces rather than monomer diffusion into the
679
cellulose. On membranes, 0 µm is at the top surface while 30 µm is the absolute back surface
680
laying against the woven backing. The thickness of the hand-cast film far-exceeded the
681
commercial membrane and had both sides exposed during SI-ATRP. Thus, we present as a
682
control only some sample points from the film.
683
For fair comparison, all samples with brushes were synthesized in the same reaction vessel at
684
the same time, producing chains on the film of ~4 kg mol-1. The two blank controls, bare
685
membranes and AC/BiBB-bonded membranes, read as having effectively 0 uranium across the
686
membrane. The dense film with brush layer had a uranium count of 105 on the outer surface
687
where chains grew and around 30 counts across the interior, indicating the ability of some
688
monomer to diffuse within the cellulose and bond to some initiator sites or adsorb to cellulose.
689
Without the use of a sealing wafer, the uranium content measured across the thickness of both 30
690
kDa and 100 kDa membranes with brushes was roughly constant, exceeding, on average, 1200
691
counts.
692
When the sealing wafer was used, both membrane types show higher counts at the surface
693
and rapidly diminishing counts within the interior. Notably, both membrane types have interior
694
uranium counts below that of samples produced without a sealing wafer but above the blank
695
controls, averaging 159 ± 69 and 764 ± 380 uranium counts for the 30 and 100 kDa MWCO,
28
696
respectively. For the 30 kDa MWCO membrane, this relatively low value suggests that
697
significantly shorter chains grew within the interior (one to a few monomers per chain) whereas
698
the membrane with larger pores likely has chains exceeding 1 kg mol-1 but below 4 kg mol-1
699
throughout its thickness.
700
The top-surface counts for membranes exceed that of the dense film. If we consider the fact
701
that the targeted EDX sample points slightly drifted during the 1 min of measuring time, we posit
702
that scans covered thicknesses exceeding that of just the thin surface brush. Thus, the
703
discrepancy in surface uranium counts of film and membranes with some pore penetration of
704
growing brush is reasonable. The discrepancy between top-surface uranium count for 100 kDa
705
MWCO and 30 kDa MWCO membranes can similarly be explained. That is, reaction solution
706
could penetrate for a longer duration into the larger pores of the former before chains lining pore-
707
mouths grew long enough to block pores. Figures 7b and c schematically illustrate the control
708
achieved with the use of a sealing wafer.
29
709 710
Figure 7. Elucidation of brush control from use of specially-designed sealing wafer, via targeted EDX
711
sampling. a) Results of targeted EDX sampling with uranium counts taken at various depths of different
712
membrane samples. Brush-layered samples had ~4 kg mol-1 PtBA, cleaved into PAA in 12% trifluoracetic
713
acid in dichloromethane. Before staining with uranium, bare membrane and membrane with AC/BiBB
714
bonded (30 kDa MWCO) were subjected to the same trifluoroacetic acid cleavage solution to test if this
715
process produces substantial carboxyl groups that attract uranium. Uranium counts were taken over the
716
course of 1 minute. b) Illustration of growth without the use of sealing wafer, whereby polymerization
717
solution diffuses into the membrane from all directions, producing brush growth throughout the pores. c)
718
Illustration of growth with the use of a sealing wafer, whereby polymerization solution diffuses into the
719
membrane from only the top side of the asymmetric membrane where smaller pores are exposed.
720
30
721
3.4 Brush layers as membrane active layers
722
As a proof of the previous concepts and a demonstration of dense brush layers as
723
selective membranes, a diblock copolymer of PtBA-PHEA on a commercial 30 kDa MWCO
724
cellulose membrane was synthesized with the use of a sealing wafer to direct growth to primarily
725
the top surface. We used our findings from the development of synthesis and characterization
726
methods to inform the fabrication of these membranes. A proportion of 55 mol% BiBB acyl
727
halides was chosen to maximize the brush density while maintaining stability. Because we had
728
observed a phase of practically static growth occurring at ~4 kg mol-1, total brush length was
729
maintained to around this value to keep layers distinct and even for a more accurate report of
730
block sizes. Furthermore, near-zero water permeabilities were often observed with PtBA brushes
731
of only 1-4 kDa (Figure 4), suggesting near-full coverage of pores. Because a 30 kDa MWCO
732
membrane is expected to have an average pore diameter of ~8 nm [56], we predict that around
733
the perimeter of the average pore mouth alone, the summed molecular weight of chains of 4 kg
734
mol-1 would be ~138 kDa, far exceeding 30 kDa (Figure S11). Thus, we expected that this small
735
size of brush of only 4 kg mol-1 would demonstrate the minimum layer length to affect solute-
736
solute selectively and rejection. However, we acknowledge that for real applications, it is
737
probably necessary to increase this length to ensure full coverage of larger pores, especially if a
738
polymer is selected that greatly collapses in the filtration solvent of choice.
739
To confirm the addition of each layer in the diblock, water contact angles were measured
740
on bare and modified membranes (Figure 8b). As expected, a layer of hydrophobic PtBA
741
increased water contact angle while subsequently-added PHEA reduced it. Additionally, ATR-
742
FTIR spectra were generally consistent with successful grafting, although signal intensity was
743
low due to short brush length (Figure S12).
31
744 745
Figure 8. Brush-layered membranes produced for proof-of-concept transport studies. a) Structure of
746
polymer brush chains. b) Membrane surface water contact angles in air, calculated as an average of
747
measurements from photographs taken every ~0.16 s for 10 s each on 3 samples total. Drop size was 1
748
µL. Subscripts represent the number average molecular weight of each polymer layer (kg mol-1), assumed
749
to be the same as the length of homogeneous polymers simultaneously produced. Brush layers were
750
synthesized atop a commercial 30 kDa MWCO cellulose membrane using 55% acyl halides from the
751
initiator BiBB (45% from crosslinker AC).
752 753
To demonstrate the selective capabilities of these brush layers, we measured flux of solutes
754
of varying octanol/water partition coefficients in a counter-diffusion experiment with organic
755
solutes on one side of a diffusion cell and saline solution in the opposite chamber. 32
756
Concentrations were balanced for a net-zero osmotic pressure difference between the chambers.
757
For fair comparison, the organic solutes chosen, including L-asparagine, hydroquinone, and
758
thymol, are all neutral at the experimental operating pH and fall within 110-150 Da in size
759
(Figure 9a). Sodium chloride was counter-diffused for reference.
760
Changes in diffusion or flux of these solutes indicated successful membrane modification and
761
altered selectivity (Figure 9b). Solute fluxes through each additional layer correlate well with the
762
octanol/water partition coefficients (LogP) for each solute (Figure 9a, b). The addition of a short,
763
relatively hydrophobic brush, PtBA of ~1.5 kg mol-1, slightly decreased the diffusion rates of all
764
solutes while reducing transport of sodium chloride below all others. With the addition of a
765
longer, relatively hydrophilic PHEA layer of ~2.5 kg mol-1, all diffusion rates reduced and
766
thymol (the most hydrophobic solute) dropped to the slowest diffusion rate. When a longer PtBA
767
layer of ~3.0 kg mol-1 was first added, the transport rate of the most hydrophilic solute L-
768
asparagine significantly dropped two orders of magnitude from the membrane with
769
crosslinker/initiator. Interestingly, when this long hydrophobic brush was followed by a short
770
PHEA layer of around ~1.0 kg mol-1, fluxes of all solutes somewhat increased. It is possible that
771
a layer of solely PtBA collapses completely in water while a diblock copolymer of PtBA and
772
PHEA aligns into a looser, lamellar configuration. Another explanation is that the chains lining
773
the openings of pores are short enough for a hydrophilic layer to produce an effectively smaller
774
PHEA-filled pore. As water is a better solvent for PHEA, this less-dense pore would allow all
775
solutes through more easily.
776
We also measured permeability and rejection of a bare and modified 30 kDa MWCO
777
membrane with a dead-end permeation cell, simulating ultrafiltration and reverse osmosis. With
778
the addition of just a PtBA brush of ~4 kg mol-1, rejection of lysozyme, a globular protein with a
779
molar mass of 14.3 kDa, drastically increases from 18 to 97%. Permeability also noticeably
780
decreases, from 290 to 1.1 Lm-2h-1bar-1, suggesting nearly full coverage of the membrane pores
781
(Figure 9c). However, NaCl was not rejected before or after brush layer formation, suggesting
782
the presence of some small defects (pores) that are smaller than lysozyme.
783
We showed here that a relatively small brush molecular weight can alter transport rates,
784
suggesting SI-ATRP may be used to produce the thinnest of ultrathin composite membranes. For
785
real applications, thicker polymer blocks are likely necessary, but this system allows for a
786
highly-controlled optimization of active layer thickness. These proofs of concept demonstrate 33
787
that selectivity and permeability can be altered by strategically choosing patterns of specific
788
polymers, grown to particular lengths.
789 790
Figure 9. Changes in solute flux and rejections of various solutes through different brush layers on a
791
commercial 30 kDa MWCO cellulose membrane. a) Pertinent characteristics of solutes involved in
792
transport experiments for diblock copolymer brush-layered cellulose membranes, including molecular
793
weight and octanol/water partition coefficients (LogP), which were obtained from PubChem. b) Small
794
molecule diffusion through brush-layer membranes. Fluxes of solutes with varying octanol/water partition
795
coefficients through bare and modified membranes were determined through use of a Valia-Chien-type
796
diffusion cell with 10 mL chambers and a 7 mm orifice diameter, set at ambient temperature with constant 34
797
stirring. One chamber was charged with all organic solutes at a concentration 0.133 mM each, while the
798
opposite side contained 0.2 mM sodium chloride to balance osmotic pressure. Changes in organic solute
799
concentration on the saline side were detected through use of an HPLC column, while changes in salt
800
concentration on the organic side were detected through conductivity meter probe. The average of
801
quadruplicate data with standard deviation as error bars is depicted. c) Pressure-driven filtration through
802
brush-layer membranes. Pure water permeability and rejections of sodium chloride and lysozyme were
803
measured for bare cellulose membrane and a membrane with a PtBA brush through using a stirred dead-
804
end cell with 7 mm diameter orifice, driven with 7 bar and 2 bar hydraulic pressure for membranes
805
without and with brushes, respectively. Saline feed was 20 mM in sodium chloride while lysozyme
806
solution was 35 µM. Lysozyme is a globular protein with a molar mass of 14.3 kDa. Conductivity
807
measurements for retentate and permeate were used to determine salt rejection while TOC readings were
808
used to determine lysozyme concentrations. The average of triplicate data with standard deviation as error
809
bars is depicted. For membrane types, subscripts represent the number average molecular weight of each
810
polymer layer (kg mol-1), assumed to be the same as the length of homogeneous polymers simultaneously
811
produced. All membranes reacted in AC/BiBB solutions containing 55% BiBB acyl halides (45% from
812
crosslinker AC).
813
4. Conclusion
814
Synthesizing reproducible, useful brush-layered membranes necessitates the development
815
of new, more robust techniques to control and resolve the location, density, and length of SI-
816
ATRP-produced brushes on porous polymeric supports. We demonstrated here the use of
817
competitively-bound crosslinker and initiator to control brush density. A certain proportion of
818
crosslinker was necessary to stabilize the cellulose, limiting density control within a certain
819
range. We showed that brush density could be measured by use of a silver ion probe and
820
inductively-coupled plasma mass spectrometry. We found that the ratios of initiator and
821
crosslinker acyl halide content used in the reaction solution did not directly match grafting
822
density, as other factors like binding rate for each type of molecule and steric hindrance of
823
growing brushes influenced the resultant density. We elucidated unique growth behavior on a
824
nonideal polymeric surface, experimentally demonstrating phases of seemingly faster and slower
825
ATRP heterogeneous growth rates compared to homogeneous rates and observing two distinct
826
populations. Such growth observations support previously-proposed growth mechanisms. We
827
showed that location of brushes could be partially controlled through use of sealing wafers if the
828
base porous support is asymmetric with relatively small top-surface pores. Targeted STEM-EDX 35
829
measurements throughout the cross-sections of samples with uranium-tagged PAA brushes
830
allowed direct quantitation of brush location control. Finally, after identifying brush density and
831
length limitations, we produced brush homopolymer and copolymer membranes that demonstrate
832
the possibility of using dense brushes as selective layers, tailored by controlled brush length and
833
type.
834 835
Acknowledgements
836 837
This material is based upon work supported by the National Science Foundation Graduate
838
Research Fellowship Program under Grant No. DGE-1752134 and the 2018-2020 NWRI-AMTA
839
Fellowship for Membrane Technology awarded to C.J.P. Any opinions, findings, and
840
conclusions or recommendations expressed in this material are those of the authors and do not
841
necessarily reflect the views of the National Science Foundation, National Water Research
842
Institute, or the American Membrane Technology Association. Facilities used for AFM, TEM,
843
and SEM were provided by the Yale Institute of Nanoscale and Quantum Engineering (YINQE).
844
TOC and ICP-MS analyzers were provided by the Yale Analytical and Stable Isotope Center
845
(YASIC). The Yale West Campus Materials Characterization Core provided equipment for XPS
846
measurements, while the Yale Chemical and Biophysical Instrumentation Center (CBIC)
847
provided FTIR and NMR equipment. Additionally, the authors would like to thank Dr. Chanhee
848
Boo for conducting XPS.
849
36
850
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987
39
Highlights •
Dense acrylate-based brush layers grown from cellulose UF membranes by SI-ATRP
•
Custom reaction wafer allowed for brush to be located mainly on top surface
•
Cleavage with SEC showed two divergent polymer populations in brush layers
•
Grafting density determined through silver bind-and-elute method
•
Solute fluxes measured through brush layers of homo- and block polymers
Conflict of Interest and Authorship Conformation Form Please check the following as appropriate:
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
o
The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
Author’s name Menachem Elimelech Cassandra Porter Mingjiang Zhong Jay Werber Yan-fang Guan Cody Ritt
Affiliation Yale Yale Yale Univ of Minnesota Yale/USCTC Yale
S0376-7388(19)33112-6
DOI:
https://doi.org/10.1016/j.memsci.2019.117719
Reference:
MEMSCI 117719
To appear in:
Journal of Membrane Science
Received Date: 7 October 2019 Revised Date:
30 November 2019
Accepted Date: 3 December 2019
Please cite this article as: C.J. Porter, J.R. Werber, C.L. Ritt, Y.-F. Guan, M. Zhong, M. Elimelech, Controlled grafting of polymer brush layers from porous cellulosic membranes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117719. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Statement
Menachem Elimelech research Cassandra Porter Mingjiang Zhong supervise the ATRP work Jay Werber Yan-fang Guan Cody Ritt
Adviser of C Porter, supervised First author, carried bulk of work Secondary adviser to C Porter; Analyzes data with C Porter Helped with ATRP work carried out all TEM work
Controlled Grafting of Polymer Brush Layers from Porous Cellulosic Membranes
Journal of Membrane Science Revised: November 30 , 2019 Cassandra J. Portera, Jay R. Werbera,b, Cody L. Ritta, Yan-Fang Guana,c, Mingjiang Zhonga, Menachem Elimelecha*
Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, United States b. Department of Chemistry, University of Minnesota, Minneapolis, MN 55455-0431, United States c. CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science & Technology of China, Hefei 230026, China a.
* Corresponding author: Menachem Elimelech, Email: [email protected], Phone: (203) 432-2789
Declarations of interest: none.
1
Graphical Abstract 1
Abstract
2
Dense polymer brushes on porous supports present powerful routes toward fouling-resistant
3
coatings and ultra-thin, mechanically-stable selective layers. However, characterization of
4
polymer chains grafted from nonideal, planar substrates often relies on indirect methods to
5
determine the length, density, and location of polymeric brushes. In this study, we explore new
6
methods to control and characterize the properties of polymer brush layers. Starting from hand-
7
cast cellulose films and commercial cellulose membranes, we use surface-initiated atom transfer
8
radical polymerization to graft polymeric chains. We tailor brush density and simultaneously
9
stabilize the support by varying the proportions of initiator and crosslinker esterified to the
10
cellulose substrate. On films with grafted polyacrylic acid (PAA) chains, we use the silver-
11
binding method to determine brush density. We analyze brush growth behavior by directly
12
cleaving chains from the surface of films for analysis by size exclusion chromatography,
13
uniquely observing the divergence of two distinct populations of differing length beyond a
14
certain molecular weight. Such behavior would be important to consider if the alignment of
15
block copolymer layers is absolutely necessary in an application. By controlled contact of
16
polymerization solution with the top surface, we show that brush growth can be partially directed 2
17
to the top surface for asymmetric cellulose membranes with relatively low molecular weight
18
cutoffs. In a semi-quantitative method to locate brush growth, depleted uranium was bonded to
19
grafted PAA chains, and targeted emission x-ray spectroscopy was conducted for comparison of
20
uranium content in various regions. Finally, we used our developed methods and findings to
21
inform the synthesis of homopolymer and diblock copolymer brush layers, which we then used
22
as selective layers in pressure-driven filtration and diffusion cell experiments.
23
Keywords
24 25
Surface-initiated atom transfer radical polymerization; thin-film composite; selectivity; grafting density; polymer brush
26
1. Introduction
27
The inherent surface properties of membrane-forming materials can be disadvantageous in
28
certain applications. For instance, the surface chemistry of aromatic polyamide used in reverse
29
osmosis desalination membranes can promote fouling, causing flux decline and diminishing
30
membrane performance, while some membrane materials used in biomedical applications are not
31
biocompatible unless modified [1]. Meanwhile, insufficient hydrophobicity of membranes used
32
in membrane distillation can induce pore wetting, which reduces the thermal driving force and
33
can cause salt break-through [2]. Catalytic, reactive, and adsorptive membranes may not
34
inherently provide sufficient functional groups for binding the desired ligands and reactive
35
species. In all these situations, the addition of polymer brushes of carefully chosen composition
36
can alter the surface characteristics of the membrane material to improve performance [3, 4].
37
Additionally, densely-grafted brush layers grown on the surface of porous supports could
38
provide alternative routes to produce ultrathin selective layers with compositions tailored for
39
specific applications. All these applications of brush layers necessitate precise addition of the
40
polymer chains to beneficially modify the base membrane without compromising other
41
performance requirements.
42
Atom transfer radical polymerization (ATRP) is a particularly attractive method for brush
43
growth. The controlled monomer-by-monomer addition during the reaction can produce
44
relatively monodisperse polymers with control over molecular weight, ranging from ~0.5 to
45
~100 kDa [5]. Advantageously, the retention of the initiating halogen at the polymer end allows
46
for the synthesis of block copolymers through sequential reactions involving different monomers 3
47
[6, 7]. By tethering an initiator to a membrane substrate, polymers may be grafted from a surface
48
with highly controlled, consistent lengths in surface-initiated atom transfer radical
49
polymerization (SI-ATRP), which can produce a higher brush density than top down “grafting-
50
onto” methods in which pre-formed polymers are bound to a surface [8].
51
ATRP-produced polymers “grafted-onto” and “-from” surfaces have been used to reduce
52
biofouling by increasing hydrophilicity [9, 10], to produce stimuli-responsive layers [11], and to
53
increase surface energy in pervaporation [12]. In many cases, these applications do not require
54
relatively monodisperse polymers and/or full surface coverage and thus could have been
55
achieved through simpler synthesis schemes than using highly-controlled ATRP. The advantages
56
of organic-phase SI-ATRP were demonstrated in one study by producing uniform coverage of
57
gold surfaces with nanoscale-thick poly(2-vinylpyridine) films with controlled grafting density
58
and polymer size to optimize biomolecular adsorption [13]. SI-ATRP was used in another study
59
to modify (decrease) the pore size of cellulose ultrafiltration membranes, with hydrophilic
60
poly[poly(ethylene glycol methacrylate)] grafted throughout the cellulose pore structure [14].
61
Dextran rejection curves shifted uniformly to lower molecular weights, suggesting controlled
62
modification of pore sizes. Membrane active-layer preparation involving ATRP has focused
63
more on producing block copolymers in homogeneous reactions that are later self-assembled into
64
patterned membranes, especially for anion- and proton-exchange membranes for fuel cells [15,
65
16]. Although poly(ethylene glycol dimethacrylate) polymer brushes grown directly from porous
66
alumina have shown high selectivity of CO2 over CH4 [3], SI-ATRP-derived active layers
67
comprising dense polymeric brushes that are robustly supported to withstand applied pressures in
68
desalination and other processes beyond gas separations have yet to be fully explored. These
69
layers would wholly exploit the unique advantages of SI-ATRP on porous substrates, requiring
70
even layering and high grafting density.
71
There is a critical need for more control of SI-ATRP when synthesizing brush layers in
72
membrane applications. For example, in the synthesis of selective layers, brush growth would
73
need to be directed to the top surface of a porous support, as opposed to throughout the
74
membrane thickness. The support layer would also need a high density of binding sites for
75
initiator and a high degree of solvent stability in the organic solvents used for ATRP.
76
Additionally, improved characterization techniques are needed for assessing brush layer
77
properties. Previous characterization methods of SI-ATRP-produced brushes have relied heavily 4
78
on assumptions which may not be entirely accurate, especially when such polymers were grown
79
on nonideal, relatively rough surfaces such as porous membranes [17-31]. To date, a systematic
80
study of the methods to govern and measure SI-ATRP growth rates as well as verify and control
81
location and density of brush-layer growth on nonideal membranes and polymer films has not
82
been reported.
83
This study focuses on the development of such techniques. Through direct cleavage and
84
measurement of brush size, we show the unique growth characteristics of polymer chains grown
85
from the nonideal surface of a dense cellulose film, which contains both concave and convex
86
regions on a nanoscale. A more direct measurement of brush density is developed, whereby
87
silver ions are bonded to the negatively-charged sidechains of polyacrylic acid (PAA) brushes,
88
eluted, and measured in inductively-coupled mass spectrometry (ICP-MS). Using commercial,
89
asymmetric regenerated cellulose ultrafiltration membranes as substrates, growth location is
90
controlled by exploiting steric hindrance with a specially designed sealing wafer. Regions of
91
growth are directly verified and quantified through uranyl nitrate staining and scanning
92
transmission electron energy-dispersive x-ray microscopy (STEM-EDX). Finally, as a proof of
93
concept, we demonstrate that SI-ATRP-produced brushes can serve as effective selective layers
94
in pressure-driven filtration and diffusion experiments.
95
2. Materials and methods
96
2.1 Chemicals and materials
97
Cellulose acetate (CA) of ~50,000 Da, α-bromoisobutyryl bromide (BiBB), adipoyl chloride
98
(AC), triethylamine (TEA), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), copper(I)
99
and copper(II) bromides (Cu(I) and Cu(II)), ethyl α-bromoisobutyrate (EBiB), tert-butyl acrylate
100
(tBA), diethyl ether, phosphoric, glacial acetic, and trifluoroacetic acids, and silver nitrate were
101
purchased from Sigma-Aldrich (St. Louis, MO USA). Commercial regenerated cellulose
102
ultrafiltration disks (Ultracel) of 30 and 100 kDa molecular weight cut-off (MWCO) were
103
obtained from USA-based EMD Millipore (Jaffrey, NH USA) while benzyl chloride was a
104
German EMD Millipore product (Darmstadt, Germany). Acetone, dichloromethane (DCM), and
105
potassium hydroxide were of Macron Fine Chemicals brand while hexanes, 2-propanol,
106
methanol, nitric acid, and HPLC-grade acetonitrile were of J. T. Baker brand, both from Avantor
107
Performance Materials (Center Valley, PA USA). Magnesium perchlorate hexahydrate was from 5
108
Alfa Aesar (Word Hill, MA USA) while n-butyl acrylate (nBA), 2-hydroxy ethyl acrylate
109
(HEA), and dimethyl sulfoxide (DMSO) came from Alfa Aesar (Heysham, England). Ethanol
110
(EtOH) was purchased from Decon Labs (King of Prussia, PA USA). HPLC-grade
111
tetrahydrofuran (THF) and L(+)-ascorbic acid came from Fisher Scientific (Fair Lawn, NJ USA).
112
Dimethyl formamide (DMF) and 0.45 µm MWCO PTFE syringe filters were purchased from
113
VWR International (Radnor, PA USA). 1,1,3,3-tetramethylguanidine (TMG) was provided by
114
the Tokyo Chemical Industry (Portland, OR USA). Uranyl nitrate was purchased from SPI
115
Supplies, a division of Structure Probe (West Chester, PA USA). Alumina basic and neutral were
116
purchased from Sorbtech (Norcross, GA USA). LR White Resin came from the London Resin
117
Company (Reading, England).
118
Cellulose filter paper was purchased from Whatman International (Maidstone, England).
119
Chemical and organic-solvent resistant rubber and polytetrafluoroethylene sheets, blocks, and
120
other hardware for fabricating specially-designed membrane sealing wafers as well as a dead-end
121
cell were purchased from McMaster-Carr (Princeton, NJ USA). Deionized ultra-filtered (DI)
122
water was prepared through passage in a Milli-Q system with Elix Technology from EMD
123
Millipore and utilized throughout synthesis and experimentation.
Nitrogen was provided by Airgas East (Salem, NH USA).
124
Chemicals and materials were used as purchased, except monomers, Cu(I), and commercial
125
cellulose membranes. Monomers were passed through a basic alumina column to remove
126
inhibitor before usage. Cu(I) was prepared by stirring a suspension of 1 molar equivalent Cu(I)
127
with 2 molar equivalents L(+)-ascorbic acid in DI water for 15 min and filtering out the white
128
product. After thorough, consecutive washing with DI water, glacial acetic acid, and diethyl
129
ether, the product was fully vacuum-dried and stored in a glovebox under nitrogen and darkness
130
until use. Before any modification or testing, commercial membranes were soaked overnight in
131
1:1 2-propanol:DI water on a shake plate to remove glycerol and wet the membrane pores.
132 133
2.2 Production of dense cellulose films
134
Dense cellulose films were produced through long-established methods [32]. Specifically, a
135
dopant consisting of 15 wt% CA, 67 wt% acetone, and 18 wt% nonsolvent (10.5 wt%
136
magnesium perchlorate in water) was prepared by stirring at 70 °C until all polymer was
137
dissolved. The solution was cooled overnight in a refrigerator. The dopant was cast cold via a
138
doctor blade onto chilled glass plates to a thickness of 0.413 mm. Nonsolvent and solvent were 6
139
evaporated off for 1 hr, immersed in an ice bath for 15 minutes to precipitate the polymer, and
140
peeled from the glass. To regenerate cellulose by conversion of acetyl groups to hydroxyl
141
groups, films were set in 0.05 M sodium hydroxide in ethanol for 12 hours and rinsed several
142
times in DI water [33].
143
Cellulose films and commercial membranes were stored in DI water in a refrigerator to
144
prevent bacterial growth and pore or structural collapse. We observed that the pores of wet
145
cellulose membranes collapse when dried from water (Figure S1). Therefore, anytime a cellulose
146
sample was dried, it was first exchanged from water to 2-propanol to hexane.
147 148
2.3 Initiator and crosslinker binding
149
Cellulose films or commercial membranes were cut into circles 2 cm in diameter.
150
Approximately 20 of these samples per initiator and crosslinker binding reaction were patted
151
with lint-free wipes to remove excess water and immediately soaked in THF. Meanwhile, a
152
solution of 40 mL of THF with 1.10 mL TEA was bubbled with nitrogen in a cylindrical jar with
153
septum screw cap for 30 minutes over ice to mitigate evaporation of solvent. Membranes or films
154
were added, and this solution was bubbled an additional 30 minutes. AC as crosslinker and BiBB
155
as initiator were pre-mixed in a separate vial in the proportion of interest, and 1 mL of this
156
solution was added slowly dropwise to the still-bubbling solution submerged in ice water. The
157
formation of a white paste indicated that the reaction was proceeding, as TEA neutralizes the
158
solution by reacting with the released hydrogen ions and halogens to form a precipitant
159
triethylammonium chloride/bromide. Bubbling needles were removed, and the cap was covered
160
in Parafilm and tape to ensure no air seeped within the vessel. The reaction proceeded on a shake
161
plate for 12 hours in a 40 °C oven for full binding (Figure S2). Membranes were consecutively
162
rinsed in fresh baths of THF, acetone, 2-propanol, and DI water on a shake plate at room
163
temperature for 1 h in each solvent.
164 165
2.4 Growth of acrylate polymers through SI-ATRP
166
For the SI-ATRP reactions in this work, a homogeneous ATRP reaction with initiator EBiB
167
was conducted simultaneously in the same vessel as the SI-ATRP reaction. Proportions of
168
reagents were chosen to achieve targeted polymer molecular weights. Table S1 provides some
169
proportions of reagents and reaction conditions with resultant homogeneous polymer sizes and 7
170
dispersities, with tBA and HEA as the monomers. Figure S3 provides rates of butyl acrylate
171
growth when only proportions of Cu(I):Cu(II) are altered while other reagent quantities are held
172
constant.
173
In a typical SI-ATRP reaction, DMF was thoroughly mixed for an hour with ligand
174
PMDETA and Cu(II) within a cylindrical vessel with septum screw cap. Meanwhile, cellulose
175
membrane or film swatches with AC/BiBB bonded were patted with lint free wipes. Depending
176
on the particular studies with these samples, these swatches were either inserted within specially
177
designed sealing wafers to control brush growth location or left unsealed. The use of such a
178
sealing wafer will be further discussed in Section 2.7. Sealed or unsealed samples were set to
179
soak in DMF.
180
Meanwhile, monomer with inhibitor removed was added to the reaction solution before
181
bubbling with nitrogen for 10 min per 10 mL of reaction solution. EBiB initiator was added, and
182
membranes/films were rapidly added through the open cap to ensure minimal incorporation of
183
air. The solution was bubbled an additional 5 minutes per 10 mL of solution. In a small glass
184
vial, Cu(I) was suspended in 1 mL of DMF and rapidly injected into the reaction vessel through
185
the septum top. With nitrogen still purging, the reaction was set to proceed in an oil bath at 70 °C
186
for 45 min.
187
To quench the reaction, membranes were immediately set to soak in fresh DMF for 1h.
188
Meanwhile, to measure relative number average molecular weight (Mn), weight average
189
molecular weight (Mw), and dispersity index (Ð), a couple drops of the homogeneous reaction
190
solution were mixed with 2 mL of THF and filtered through neutral alumina. This filtered
191
solution was run through a SEC column (EcoSEC HLC-8320GPC, Tosoh Bioscience, Tokyo,
192
Japan) with THF mobile phase and polystyrene (PS) stationary phase using PS standards for
193
calibration. Resultant peaks were analyzed with equipment-provided software (EcoSEC
194
Analysis). Membranes or films were further rinsed sequentially in fresh DMF, acetone, 2-
195
propanol, and finally DI water baths on a shake plate for a half hour each and stored in DI water
196
in a refrigerator.
197 198
2.5 Polymer growth rate determination
199
Heterogeneous polymer growth rate was compared to homogeneous through direct cleavage
200
of poly(tert-butyl acrylate) (PtBA) brushes. Larger quantities of samples were used to have 8
201
enough polymer sample to be detectable in the SEC column. In glass vials, ~9 cm2 of film with
202
PtBA brushes were submerged overnight in 2 mL of 5 vol% potassium hydroxide in water to
203
hydrolyze ester bonds and cleave off the brush layers. This produced cleaved polyacrylic acid
204
(PAA) chains, which were incompatible with the SEC column. Thus, PAA was esterified into
205
poly(benzyl acrylate) (PBzA) through a modified version of a previously-described method [34].
206
In this method, the basic solution with cleaved chains was neutralized with hydrochloric acid,
207
and ~1.5 mL of water were evaporated off to reduce the presence of water but ensure polymer
208
was not trapped in dry salt matrices. To this, 2 mL of DMSO, 126 µL of TMG, and 172 µL of
209
benzyl chloride were added. The reaction proceeded at least 10 h on a shake plate at room
210
temperature. Upon completion, the solution was neutralized with 114 µL acetic acid. The
211
polymer was precipitated in excess of 50:50 % MeOH:water solution and recovered by vacuum
212
filtration. The polymer was rinsed twice with MeOH and thoroughly dried at 40°C. In
213
preparation for SEC, the polymer was re-dissolved in 2 mL of THF and filtered through neutral
214
alumina.
215
polymer and converted polymer was prepared (Figure S4). For this calibration, homogeneously-
216
grown PtBA was measured in SEC, cleaved into PAA, converted into PBzA through the method
217
described above, and rerun through SEC.
Because PBzA was produced as opposed to PtBA, a calibration between grown
218 219
2.6 Brush density determination
220
A method was developed in our group whereby silver ions were used to quantify the density
221
of carboxyl functional groups in polyamide reverse-osmosis membranes, during which 1:1
222
coordination of carboxyl groups with silver ions and complete elution of silver were verified
223
[37]. Here, we applied this silver bind-and-elute technique to determine the density of polymer
224
chains on a surface. In general, we produced grafted chains with one carboxyl group per repeat
225
unit, i.e., PAA, and quantified the carboxyl density using the silver ion probe method. The brush
226
length was determined through the direct cleavage methods described in the previous section.
227
Subsequently, brush surface density σ was determined through
228
=
,
[1]
229
where Ns is the total number of eluted silver probes (CS,EVE, or the concentration of silver eluted
230
from brush-layer carboxyl groups multiplied by the solution volume), Mn is the measured chain 9
231
molecular weight, Mn,R is the repeat unit molecular weight, and Ap is the apparent area of the
232
dense film subjected to the polymerization reaction. CS,E was determined from the silver
233
concentration measured in the elution solution minus the average concentration of silver eluted
234
from three blanks. Blanks were samples with only AC/BiBB bonded to account for binding of
235
silver on the cellulose and any residual acyl chlorides that would convert to carboxyl groups in
236
water.
237
More specifically, AC:BiBB ratios were varied to control brush density on dense cellulose
238
films. Because negatively-charged polymers cannot be grown in traditional ATRP reactions due
239
to unwanted interactions with charged metal activators/deactivators, PtBA chains were grown
240
and selectively cleaved at their tert-butyl ester bonds to produce carboxyl sidechains through a 6
241
h reaction on shake plate in 12 vol% trifluoroacetic acid (TFA) in DCM, a commonly used
242
deprotection system [38]. The same reaction conditions resulted in ~99% hydrolysis of tert-butyl
243
ester bonds for PtBA in bulk solution after 6 h of reaction time, verified through proton nuclear
244
magnetic resonance (1H-NMR) analysis (Agilent DD2 400 MHz NMR Spectrometer) (Figure
245
S5). After cleavage, films were rinsed in fresh DCM, water, and finally acetone before fully
246
drying.
247
For silver binding and elution, the method described elsewhere was used [37], with silver
248
solutions set to pH 10. Because the silver elution solution of 1% nitric acid in water removes
249
bound ions but also potentially cleaves the ester bonds between chains and support, repeated
250
silver binding and elution could not be conducted multiple times on each sample. Elution
251
solutions were run through ICP-MS (ELAN DRC-e, Perkin Elmer, Waltham, MA) to determine
252
silver concentration. Serially-diluted solutions of 0-100 µg L-1 silver nitrate in 1% nitric acid in
253
water were used to produce a calibration curve, while 40 µg L-1 indium nitrate was used as an
254
internal standard. The full method is represented in Figure 1.
255
10
256 257
Figure 1. Methodology for determining brush density. After binding initiator, poly(tert-butyl acrylate)
258
chains are grown from the support layer via surface-initiated atom transfer radical polymerization (SI-
259
ATRP). Ester bonds connected to tert-butyl sidechains are selectively cleaved into carboxyl groups in
260
dichloromethane with 12% trifluoroacetic acid. Positively-charged silver ions electrostatically bind to
261
deprotonated carboxyl groups in basic conditions. In a set volume of 1% nitric acid in water, bound silver
262
is eluted, and its concentration is measured in inductively coupled mass spectrometry. After cleaving and
263
directly measuring brush molecular weight, brush density can be determined with Equation 1.
264
2.7 Brush location control and verification
265
A special sealing wafer was designed to fit within the opening of the septum-capped jar to
266
ensure a controlled, oxygen-free reaction environment. This frame was crafted from solvent-
267
stable, high-density materials and consisted of laser-cut rubber and polytetrafluoroethylene rings
268
and disks, sandwiched together with polytetrafluoroethylene screws and nuts (McMaster Carr)
269
(Figure 2).
270
Utilizing STEM-EDX, we developed a more direct, semi-quantitative method of determining
271
growth location than typically employed. However, the brush layers in our study are not
272
elementally distinctive enough to distinguish from the cellulosic support. Therefore, PtBA chains
273
grafted-from films and membranes, with and without the use of a sealing wafer, were selectively
274
hydrolyzed to PAA, as described in Section 2.6. To enhance elemental contrast, depleted 11
275
uranium from uranyl nitrate was bonded to the carboxyl groups of the PAA layer using a
276
previously described procedure [40].
277
Small wedges were embedded in the water-miscible acrylic resin LR White as previously
278
reported [41]. After curing at 65 °C for 24 hours, 90-nm-thick cross-sections were sliced with a
279
diamond knife (2.5 mm Ultra 45°, Diatome) using a Leica Ultramicrotome UC7 (Leica
280
Microsystems) and mounted onto carbon/formvar coated copper grids (Carbon Type-B, Ted
281
Pella). Dark-field images and STEM-EDX elemental mapping were obtained at 14000x
282
magnification using an FEI Tecnai Osiris 200kV S/TEM with 0.25 nm point resolution. Uranium
283
maps were each produced with 3 minutes of scan time. To identify elemental spectra and the
284
relative abundance of uranium, targeted EDX measurements were taken at sample points
285
throughout the thickness of the membranes and films using 1 minute of sample time.
286
287 288
Figure 2. Methodology to control location of brush growth. a) Components of a sealing wafer that fits
289
within a septum-capped vessel for controlled atmospheric conditions. The top (skin) surface of an
290
asymmetric membrane can be exposed to the reaction solution so chains grow predominantly on the top
291
surface. The relatively small pores of the porous membrane prevent easy access of reactants to the
292
membrane interior. b) Photographs of the sealing wafer, showing exposed membrane on the ring side and
293
the sealed-off side of the membrane on the disk side.
294
12
295
2.8 Film and membrane characterization
296
General characterization of bare and modified cellulose membranes included SEM imaging,
297
water contact angles, water permeability, and roughness. To ensure production of dense films
298
rather than membranes, top and bottom surface morphology of cast films and membranes for
299
comparison were sputter coated with 5 nm of iridium (BTT-IV Evaporator, Denton Vacuum,
300
LLC) and examined by scanning electron microscopy (SEM, Hitachi SU-70). For comparison,
301
films and hand-cast membranes were formed, although only films were used in experiments.
302
Membranes were formed by immersion precipitation without any solvent evaporation time.
303
Conveniently, cellulose samples with pores, confirmed via scanning electron microscopy,
304
appeared white to the naked eye once dried, while dense films appeared clear (Figure S1).
305
For static fluid contact angle testing, samples were taped to microscope slides with double-
306
sided tape. Contact angles for DI water were measured via sessile drop method with a contact
307
angle goniometer (OneAttension, Biolin) by photographing the sideview of a 1 µL droplet sitting
308
on the surface every ~0.16 s with digital camera for 10 s total post drop placement. Left and
309
right-side contact angles were measured in processing software (OneAttension software), and the
310
measurements of three drops per sample type were averaged together.
311
FTIR measurements were taken with an FTIR/Raman Thermo Nicolet 6700 with commercial
312
bare and modified membrane top surfaces pressed firmly against a diamond attenuated total
313
reflection cell. Before measurement, samples were exchanged through solvent and dried for 24 h
314
in a vacuum oven at 40 °C to remove any water and solvent. Blank readings comprised 4 scans
315
while 32 scans were taken for samples. Samples were measured twice on two different locations
316
to ensure consistent reading.
317
Film roughness was characterized in air with atomic force microscopy (AFM, Dimension
318
FastScan, Bruker) using PeakForce Tapping mode and a silicon-tipped nitride lever probe
319
(FASTSCAN-B, Bruker). Sections of 1 µm2 were scanned at a rate of 2.90 Hz with 496 samples
320
per line and 512 histogram bins. In data analysis software (NanoScope Analysis 1.9, Bruker),
321
root-mean-square roughness, Rq, and maximum roughness, Rmax, of each were determined. Four
322
scans at different locations of each surface type were averaged for the reported value.
323
Water permeability was measured with a specially-made, rubber-bodied dead-end cell for
324
higher pressures (maximum of 14 bar) and relatively small sample sizes, with a 7-mm diameter
325
exit orifice and a 30-mL chamber. Permeability testing was conducted at 2.07 bar (30 psi) for 13
326
membranes without brushes and 6.89 bar (100 psi) for membranes with brushes. For loose
327
membranes (i.e. permeability exceeded 100 Lm-2h-1bar-1), permeability was tracked continuously
328
and recorded once steady state was reached. For tight membranes, the system was allowed to
329
compress overnight before permeate was collected for 48 h. Water permeabilities of 3-4 samples
330
were averaged.
331 332
2.9 Solute transport rate and rejection
333
To demonstrate how brush layers with varying hydrophobicity affect solute transport rates, a
334
variety of solutes with varying octanol/water partition coefficients were allowed to diffuse
335
through bare and modified commercial 30 kDa MWCO membranes. Two aqueous solutions (pH
336
6.0 ± 0.2) were prepared, one containing the three organic solutes—L-asparagine, hydroquinone,
337
and thymol— at 0.13 mM each and another containing sodium chloride at 0.20 mM.
338
Concentrations were chosen to eliminate osmotic pressure effects during experimentation and
339
ensure the full dissolution of the most hydrophobic solute utilized, thymol.
340
A membrane sample was pinch-clamped at the flat-ground joint of an unjacketed Valia-
341
Chien diffusion cell with a 7 mm orifice and 10 mL chambers with stir bars (PermeGear). The
342
chamber facing the active membrane layer was loaded with solution containing organic solutes
343
while the opposite chamber contained the saline solution. To come to diffusive steady-state, the
344
system sat stirring for 2 h for samples without brushes and 4 h for those with brushes before
345
taking initial samples. Final solution samples were taken after ~10 h for brush-free swatches and
346
~20 h for brush-covered swatches. These time points were selected after preliminary studies with
347
each membrane type in which concentration changes were measured more continuously to
348
determine when a linear increase in concentration was reached and when concentration changes
349
of all solutes significantly overcame measurement error. Transport rate was calculated as the
350
change in total species (molar concentration multiplied by cell volume) per time per membrane
351
area. Four samples of each membrane type were tested.
352
Organic solute concentrations in the saline side of the chamber were determined through
353
high-performance liquid chromatography (HPLC, 1260 Infinity LC System, Agilent
354
Technologies) with mobile phase consisting of 40% acetonitrile and 60% of 0.1% phosphoric
355
acid in water. Calibration curves were developed prior to diffusion experiments, pinpointing
356
retention time and detection wavelength, and data was analyzed with curve integration software 14
357
(OpenLAB CDS EZChrom Edition, Agilent Technologies). A conductivity probe (Con 2700,
358
Oaklon) was used to measure conductivity of the solution on the organic side and compute
359
sodium chloride concentration from conductivity-derived calibration curves.
360
Rejection studies were conducted in the same custom dead-end cell used for water
361
permeability testing, with an added stir bar. Sodium chloride and lysozyme solutions, run
362
separately, were 20 mM and 35 µM, respectively. As in water permeability tests, hydraulic
363
driving pressures of 2.07 bar (30 psi) and 6.89 bar (100 psi) were used for membranes without
364
and with brush layers, respectively. At least 8 mL of permeate were collected for a more accurate
365
representation of observed rejection, calculated through = 1−
366
[2]
367
where Cp is the concentration in the permeate and CF is the concentration in the feed. In the case
368
of high rejection, as with rejection of lysozyme by brush-layered membranes, CF was
369
approximated as the average of the initial feed concentration and the final retentate
370
concentration. Lysozyme concentration was determined by measuring total organic carbon
371
concentration (Shimadzu TOC Analyzer) while a conductivity probe was again used for sodium
372
chloride concentration.
373
3. Results and Discussion
374
3.1 Crosslinker maintains support-layer stability while tailoring brush density
375
A cellulosic support was chosen for growing SI-ATRP-produced brush layers because it
376
provides a high density of hydroxyl groups for bonding initiator in a relatively straightforward
377
manner. However, these hydroxyl groups are also responsible for the hydrogen bonds that render
378
cellulose as highly solvent stable. Thus, we hypothesized that without the addition of a
379
crosslinker during initiator binding, cellulose would lose stability as hydroxyl groups would be
380
fully occupied by initiator BiBB (Figure 3a,c). For a more insightful comparison on how the
381
proportion of crosslinker versus initiator affects brush density, the mole percentage of acyl
382
halides attributed to AC versus BiBB was used during analyses instead of volumetric or molar
383
portions of the reagents. Acyl halides are the functional groups at which these molecules esterify
384
with cellulose hydroxyl groups. Every crosslinker provided two acyl halides while every initiator
385
provided one. Thus, for instance, 25:75 vol% AC:BiBB at room temperature would be defined as
386
36:64 mol% AC:BiBB acyl halides. Note that these percentages refer only to those in the binding 15
387
solution and not to the final bonded AC:BiBB on the membrane as differing steric hindrances
388
and reactivities are sure to influence final proportions.
389 390
Figure 3. Stabilization of cellulose support membrane and simultaneous control of brush density. a)
391
Cellulose structure containing an abundance of hydroxyl groups, which hydrogen-bond in the presence of
392
harsh organic solvents, stabilizing this polymer in typical ATRP environments. b) SI-ATRP initiator α-
393
bromoisobutyryl bromide (BiBB) and crosslinker adipoyl chloride (AC) to be bonded to the cellulose
394
support membrane through an esterification reaction. c) The occupation of all hydroxyl groups with
395
initiator α-bromoisobutyryl bromide. Hydrogen bonds are eliminated, decreasing stability in organic
396
solvents. d) Hypothetical configuration of cellulose when AC as crosslinker is incorporated. By changing
397
the ratio of AC:BiBB, the support membrane can be stabilized and the brush density can be
398
simultaneously controlled within a certain range. Note that this is just an illustration and does not
399
represent exact bonding patterns.
400
As predicted, crosslinker was necessary to stabilize cellulose, evident in both
401
permeability testing on membranes and brush density determination on films (Figure 4). One can
402
expect that water permeability will diminish as a hydrophobic PtBA brush is grown on the
403
surface of a porous support. Indeed, in cases where BiBB acyl halide content was less than or 16
404
equal to 55%, with the remaining acyl halides attributed to crosslinker, permeability with the
405
addition of PtBA brushes is below that of the bare membranes (Figures 4a-b). However, at 64
406
mol% BiBB acyl halides, permeabilities for several membrane samples far exceeded that of the
407
bare membrane for both 30 kDa MWCO (Figure 4a) and 100 kDa MWCO membranes (Figure
408
4b) with PtBA brush. This suggests that too many stabilizing hydroxyl groups were occupied by
409
initiator, leading to membrane instability. As the membrane is exposed to DMF during SI-ATRP,
410
cellulose with depleted hydroxyl content is more easily dissolved.
411
In many studies, brush density, or the number of chains per unit surface area, has been
412
estimated by measuring dry brush layer thickness via ellipsometry on a harder substrate [18, 42]
413
or degree of polymerization [24]. These methods typically necessitate the growth of a relatively
414
long brush to overcome detection limits. Furthermore, in order to calculate brush density, the
415
heterogeneous polymer length (i.e., on the surface) is assumed to equal homogeneous length
416
(i.e., in solution). For optimization of a brush-layered membrane technology, an alternative, more
417
sensitive method to measure brush density may be helpful.
418
We directly measured the brush density by first binding-and-eluting silver ions to PAA
419
brush layers grown from dense cellulose films, thereby quantifying the total number of
420
carboxylic acid groups on the film surface. Chain length (~4 kg mol-1) was then measured by
421
SEC for equivalent samples following cleavage of the polymer brush from the surface. The
422
combination of the two methods allowed for direct measurement of brush density, as described
423
in Section 2.6. Using this method, we found that the crosslinker used for stabilization
424
simultaneously controlled brush density. We observed a reasonable positive trend between brush
425
density and initiator content used during AC/BiBB bonding (Figure 4c). However, at 64 mol%
426
BiBB acyl halides, brush density drastically dropped, again suggesting instability at this initiator
427
density as both cellulose and attached brush dissolve in the ATRP solvent. The maximum BiBB
428
acyl halide content that maintained membrane stability was 55 mol%, with a maximum observed
429
brush density of 1.70 ± 0.34 chains nm-2.
430
To evaluate the reasonableness of these observed densities, we compared our measured
431
density to the theoretical maximum brush density. Using atomic bromine percentages read from
432
x-ray photo electron spectroscopy (XPS) and hypothetically assuming that all bonded initiator
433
actually produces a polymer chain without inducing membrane instability, we can expect a
434
maximum of 7.0 chain nm-2 (see Section S1 and Figures S2, S6). If we assume that acyl halides 17
435
of both crosslinker and initiator have an equal probability of bonding, the use of 16, 23, 37, and
436
55 mol% BiBB acyl halides would produce around 1.13, 1.62, 2.61, and 3.88 chains nm-2,
437
respectively.
438
For all densities, we observed fewer brushes than theoretical (Figure S6). When
439
considering individual acyl halide groups, the slightly higher reactivity of acyl bromides over
440
acyl chlorides, based on bromine’s higher propensity to leave due to a lower halide-carbon bond
441
energy of 285 versus chlorine’s 327 kJ mol-1 [43], would tend to skew theoretical maximum in
442
the opposite direction (i.e., observed density would exceed our calculated maximum that
443
assumed equal acyl halide reactivity). However, it is possible that the presence of two acyl
444
chloride groups per one adipoyl chloride molecule could further enhance acyl chloride reaction
445
rates. That is, following reaction of the first acyl chloride, the second acyl chloride reacts soon
446
after owing to the proximity of neighboring hydroxyl groups on the surface of cellulose film.
447
Steric hindrance is likely another limiting factor. We observed a relatively high discrepancy
448
between theoretical and observed brush density at higher proportions of initiator, as crowded,
449
growing brushes would reduce effective brush density. Due to a combination of the above
450
effects, the discrepancy between theoretical and observed brush densities was non-linear. The
451
lowest discrepancy occurred at an intermediate initiator density (~30 mol% BiBB acyl halides).
452
Several previous studies with poly(methyl methacrylate) chains of much larger molecular
453
weights (i.e. 10-100 kg mol-1), grown on silicon wafers, all reported lower maximum brush
454
densities of 0.3-0.8 chains nm-2 [18-23]. However, by using brush thickness determined from
455
ellipsometry and assuming equal lengths for chains produced in bulk and on the surface, these
456
studies did not take into account dispersity and the possibility of divergent polymer populations
457
when calculating density. In our study, the density was also possibly measured as higher than
458
actual due to a few effects, but none of them account for the magnitude of the difference. First,
459
the surface roughness of the film increases the surface area for binding. However, from AFM
460
images using NanoScope Analysis software, we found that the actual surface area was only 6%
461
greater than the projected area and thus cannot fully explain the discrepancy. Second, our use of
462
PS standards in the SEC column may introduce a systematic error in measuring molecular
463
weights of acrylates. However, based on intrinsic viscosities of the relevant polymers, the error is
464
expected to be minimal (see Section S3 and Figure S7). The final possible source of error is that
465
trace quantities of free PtBA adsorbed within the film polymer matrix, to which silver was 18
466
electrostatically attracted after PtBA hydrolysis into PAA (see Section 3.2). While this
467
adsorption was likely minimal due to extensive rinsing, trace internally-bonded silver could
468
increase total eluted silver given a thick enough film. In the absence of polymer brushes, silver
469
binding was minimal (typically 1-2 orders of magnitude less than for brush-containing samples),
470
as determined for unmodified cellulose membranes and initiator/crosslinker-modified cellulose
471
membranes.
472
19
473
Figure 4. Observed control of support layer stability and brush density through competitive binding of
474
crosslinker and initiator. Verification of membrane stability by testing pure water permeability for a) 30
475
kDa MWCO and b) 100 kDa MWCO commercial cellulose membranes, subjected to initiator/crosslinker
476
(AC/BiBB) binding solutions of various BiBB content (as mol% of acyl halide groups) and different
477
lengths of grafted poly(tert-butyl acrylate) (PtBA) chains. For (a-b), the surface polymer length was
478
assumed to match that of a homogeneous reaction occurring simultaneously in the same environment.
479
Each data point represents an individual membrane sample. Water permeabilities below that of the bare
480
membrane (gray dashed lines) suggest membrane chemical stability in harsh organic solvents while
481
increased permeability suggests instability. c) Brush density on dense cellulose films resulting after
482
reaction with AC/BiBB solutions with varying BiBB content. The average of four samples with standard
483
deviation are presented for each data point. Note that BiBB contributes one acyl halide per molecule
484
while the crosslinker AC contributes two. Acyl halides are the functional groups at which these molecules
485
esterify with cellulose hydroxyl groups.
486
3.2 SI-ATRP growth behavior from a nonideal substrate
487
Simultaneous homogeneous reactions of polymer conducted in the same environment as the
488
heterogenous, surface-initiated polymerizations have frequently been used as indicators for
489
relative dispersity and growth rates. Measuring polymers from the bulk reaction is preferential
490
since they may be run through a size-exclusion chromatography (SEC) column while the product
491
consisting of substrate with tethered brush is left intact. There have been claims that a
492
simultaneous homogenous polymerization results in similar polymer length with comparable
493
dispersity index as the heterogeneous reaction [27, 42, 44-46]. To investigate such claims,
494
spherical nanoparticles have previously been employed since they maximize surface to volume
495
ratio, producing sufficiently large polymer populations for cleavage and direct measurement in a
496
SEC column [47]. Growth on nanoparticles has been shown to be similar to the homogeneous
497
reaction. However, the positive curvature of spherical particles means intertwining chains and
498
radical-crowding reduce as growth progresses [18].
499
Most comparisons of SI-ATRP on planar surfaces using direct cleavage of brushes have been
500
performed on smooth gold substrates rather than nonideal, relatively rough surfaces [48-50].
501
However, it is evident that topology of a substrate influences growth rate and dispersity.
502
Simulations of polymer growth on concave topologies like channels (pores) and inside spheres
503
have revealed that grafted polymer chains under higher physical confinement grow slower than
504
polymers in homogeneous reactions, even when radical-radical termination was not considered 20
505
[31]. Several empirical studies have reported that SI-ATRP conducted on flat surfaces is slower
506
than in the bulk [24-27], while some others have reported faster surface growth [28-30]. Two
507
schools of thought have formed that may explain slower growth observations: (1) “School of
508
Propagation” posits that crowding of chains causes radical burying, inducing a premature glassy
509
state; and (2) “School of Termination” posits that radical ends easily terminate each other when
510
initiators are forced into alignment on a plane [47]. Because the theoretical proportion of
511
activated radical ends at any one time is so low compared to dormant ends, or [P•]/[PX] = 10−4 to
512
10−6, the distance between them renders the “School of Termination” implausible by many
513
scientists [51]. However, with mixed results of growth phenomena that cannot be fully explained
514
by either school of thought [52], it was proposed that there is an induced rapid “hopping” of
515
radicals across the plane of aligned initiator ends, which could increase the chances of radical-
516
radical interactions and termination [51, 53] or increase growth rate depending on brush density
517
and exactly how “hopping” and termination mechanisms work.
518
In summary, literature results suggest that chains on convex surfaces grow similarly to in the
519
bulk. In contrast, concave and flat surfaces present ample opportunity for entanglement and
520
radical-radical interactions to slow growth and increase dispersity. Because topology has such an
521
apparent effect on growth behavior, we characterized our cellulose films in AFM in order to
522
identify under what category of surface morphology they fell. Cellulose films were used instead
523
of membranes to eliminate the complicated and inconsistent effects of asymmetric, polydisperse
524
pores. Our dry cellulose films exhibited a root-mean-square roughness Rq of 8.5 ± 2.3 nm and a
525
maximum roughness Rmax of 73.1 ± 20.5 nm (Figure 5a). Thus, the film topological peaks and
526
valleys possess considerable height difference on the nanoscale, especially when considering that
527
grafted polymeric chains produced in this study are only from 4–30 kg mol-1. Free-floating
528
polymers of the same composition have a radius of gyration of only 5–15 nm in good solvent
529
(tetrahydrofuran) at room temperature [54], which is comparable to the scale of roughness of the
530
cellulose film, although we can expect some stretching of crowded, densely-grafted chains.
531
Roughness of bare cellulose is evident in the Peak Force Error images that produce a more
532
visually comprehensible contour map while Height Sensor readings layout exact depths (Figure
533
5a). With a PtBA brush size of Mn ~ 4 kg mol-1, roughness somewhat decreases, with a Rq of 7.6
534
± 0.7 nm and a Rmax of 64.2 ± 10.0 nm (Figure S8).
21
535
A final factor for growth rate concerns initiator density. Intuitively, it makes sense that for
536
any ATRP reaction, whether surface-initiated or homogeneous, the more initiator there is for a
537
given amount of monomer and set ratio of activating to deactivating metal/ligand complexes, the
538
slower the chain growth occurs, as a set amount of monomer must be distributed amongst more
539
growing polymer chains. We indeed observed that homogeneous reactions with more initiator
540
would grow slower and reach shorter final lengths than reactions with less EBiB (Table S1).
541
Several studies utilizing direct brush cleavage, including a couple performed on polymeric
542
supports, found that initiator density can affect heterogeneous growth rates [28, 30, 49, 50].
543
Previously in molecular dynamics simulations, it was shown that whether heterogeneous
544
reactions grow similarly to or slower than a simultaneous homogeneous reaction primarily
545
depends on surface initiator density, even without factoring in termination reactions and chain
546
entanglement. That is, regions of higher local initiator density decreased the probability of each
547
chain to collide with reagents as crowded halogens competed with one another for a local supply
548
of a set amount of reagents [55].
549
From direct cleavage of brushes at various stages of growth, we empirically demonstrate—
550
for the first time—that both faster and slower surface growth accurately describe behavior on a
551
planar but nonideal (i.e. relatively rough) surface depending on the time-point of sampling. Four
552
distinct phases of growth occurred (Figure 5b,c). Considering previous empirical and modelling
553
findings on SI-ATRP growth behavior, we propose the following explanations for our growth
554
observations, keeping in mind three main factors dictating growth rates: 1) hopping of radicals,
555
2) entanglement and burying of chain ends, and 3) local initiator/radical density.
22
556 557
Figure 5. Growth behavior of brushes on nonideal topology. a) Characterization of dense cellulose film
558
topology through atomic force microscopy (AFM). Root-mean-square (Rq), maximum roughness (Rmax),
559
and both height sensor and peak force error 2D images are presented. b) Growth of homogeneous PtBA
560
chains in bulk solution compared to simultaneously-grown brushes on dense cellulose films. Brushes
561
were cleaved in highly basic conditions, esterified into benzyl acrylate, and run directly through a size-
562
exclusion chromatography (SEC) column with polystyrene stationary phase and tetrahydrofuran mobile
563
phase. c) Illustration of four distinct phases of SI-ATRP growth on a nonideal surface, coordinated with 23
564
phase colors and number labels in (b). Chain-end spheres represent radicals while x’s are dormant,
565
halogen-capped ends. d) Homogeneous SEC traces and e) sample heterogeneous SEC traces with colors
566
corresponding to homogeneous population molecular weight, with homogeneous Mn in the legend. The
567
heterogeneous population at 15 kg mol-1 homogeneous molecular weight is a sample from right before
568
population divergence, while the double population at 16 kg mol-1 is right after divergence. The blank,
569
which is only represented in (e), consists of a film with just crosslinker and initiator bonded, subjected to
570
all cleavage and conversion reactions. Beyond a time of 8.75 min in the column, other molecules like
571
cleaved crosslinker and reagents involved in conversions are detected. Polymer populations could not be
572
detected below ~3.5 kg mol-1 due to the presence of these residual molecules. Notice that homogeneous
573
signal is orders of magnitude larger than the achieved traces of heterogeneous populations. To increase
574
heterogeneous signal, a much larger dense film surface area would be needed.
575 576
In phase 1, growth on the surface appears similar to the homogeneous rate in the bulk
577
solution, although this conclusion relies mostly on the initial data point. Direct measurements of
578
molar mass below ~3.5 kg mol-1 were not possible due to the limited amount of cleaved polymer
579
and the presence of other residual molecules (Figure 5e). However, if phase 1 truly represents
580
similar homogeneous and heterogeneous growth rate, there are possibly two competing factors
581
during this phase, whereby the alignment of initiator points induces rapid radical hopping along
582
this plane while the effectively denser initiator on the surface keeps growth slower per polymer
583
chain than in the bulk. In phase 2 at around 4 kg mol-1, the chains seem to reach a critical length
584
where intertwining can occur, diminishing radical alignment and increasing radical burying.
585
Reagents approaching these tangled chains are likely consumed by the bulk (homogeneous)
586
reaction. Therefore, growth slowed to a near standstill. In phase 3, some chains have had enough
587
time to escape entanglement, producing a longer population. We hypothesize that these chains
588
are located at the peaks of convex features, where less spatial crowding occurs, while chains
589
within concave features are more likely to stay entangled. The apparent rapid growth of the
590
longer population could be explained by the hopping of radical ends because the chains are long
591
enough and free enough to sway and contact one another. Additionally, at a now effectively
592
lower halogen density along this plane, hopping radicals in the longer population no longer
593
compete with rapid monomer depletion, inducing rapid growth. In phase 4, both long and short
594
populations are now at effectively lower initiator concentrations with diminished alignment due
595
to increased radii of gyration. Thus, these populations have the same monomer depletion rate and 24
596
probability of radical hopping as the bulk solution and grow at a similar rate. Because the lower
597
population begins to show observable growth, we believe little to no termination occurred during
598
the slowed growth in phase 2.
599
Neither stir rates nor brush density within the ranges considered affected growth (Figure S9).
600
These observations suggest that Brownian motion modulates reagent collision rate more than
601
convection and that effective brush density for various populations is self-regulated for a given
602
nonideal surface. Although longer chains are likely located on convex regions while shorter ones
603
are within concave regions, it is even possible that divergence of populations could occur on
604
ideal, ultra-smooth flat surfaces because it is currently unclear if the entrapment that produces a
605
shorter population is spatially coordinated with concave regions or just statistically random. As
606
mentioned in Section 3.1, the observed smaller, highly-consistent brush densities of much longer
607
brushes, calculated through ellipsometry with the assumption that brush lengths equal that of
608
homogenous chains, support the idea of a self-regulated, effective brush density, even on ultra-
609
smooth supports. Our observed growth behaviors help explain discrepancies in previous SI-
610
ATRP studies as well as possibly bolster some prior phenomena observed in both empirical and
611
model studies. In our proposed mechanisms, we have uniquely stressed the influence of local
612
initiator density coupled with hopping of radicals across aligned initiator points.
613 614
3.3 Control and determination of brush location
615
In order to produce a desired active layer, SI-ATRP must be directed to certain locations on
616
the membrane substrate, namely top-surface only or within pores. Prior to this study, such
617
control has not been well achieved and verified. Recently, a method of utilizing a viscous pore-
618
filling solvent during initiator binding was developed to putatively prevent SI-ATRP polymer
619
brushes from growing inside the pores of ultrafiltration cellulose membranes [17]. Pore filling
620
was performed before initiator binding so poly(2-hydroxyethyl methacrylate) chains would
621
ideally only grow on the top surface. Highly viscous glycerol worked most effectively. However,
622
the only means of confirming brush location was based on preserved permeability and MWCO,
623
which could have resulted from several other factors. When filling cellulose pores with an
624
especially viscous liquid that also contains hydroxyl groups, it is very difficult to control where
625
this liquid goes. We have seen in SEM that glycerol-filled membranes are covered in glycerol,
626
which would reduce overall growth of tethered polymers. Additionally, glycerol’s hydroxyl 25
627
groups would also bind to the initiator through esterification, reducing the effective
628
concentration of initiator and subsequently its bonding density. Finally, initiator binding was
629
only performed for a minute at room temperature to minimize the time for glycerol to diffuse
630
into the bulk solution and to be consumed by reaction with the initiator. We have seen through
631
XPS analysis that initiator binding is not instantaneous, with full binding not reached until
632
around 12 h of reaction at 35 °C (Figure S2).
633
We hypothesized that with asymmetric membranes, steric hindrance of reagents in ATRP
634
reactions can be exploited to control location of brush growth on membranes. The top surface of
635
a commercial asymmetric cellulose with much smaller pores than the bottom can be directly
636
contacted with the reaction solution while the bottom is sealed-off in a specially-designed sealing
637
wafer. Growth was predicted to occur throughout the pores without use of the sealing wafer.
638
Following PtBA brush synthesis on commercial membranes of 30 and 100 kDa MWCO, with
639
and without the use of our sealing wafer, we first examined permeability as an indirect indicator
640
for brush location. However, we observed that membranes subjected to growth without the use of
641
a sealing wafer simply showed more variable permeability, averaging to a slightly higher
642
permeability than membranes produced with a sealing wafer but with large error bars, resulting
643
in an insignificant difference (Figure S10). Next, we speculated that if we cleaved PtBA chains
644
into PAA and stained the membranes by electrostatic attraction of uranium ions with carboxyl
645
groups, we could use STEM-EDX to visually map locations where uranium and thus also
646
brushes were present (Figure 6b, d, f, and h). Through this more direct yet qualitative method, it
647
initially appeared that growth location was not controlled at all, because samples produced with
648
the usage of the sealing wafer (Figure 6b, d) and without (Figure 6f) had uranium detected across
649
the entire depth of the membrane. Controls of bare cellulose membranes and membranes with
650
crosslinker and initiator were soaked in the trifluoroacetic acid solution used for t-butyl ester
651
cleavage and uranium staining solution to ensure these procedures did not incidentally cleave
652
other areas of the membrane and produce carboxyl groups. These samples both read as having
653
effectively no uranium (Figure 6h). It became apparent that visual mapping typically indicates
654
areas where uranium is present but does not clearly depict the quantity of uranium.
655 656 657 26
658 659
Figure 6. Visual analysis of brush location control from use of a specially-designed sealing wafer. For
660
brush detection, ~4 kg mol PtBA brushes were hydrolyzed to PAA, followed by binding of uranyl ions.
661
(a-d) Cross-sectional images in STEM-mode of a 30 kDa MWCO commercial cellulose membrane with
662
PAA brush produced using a sealing wafer, showing a) dark field mode and b) EDX mapping of uranium
663
of the top-surface sideview as well as c) dark field mode and d) EDX mapping of uranium of the bottom-
664
surface sideview. (e-f) Cross-sectional images in STEM-mode of a 30 kDa MWCO commercial cellulose
-1
27
665
membrane with PAA brush produced without using a sealing wafer, showing e) dark field mode and f)
666
EDX mapping of uranium of the bottom-surface sideview. g) STEM-mode, dark field cross-sectional
667
image of the top surface sideview of a dense cellulose film with PAA brush. Images (a-g) are taken at 14
668
kx magnification. Note that at select positions of targeted EDX, a white spot typically appears from the
669
burning of the TEM beam. Uranium visual maps were produced after 3 minutes of scan time. h) Typical
670
uranium mapping for samples that read as not having any uranium, even in targeted EDX sampling. Here,
671
the reading is from a bare cellulose membrane after being subjected to the tert-butyl ester bond cleavage
672
solution and uranium staining procedure.
673
Targeted EDX was next used to measure relative counts of uranium throughout the thickness
674
of the membrane, producing further insight on location control (Figure 7a). Some EDX sample
675
points are apparent as bright white spots in the dark-field images, where the beam has etched the
676
material (Figure 6a, c, e, and g). For all reported interior points except film measurements, we
677
targeted surfaces of the membrane and pore walls as opposed to cut areas within the membrane
678
polymer for better readings of brush growth on surfaces rather than monomer diffusion into the
679
cellulose. On membranes, 0 µm is at the top surface while 30 µm is the absolute back surface
680
laying against the woven backing. The thickness of the hand-cast film far-exceeded the
681
commercial membrane and had both sides exposed during SI-ATRP. Thus, we present as a
682
control only some sample points from the film.
683
For fair comparison, all samples with brushes were synthesized in the same reaction vessel at
684
the same time, producing chains on the film of ~4 kg mol-1. The two blank controls, bare
685
membranes and AC/BiBB-bonded membranes, read as having effectively 0 uranium across the
686
membrane. The dense film with brush layer had a uranium count of 105 on the outer surface
687
where chains grew and around 30 counts across the interior, indicating the ability of some
688
monomer to diffuse within the cellulose and bond to some initiator sites or adsorb to cellulose.
689
Without the use of a sealing wafer, the uranium content measured across the thickness of both 30
690
kDa and 100 kDa membranes with brushes was roughly constant, exceeding, on average, 1200
691
counts.
692
When the sealing wafer was used, both membrane types show higher counts at the surface
693
and rapidly diminishing counts within the interior. Notably, both membrane types have interior
694
uranium counts below that of samples produced without a sealing wafer but above the blank
695
controls, averaging 159 ± 69 and 764 ± 380 uranium counts for the 30 and 100 kDa MWCO,
28
696
respectively. For the 30 kDa MWCO membrane, this relatively low value suggests that
697
significantly shorter chains grew within the interior (one to a few monomers per chain) whereas
698
the membrane with larger pores likely has chains exceeding 1 kg mol-1 but below 4 kg mol-1
699
throughout its thickness.
700
The top-surface counts for membranes exceed that of the dense film. If we consider the fact
701
that the targeted EDX sample points slightly drifted during the 1 min of measuring time, we posit
702
that scans covered thicknesses exceeding that of just the thin surface brush. Thus, the
703
discrepancy in surface uranium counts of film and membranes with some pore penetration of
704
growing brush is reasonable. The discrepancy between top-surface uranium count for 100 kDa
705
MWCO and 30 kDa MWCO membranes can similarly be explained. That is, reaction solution
706
could penetrate for a longer duration into the larger pores of the former before chains lining pore-
707
mouths grew long enough to block pores. Figures 7b and c schematically illustrate the control
708
achieved with the use of a sealing wafer.
29
709 710
Figure 7. Elucidation of brush control from use of specially-designed sealing wafer, via targeted EDX
711
sampling. a) Results of targeted EDX sampling with uranium counts taken at various depths of different
712
membrane samples. Brush-layered samples had ~4 kg mol-1 PtBA, cleaved into PAA in 12% trifluoracetic
713
acid in dichloromethane. Before staining with uranium, bare membrane and membrane with AC/BiBB
714
bonded (30 kDa MWCO) were subjected to the same trifluoroacetic acid cleavage solution to test if this
715
process produces substantial carboxyl groups that attract uranium. Uranium counts were taken over the
716
course of 1 minute. b) Illustration of growth without the use of sealing wafer, whereby polymerization
717
solution diffuses into the membrane from all directions, producing brush growth throughout the pores. c)
718
Illustration of growth with the use of a sealing wafer, whereby polymerization solution diffuses into the
719
membrane from only the top side of the asymmetric membrane where smaller pores are exposed.
720
30
721
3.4 Brush layers as membrane active layers
722
As a proof of the previous concepts and a demonstration of dense brush layers as
723
selective membranes, a diblock copolymer of PtBA-PHEA on a commercial 30 kDa MWCO
724
cellulose membrane was synthesized with the use of a sealing wafer to direct growth to primarily
725
the top surface. We used our findings from the development of synthesis and characterization
726
methods to inform the fabrication of these membranes. A proportion of 55 mol% BiBB acyl
727
halides was chosen to maximize the brush density while maintaining stability. Because we had
728
observed a phase of practically static growth occurring at ~4 kg mol-1, total brush length was
729
maintained to around this value to keep layers distinct and even for a more accurate report of
730
block sizes. Furthermore, near-zero water permeabilities were often observed with PtBA brushes
731
of only 1-4 kDa (Figure 4), suggesting near-full coverage of pores. Because a 30 kDa MWCO
732
membrane is expected to have an average pore diameter of ~8 nm [56], we predict that around
733
the perimeter of the average pore mouth alone, the summed molecular weight of chains of 4 kg
734
mol-1 would be ~138 kDa, far exceeding 30 kDa (Figure S11). Thus, we expected that this small
735
size of brush of only 4 kg mol-1 would demonstrate the minimum layer length to affect solute-
736
solute selectively and rejection. However, we acknowledge that for real applications, it is
737
probably necessary to increase this length to ensure full coverage of larger pores, especially if a
738
polymer is selected that greatly collapses in the filtration solvent of choice.
739
To confirm the addition of each layer in the diblock, water contact angles were measured
740
on bare and modified membranes (Figure 8b). As expected, a layer of hydrophobic PtBA
741
increased water contact angle while subsequently-added PHEA reduced it. Additionally, ATR-
742
FTIR spectra were generally consistent with successful grafting, although signal intensity was
743
low due to short brush length (Figure S12).
31
744 745
Figure 8. Brush-layered membranes produced for proof-of-concept transport studies. a) Structure of
746
polymer brush chains. b) Membrane surface water contact angles in air, calculated as an average of
747
measurements from photographs taken every ~0.16 s for 10 s each on 3 samples total. Drop size was 1
748
µL. Subscripts represent the number average molecular weight of each polymer layer (kg mol-1), assumed
749
to be the same as the length of homogeneous polymers simultaneously produced. Brush layers were
750
synthesized atop a commercial 30 kDa MWCO cellulose membrane using 55% acyl halides from the
751
initiator BiBB (45% from crosslinker AC).
752 753
To demonstrate the selective capabilities of these brush layers, we measured flux of solutes
754
of varying octanol/water partition coefficients in a counter-diffusion experiment with organic
755
solutes on one side of a diffusion cell and saline solution in the opposite chamber. 32
756
Concentrations were balanced for a net-zero osmotic pressure difference between the chambers.
757
For fair comparison, the organic solutes chosen, including L-asparagine, hydroquinone, and
758
thymol, are all neutral at the experimental operating pH and fall within 110-150 Da in size
759
(Figure 9a). Sodium chloride was counter-diffused for reference.
760
Changes in diffusion or flux of these solutes indicated successful membrane modification and
761
altered selectivity (Figure 9b). Solute fluxes through each additional layer correlate well with the
762
octanol/water partition coefficients (LogP) for each solute (Figure 9a, b). The addition of a short,
763
relatively hydrophobic brush, PtBA of ~1.5 kg mol-1, slightly decreased the diffusion rates of all
764
solutes while reducing transport of sodium chloride below all others. With the addition of a
765
longer, relatively hydrophilic PHEA layer of ~2.5 kg mol-1, all diffusion rates reduced and
766
thymol (the most hydrophobic solute) dropped to the slowest diffusion rate. When a longer PtBA
767
layer of ~3.0 kg mol-1 was first added, the transport rate of the most hydrophilic solute L-
768
asparagine significantly dropped two orders of magnitude from the membrane with
769
crosslinker/initiator. Interestingly, when this long hydrophobic brush was followed by a short
770
PHEA layer of around ~1.0 kg mol-1, fluxes of all solutes somewhat increased. It is possible that
771
a layer of solely PtBA collapses completely in water while a diblock copolymer of PtBA and
772
PHEA aligns into a looser, lamellar configuration. Another explanation is that the chains lining
773
the openings of pores are short enough for a hydrophilic layer to produce an effectively smaller
774
PHEA-filled pore. As water is a better solvent for PHEA, this less-dense pore would allow all
775
solutes through more easily.
776
We also measured permeability and rejection of a bare and modified 30 kDa MWCO
777
membrane with a dead-end permeation cell, simulating ultrafiltration and reverse osmosis. With
778
the addition of just a PtBA brush of ~4 kg mol-1, rejection of lysozyme, a globular protein with a
779
molar mass of 14.3 kDa, drastically increases from 18 to 97%. Permeability also noticeably
780
decreases, from 290 to 1.1 Lm-2h-1bar-1, suggesting nearly full coverage of the membrane pores
781
(Figure 9c). However, NaCl was not rejected before or after brush layer formation, suggesting
782
the presence of some small defects (pores) that are smaller than lysozyme.
783
We showed here that a relatively small brush molecular weight can alter transport rates,
784
suggesting SI-ATRP may be used to produce the thinnest of ultrathin composite membranes. For
785
real applications, thicker polymer blocks are likely necessary, but this system allows for a
786
highly-controlled optimization of active layer thickness. These proofs of concept demonstrate 33
787
that selectivity and permeability can be altered by strategically choosing patterns of specific
788
polymers, grown to particular lengths.
789 790
Figure 9. Changes in solute flux and rejections of various solutes through different brush layers on a
791
commercial 30 kDa MWCO cellulose membrane. a) Pertinent characteristics of solutes involved in
792
transport experiments for diblock copolymer brush-layered cellulose membranes, including molecular
793
weight and octanol/water partition coefficients (LogP), which were obtained from PubChem. b) Small
794
molecule diffusion through brush-layer membranes. Fluxes of solutes with varying octanol/water partition
795
coefficients through bare and modified membranes were determined through use of a Valia-Chien-type
796
diffusion cell with 10 mL chambers and a 7 mm orifice diameter, set at ambient temperature with constant 34
797
stirring. One chamber was charged with all organic solutes at a concentration 0.133 mM each, while the
798
opposite side contained 0.2 mM sodium chloride to balance osmotic pressure. Changes in organic solute
799
concentration on the saline side were detected through use of an HPLC column, while changes in salt
800
concentration on the organic side were detected through conductivity meter probe. The average of
801
quadruplicate data with standard deviation as error bars is depicted. c) Pressure-driven filtration through
802
brush-layer membranes. Pure water permeability and rejections of sodium chloride and lysozyme were
803
measured for bare cellulose membrane and a membrane with a PtBA brush through using a stirred dead-
804
end cell with 7 mm diameter orifice, driven with 7 bar and 2 bar hydraulic pressure for membranes
805
without and with brushes, respectively. Saline feed was 20 mM in sodium chloride while lysozyme
806
solution was 35 µM. Lysozyme is a globular protein with a molar mass of 14.3 kDa. Conductivity
807
measurements for retentate and permeate were used to determine salt rejection while TOC readings were
808
used to determine lysozyme concentrations. The average of triplicate data with standard deviation as error
809
bars is depicted. For membrane types, subscripts represent the number average molecular weight of each
810
polymer layer (kg mol-1), assumed to be the same as the length of homogeneous polymers simultaneously
811
produced. All membranes reacted in AC/BiBB solutions containing 55% BiBB acyl halides (45% from
812
crosslinker AC).
813
4. Conclusion
814
Synthesizing reproducible, useful brush-layered membranes necessitates the development
815
of new, more robust techniques to control and resolve the location, density, and length of SI-
816
ATRP-produced brushes on porous polymeric supports. We demonstrated here the use of
817
competitively-bound crosslinker and initiator to control brush density. A certain proportion of
818
crosslinker was necessary to stabilize the cellulose, limiting density control within a certain
819
range. We showed that brush density could be measured by use of a silver ion probe and
820
inductively-coupled plasma mass spectrometry. We found that the ratios of initiator and
821
crosslinker acyl halide content used in the reaction solution did not directly match grafting
822
density, as other factors like binding rate for each type of molecule and steric hindrance of
823
growing brushes influenced the resultant density. We elucidated unique growth behavior on a
824
nonideal polymeric surface, experimentally demonstrating phases of seemingly faster and slower
825
ATRP heterogeneous growth rates compared to homogeneous rates and observing two distinct
826
populations. Such growth observations support previously-proposed growth mechanisms. We
827
showed that location of brushes could be partially controlled through use of sealing wafers if the
828
base porous support is asymmetric with relatively small top-surface pores. Targeted STEM-EDX 35
829
measurements throughout the cross-sections of samples with uranium-tagged PAA brushes
830
allowed direct quantitation of brush location control. Finally, after identifying brush density and
831
length limitations, we produced brush homopolymer and copolymer membranes that demonstrate
832
the possibility of using dense brushes as selective layers, tailored by controlled brush length and
833
type.
834 835
Acknowledgements
836 837
This material is based upon work supported by the National Science Foundation Graduate
838
Research Fellowship Program under Grant No. DGE-1752134 and the 2018-2020 NWRI-AMTA
839
Fellowship for Membrane Technology awarded to C.J.P. Any opinions, findings, and
840
conclusions or recommendations expressed in this material are those of the authors and do not
841
necessarily reflect the views of the National Science Foundation, National Water Research
842
Institute, or the American Membrane Technology Association. Facilities used for AFM, TEM,
843
and SEM were provided by the Yale Institute of Nanoscale and Quantum Engineering (YINQE).
844
TOC and ICP-MS analyzers were provided by the Yale Analytical and Stable Isotope Center
845
(YASIC). The Yale West Campus Materials Characterization Core provided equipment for XPS
846
measurements, while the Yale Chemical and Biophysical Instrumentation Center (CBIC)
847
provided FTIR and NMR equipment. Additionally, the authors would like to thank Dr. Chanhee
848
Boo for conducting XPS.
849
36
850
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39
Highlights •
Dense acrylate-based brush layers grown from cellulose UF membranes by SI-ATRP
•
Custom reaction wafer allowed for brush to be located mainly on top surface
•
Cleavage with SEC showed two divergent polymer populations in brush layers
•
Grafting density determined through silver bind-and-elute method
•
Solute fluxes measured through brush layers of homo- and block polymers
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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
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This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
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The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
Author’s name Menachem Elimelech Cassandra Porter Mingjiang Zhong Jay Werber Yan-fang Guan Cody Ritt
Affiliation Yale Yale Yale Univ of Minnesota Yale/USCTC Yale