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nanofibrous membrane prepared through solution blow spinning for oil adsorption
Journal Pre-proof Polycaprolactone/poly(L-lactic acid) composite micro/nanofibrous membrane prepared through solution blow spinning for oil adsorption Rongguo Li, Zhiming Li, Ruochen Yang, Xueqiong Yin, Ju Lv, Li Zhu, Ruiting Yang PII:
S0254-0584(19)31153-8
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122338
Reference:
MAC 122338
To appear in:
Materials Chemistry and Physics
Received Date: 4 July 2019 Revised Date:
1 October 2019
Accepted Date: 17 October 2019
Please cite this article as: R. Li, Z. Li, R. Yang, X. Yin, J. Lv, L. Zhu, R. Yang, Polycaprolactone/ poly(L-lactic acid) composite micro/nanofibrous membrane prepared through solution blow spinning for oil adsorption, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122338. 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.
Graphical abstract
1
Polycaprolactone/Poly(L-lactic acid) composite
2
micro/nanofibrous membrane prepared through solution
3
blow spinning for oil adsorption
4 5 6
Rongguo Li, Zhiming Li, Ruochen Yang, Xueqiong Yin*, Ju Lv, Li Zhu*, Ruiting Yang
7 8
(Hainan Provincial Fine Chemical Engineering Research Center, Hainan University,
9
Haikou, Hainan, 570228, P.R. China.)
10
* Corresponding Author: [email protected] (Xueqiong Yin); [email protected] (Li
11
Zhu)
12
13
Abstract:
14
Polycaprolactone/poly (L-lactic acid) composite micro/nanofibrous membranes
15
(PPA) were prepared through solution blow spinning (SBS), using biodegradable poly
16
(L-lactic acid) (PLLA) and polycaprolactone (PCL) as the raw materials. PPA were
17
characterized with FTIR, SEM, XPS, TGA, and XRD. The effects of SBS parameters
18
on the morphology, porosity, density, mechanical property and oil adsorption
19
properties of PPA were investigated. PPA with different mass ratio of PCL/PLLA had
20
different fiber diameter and porosity. PPA with PCL/PLLA mass ratio 1:4 expressed
21
higher oil adsorption capacity than the raw materials, being 24.56 g/g, 14.54 g/g and
22
13.28 g/g to crude oil, peanut oil and diesel oil, respectively. The oil adsorption
23
capacity could remain about 50% after 10 cycles of reuse. PPA had good
24
hydrophobicity (water contact angle up to 155o and oil/water adsorption selectivity
25
26:1). The obtained PPA membrane is potential as adsorbent to separate oil from
26
oil/water mixture.
27
Keywords:
28
Polycaprolactone; Solution blow spinning; Oil adsorption
29
Introduction:
Composite
micro/nanofibrous
membrane;
Poly(L-lactic
acid);
30
Oil-water mixture is a common environmental pollutant, which widely exists in
31
petrochemical industry, manufacturing industry, transportation, everyday life, and
32
other circumstances. The oily water poses a great threat to the ecological environment
33
and human health [1, 2]. Therefore, cleaning up oil contaminants in water has become
34
an urgent issue [3]. The difficultness of treating oily wastewater is separating the oil
35
and water efficiently. Up to now, many methods have been developed to treat
36
oil-water mixture, such as chemical condensation [4], situ combustion [3],
37
gravitational sedimentation [5], air flotation [6], adsorption, etc. Comparing with
38
other methods, adsorption has the advantages of high efficiency, low cost, no need of
39
unique equipment, easy to handle, etc.
40
Micro/nanofibers are ultrafine fiber materials having a diameter between 100
41
nanometers
and
100
microns
[7].
The
membranes
being
composed
of
42
micro/nanofibers (called micro/nanofibrous membranes) have the structural features
43
of large specific surface area, high porosity, adjustable structure (such as morphology,
44
porosity, wettability, chemical components), etc.. Fibrous membranes are increasingly
45
used in the treatment of oily wastewater and oil spill, mainly through adsorption or
46
filtration [8].
47
solution blow spinning (SBS) [10], gas jet spinning [11], and solution centrifugal jet
48
spinning [12], etc. have been used to produce fibrous membrane. SBS was proposed
49
by Medeiros et al. [13] in 2009 and has become an alternative cost-effective
50
technology for the preparation of micro/nanofibrous membranes. SBS utilizes a
51
high-pressured gas stream with high velocity to draw dissolved polymer into
52
micro/nanofibers. Compared with other methods, SBS has the advantages of low
53
energy consumption, high safety, no requirement of equipment with high voltage or
54
high temperature, easy to scale up, etc. [14]. SBS is promising in preparing
55
micro/nanofibrous membrane. Zhang has prepared micro/nanofibrous polystyrene
Methods including melt blown spinning [8], electrospinning [9],
56
through SBS, which showed much higher oil adsorption capacity than the commercial
57
oil adsorbent polypropylene due to the high hydrophobicity, highly porosity and small
58
fibers [15]. SBS is also appropriate to prepare composite micro/nanofibrous
59
membrane, which is made of two or more polymers with different physical and
60
chemical properties and expresses different characteristics from the original polymers
61
[8].
62
Poly (L-lactic acid) (PLLA) is a polyester compound, which has low polarity,
63
biodegradability and good mechanical properties [16]. PLA fibrous oil-adsorbing
64
materials have been successfully prepared through electrospinning and melt blown
65
spinning. SBS has also been applied onto PLA to prepare nonwoven membrane. [17,
66
18] However, there had no reports on PLA oil adsorption material prepared through
67
SBS. Polycaprolactone (PCL) is a semicrystalline biodegradable polyester, obtained
68
from ε-caprolactone through a ring opening reaction. PCL has good property of
69
film-forming. Therefore, PCL has been used to prepare microfiltration membranes
70
and ultrafiltration membranes, which could be applied in various fields, such as daily
71
life, food, industry, and sewage treatment [19, 20]. PCL is often used as an additive in
72
resins preparation to improve their processing properties. PCL has not been reported
73
as an oil adsorbing material. The use of PCL as an oil adsorbing material or as a
74
toughening agent for other oil adsorbing materials could give a positive impact on the
75
development of oil adsorbing materials.
76
Crude oil, diesel and peanut oil are typical oil often existing in the petroleum
77
industry, transportation and kitchens, respectively. Petroleum industry, transportation,
78
kitchens are the main places to produce oily wastewater. Therefore, crude oil, diesel
79
and peanut oil were chosen as the targeted oil for adsorption experiment. [9, 21-22] In
80
this study, composite micro/nanofibrous membranes for oil adsorption were prepared
81
through SBS using PCL and PLLA as the raw materials. The effects of SBS
82
parameters on the morphology, porosity, and density of the composite membranes
83
were investigated. Adsorption performance and recyclability of the membranes on
84
crude oil, diesel and peanut oil were also measured. The schematic diagram of SBS
85
for preparation of PPA was presented in Fig. 1.
86
Fig. 1 schematic diagram of SBS for preparation of PPA
87
88
2. Experimental
89
2.1 Materials
90
Polycaprolactone (PCL, Mw=90 kDa) was purchased from Haifei Plastic
91
Chemical Co. Ltd. Poly (L-lactic acid) (PLLA, Mw=6.2 kDa) was purchased from
92
Chunjing Plastic Materials Co., Ltd.. Dichloromethane (DCM) was purchased from
93
Guangzhou Jinhua Chemical Reagent Co. Ltd (China, Guangzhou). Three oils,
94
including peanut oil (relative density 0.911g/cm3, viscosity 0.055Pa·S-1), crude oil
95
(relative density 0.95g/cm3, viscosity 0.277Pa·S-1) and diesel oil (relative density
96
0.84g/cm3, viscosity 0.001Pa·S-1), were purchased from local market. All reagents
97
were used without further purification.
98
2.2 Preparation of PCL/PLLA composite membrane (PPA)
99
Different mass ratios of PCL to PLLA (0:1, 1:0, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3 and
100
1:4) were simultaneously dissolved in 30 ml of DCM, and magnetically stirred. All
101
the spinning solution had a mass fraction of 7%. The solution was then poured into
102
the reservoir of a commercial airbrush (TAMIYA 7452, nozzle diameter: 0.5 mm).
103
After adjusting the gas pressure (0.3 MPa) and the distance (18 cm) from the airbrush
104
to the receiving plate, the trigger of the airbrush was pressed to start the spinning. The
105
prepared PPA membrane was collected on a plastic mesh and dried in a vacuum oven
106
at 50℃ for 5 hours before characterization and oil adsorption.
107
2.3 Characterization of PPA membrane
108
The PPA membrane was observed by a scanning electron microscopy (SEM,
109
S-3000N, Hitachi, Japan) and the fiber diameter in the SEM image was measured
110
using E-ruler software. The fourier transform infrared spectra (FTIR) of raw PCL, raw
111
PLLA and PPA membranes were measured by KBr method using a Paragon 1000
112
Fourier transform infrared spectrometer. The scanning range was in the range of
113
4000-400 cm-1. The wettability of the materials was characterized by measuring the
114
water contact angle (SL200B). The static water contact angle was measured 10 times
115
for each sample and the average data were taken. The structure and chemical
116
composition of raw PCL, raw PLLA and PPA membrane were analyzed by X-ray
117
photoelectron spectroscopy (XPS). The thermogravimetric analysis was carried out on
118
a thermogravimetric analyzer under the protection of nitrogen with the heating rate of
119
10oC/min, in the range of 30~600 oC. Tensile testing of PPA membranes were carried
120
out on a Electro-mechanical Universal Testing Machines (Model WDW-1 1KN) at a
121
cross head speed of 20 mm/min and a gauge length of 20 mm. Five specimens were
122
tested for each membrane [23].
123
The PPA membrane cut into squares (1 cm×1 cm) was weighed and the thickness
124
was measured with a vernier caliper. Each set of data was measured 5 times and
125
averaged. The volume was calculated according to the obtained data. PPA membrane
126
density (ρ) was calculated by equation (1).
127 128
129
ρ=
m v
(1)
The porosity (P%) of the fiber membrane was calculated by equation (2). P (%) = (1 −
ρ porous ) ×100% ρ solid
(2)
130
(where, ρsolid was the density of the cast film with the same PCL/PLLA mass ratio
131
with PPA; ρporous was the density of the PPA membrane)
132
2.4 Oil adsorption performance of PPA membrane
133
PPA cut into a certain size was placed in a beaker containing oil (peanut oil,
134
crude oil, or diesel oil) for adsorption test. After a period of time, the oil adsorbed PPA
135
was taken out with tweezers and put on a copper mesh to naturally drop off the excess
136
liquid on the surface of the membrane for 60 seconds. Then the oil-adsorbed PPA was
137
weighed quickly. The oil adsorption capacity (OAC) of PPA was calculated with
138
equation (3).
OAC ( g / g ) = (
139
Wi − 1) Wo
(3)
140
(Wo is the mass of the PPA membrane before oil adsorption; Wi is the mass of the PPA
141
membrane after oil adsorption)
142
The reusability adsorption experiment was carried out as follows: a certain
143
quality of PPA was put into the oil container. After the oil adsorption finished, the
144
membrane was removed and weighed after oil dropping naturally for 60 s. The oil
145
adsorption capacity was calculated with Eq(3). Then PPA membrane was extruded to
146
remove most of the adsorbed oil, and further soaked in n-butanol for about 5 hours.
147
Then membrane taken out from n-butanol was dried and used for next adsorption.
148
Above adsorption-desorption process was repeated for 10 times.
149
2.5. Adsorption isotherm experiment
150
The adsorption isotherm mechanism was investigated by fitting the Langmuir
151
equation (Eq(4)) and Freundlich equation (Eq(5)), respectively. During the
152
investigation, 45 mg of PPA was added to 50 mL oil aqueous solution with a rotation
153
speed of 300 r/min at 298K (25°C). The concentration of the oil (crude oil, peanut oil,
154
diesel oil) was in range of 4-80 g/L. [24].
155 156
=
+ =
(4)
+
(5)
157
Where, Ce is the equilibrium concentration of oil in aqueous solution; qe is the
158
adsorption capacity of adsorbent at equilibrium; Qm is the saturated oil adsorption
159
capacity of PPA; Kl is the Langmuir constant, representing adsorption heat in the
160
adsorption process of adsorbent; Kf and n are the constants of Freundlich isotherm
161
model at a given temperature.
162
3 Results and Discussion
163
3.1 Preparation of PCL/PLLA composite membrane (PPA)
164
Different mass ratios of PCL to PLLA were chosen to make a spinning solution
165
with a mass fraction of 7%. PPA was prepared through SBS using a common
166
commercial airbrush, and the specific SBS conditions were shown in Table 1. The
167
SEM images and fiber diameter distribution of the obtained samples were shown in
168
Fig. 2.
169
As shown in Table 1, the density and porosity of pure PCL and pure PLLA
170
membrane were 0.1064 g/cm3, 79.58% and 0.264 g/cm3, 80.09%, respectively. When
171
PCL and PLLA were blended before SBS, the obtained PPA membrane with different
172
PCL/PLLA mass ratio expressed different structure characteristics. The fiber diameter
173
and density decreased with the increase of PLLA amount, while the porosity increased.
174
The fiber diameter of pure PLLA membrane was 570 nm, whereas the diameter of
175
pure PCL membrane was 1380 nm, which indicating PLLA was easier to be spun
176
under the same SBS conditions. Therefore, with the increase of PLLA amount, the
177
composite polymer was more easily spun to fibers, resulting in smaller diameter, and
178
therefore higher porosity, low density and lower mechanical strength. As shown in
179
Table 1, pure PLLA membrane had the lowest tensile strength (0.240MPa), while pure
180
PCL membrane had the highest tensile strength (1.155 MPa) [25]. The tensile strength
181
of the PPA film increased with the proportion of PCL increasing, indicating the
182
presence of PCL increased the mechanical strength of PLLA. The water contact angle
183
first increased to 155o when the mass ratio increased to 1:1, being superhydrophobic,
184
further decreased with the increase of PLLA amount. The oil adsorption capacity of
185
the composite PPA also increased with the increase of PLLA amount. And PPA with
186
the ratio of 1:4 had best oil adsorption ability, higher than that of the raw materials.
187
For the same oil, the oil adsorption capacity is affected by the adsorbent. The fibers
188
diameter, porosity, and pore size would have impacts on the adsorption. With the
189
increase of PLLA, the porosity increased, whereas fiber diameters decreased.
190
Therefore, the accessibility of oil to the fibers and the pores increased, resulting in
191
higher oil adsorption capacity [16, 26-27]. In order to carry out the oil adsorption
192
experiments easily, PPA with PCL/PLLA ratio of 1:2 (having good hydrophobicity,
193
small diameter and operable mechanical strength) was selected for further oil
194
adsorption experiments.
195
Table 1. Spinning conditions and physiochemical characters of PPA membranes oil adsorption capacity PCL/ PLLA
Diam. (nm)
mass ratio
196
Porosit y (%)
Density 3
(g/cm )
Contact angle (°)
Tensile
(g/g)
strength (MPa)
Peanut
Crude
Diesel
oil
oil
oil
0:1
570±206
80.09
0.264
144.2±1.6
0.240
14.54
20.56
13.28
1:0
1380±406
79.58
0.1064
138.6±0.2
1.155
9.03
13.39
8.02
4:1
1140±704
78.72
0.2409
131.0±0.7
0.803
9.97
12.06
5.53
3:1
1130±509
81.33
0.2259
136.3±0.9
0.775
12.01
14.85
7.66
2:1
1090±410
82.04
0.2117
140.2±1.4
0.728
12.95
15.01
9.97
13.72
16.08
10.44
1:1
720±315
82.4
0.2105
155.0±0.2
0.627
1:2
780±269
86.26
0.1667
144.1±2.2
0.558
14.66
21.75
12.81
1:3
630±321
88.44
0.1605
134.7±1.1
0.538
17.49
22.16
14.58
1:4
520±254
88.93
0.1413
142.3±0.7
0.481
19.91
24.65
16.42
197 198
Fig. 2 SEM images and diameter distribution of PCL, PLLA and PPA with different
199
mass ratio of PCL and PLLA ((a)4:1、(b)3:1、(c)2:1、(d)1:1、(e)1:2、(f)1:3、(g)1:4).
200
To further understand the effects of spinning conditions (mass ratio of PCL and
201
PS, gas pressure and receiving distance) on the structure of PPA, an orthogonal test of
202
three factors and three levels of L9 (34) was carried out. The average diameter of
203
micro/nanofibers and the porosity of PPA were used to determine the optimal SBS
204
conditions. The levels and results of the orthogonal test were shown in supplementary
205
materials (Table S1-S3). By comparing the R1 and R2 data of the orthogonal test table,
206
the influence degree of each factor on the average diameter of the fiber and the
207
porosity was A (mass ratio) > B (spinning distance) > C (pressure).
208
3.2 Structural Characterization of PPA Membranes
209
3.2.1 FTIR results
210
In the FTIR spectrum of the raw PLLA (Fig. 3(a)), the peak at 1760 cm-1 was the
211
stretching vibration of C=O. The peaks at 2945 cm-1 and 1400 cm-1 were the
212
stretching and bending vibrations of -CH3, respectively. The stretching vibration and
213
rocking vibration peaks of –C-H appeared at 3000 cm-1 and 1300 cm-1, respectively.
214
The stretching vibration peak of C-O-C in PLLA was located at 1100 cm-1~1200 cm-1
215
[28]. In the spectrum of raw PCL, the stretching vibration and the rocking vibration
216
peak of CH2 appeared at 2957 cm-1 and 1400~1350 cm-1, respectively. The peak of
217
C=O was located at 1734 cm-1. And the anti-symmetric stretching vibration and the
218
symmetric stretching vibration of C-O-C appeared at 1250 cm-1 and 1150 cm-1,
219
respectively [29]. After SBS, the characteristic peaks of raw PCL and PLLA could be
220
observed in the spectrum of PPA. The peaks of C=O appeared at1760 and 1734 cm-1,
221
respectively, indicating PPA containing the components of PCL and PLLA.
222
3.2.2 XRD spectrum analysis
223
Pure PCL and pure PLLA were also solution blow spun to membranes. The XRD
224
patterns of PCL membrane, PLLA membrane and PPA membrane were shown in Fig.
225
3(b). All the samples showed peaks at 30°, 36°, 39°, 43°, 47° and 49°. The
226
crystallinity of PCL, PLLA, and PPA was 69.64%, 39.04% and 43.07%, respectively.
227
The crystallinity of PPA was between PCL and PLLA, indicating that adding PCL
228
enhanced the crystallinity of PLLA and therefore the mechanical strength was
229
improved (Table 1.).
230
3.2.3 Thermogravimetric analysis
231
Fig.3c and Fig.3d showed the thermogravimetric analysis (TG) and the
232
derivative thermogravimetry (DTG) curves of PCL, PLLA and PPA. It showed that
233
there was no weight loss below 100oC, indicating no moisture existing in PPA
234
membrane [30]. Besides the moisture loss below 100oC, both of raw PCL and raw
235
PLLA had only one pyrolysis temperature at range of 380-420oC and 320-360oC [31].
236
After spinning, there were two weight loss of PPA which occurred at 280~320℃ and
237
380~420oC, respectively. The results indicated that the highest weigh loss temperature
238
had no obvious change while some decomposition happened at a lower temperature
239
than PLLA. During SBS, the dissolved PCL and PLLA molecules interacted with
240
each other. The intermolecular interactions caused polymer chains rearrangement and
241
resulted in weakening of the intramolecular forces of partial bonds, such as CH3-C of
242
PLLA [32]. Therefore, the decomposition temperature decreased.
243 244
Fig. 3 FTIR spectra (a), XRD patterns (b), TGA (c) and DTG (d) of the raw PCL, raw
PLLA and PPA.
245 246
3.2.4 XPS spectral analysis
247
The XPS results of C1s of PCL, PLLA and PPA were shown in Fig. 4 and Table
248
2. Three samples all contained only three elements (C, H, and O). Since the three
249
samples contained only three kinds of carbon state (C-H（C-C） 、C-O and C=O), the
250
XPS pattern of PCL had three peaks at around 284.79ev, 286.4ev, 288.8eV,
251
corresponding to C-H（C-C）, C-O and C=O, respectively [33]. The binding energy of
252
C-H（C-C）and C=O in PLLA and PPA also appeared at 284.79 eV and 288.8 eV,
253
whereas the binding energy of C-O varied a little. The binding energy of C-O in
254
PLLA and PPA appeared at 286.78 eV and 286.83 eV. The difference between PCL
255
and PLLA was due to the long alkane group attaching to C-O in PCL decreased the
256
binding energy of C-O [34]. The C-O binding energy difference of PPA might be due
257
to the deviation from multi-peak fitting. According to the area of the peaks, the
258
contents of C-H（C-C）、C-O, C=O in PCL and PLLA were 69.74%, 15.55%, 14.71%
259
and 37.21% 37.21%, 28.58%, respectively, while 53.47%, 22.35%, 24.18% in PPA.
260
The contents of C-H/C-C, C-O and C=O in PPA was between those of PCL and PLLA.
261
The results indicated that PPA was the composite membrane of PCL and PLLA. The
262
unchanged location of the peaks after spinning expressed that no chemical structure
263
change occurred during SBS.
Table 2. Attributes of XPS peaks of raw PCL, raw PLLA and PPA.
264 Sample
PCL
PLLA
C-H/C-C
C-O
C=O
C-H/C-C
C-O
Area (%)
69.74
15.55
14.71
37.21
37.21
Binding
284.79
286.4
288.8
288.79
286.78
PPA C=O
C-H/C-C
C-O
C=O
28.58
53.47
22.35
24.18
288.8
284.79
286.83
288.8
energy (eV)
265
266 267 268
Fig. 4 XPS patterns of the raw PCL, raw PLLA and PPA 3.2.5 Wettability of PPA membrane
269
The wettability of a material plays an important role in oil adsorption and
270
oil-water separation process. The wettability of PCL/PLLA cast film and PPA was
271
characterized through measuring the static water contact angle (WCA), which was
272
shown in Fig.5. As shown in Fig. 5, the WCA of PCL/PLLA cast film with the ratio of
273
PCL/PLLA 1:2 was 68°(Fig. 5a), whereas that of PPA was 144.1° (Fig. 5b). The
274
images of PPA in contact with water and oils (crude oil, diesel oil and peanut oil) were
275
shown in Fig. 5c. The water droplet on PPA surface was almost spherical, while the
276
oils all spread on the surface widely.
277
Compared with PCL/PLLA cast film, PPA membrane had higher hydrophobicity
278
and lipophilicity. As shown in Table 1, the WCA of PPA varied with the ratio of
279
PCL/PLLA. The hydrophobicity is affected by the membrane morphology and
280
chemical structure. The spun membrane had higher porosity than the cast film, which
281
resulted in more hydrophobic air enclosed inside the membrane and therefore higher
282
hydrophobicity [35, 36]. Regarding all the spun PPA membrane, the WCA of PPA
283
increased with the increase of PLLA content and reached the highest 155° when the
284
ratio of PCL/PLLA being 1:1. With the increase of PLLA content, the porosity of PPA
285
membrane increased which would lead higher hydrophobicity [27, 37]. On the other
286
hand, the increase of less hydrophobic PLLA in PPA would lead less hydrophobicity.
287
Therefore, the WCA of PPA reached highest at PCL/PLLA ratio of 1:1.
288 289
Fig. 5 Water contact angle of the PCL/PLLA cast film (a) and PPA (b), images of
290
PPA contacting with water and oils (c), oil adsorption capacity (d) and reusability (e)
291
of PPA.
292
3.3 Oil adsorption performance measurement
293
The aliphatic chains of PLLA and PCL endow PPA membrane strong
294
lipophilicity and hydrophobicity. The oil adsorption mechanism is shown in Fig. 6.
295
The oleophilic groups, such as methyl segments of PLLA and CH2 of PCL can capture
296
oil molecules [38]. Moreover, the micro/nano structure and high surface area have
297
enhancing effects on the lipophilic-hydrophobicity of the micro/nanofibrous
298
membrane [39]. Due to the capillary action, the oil rapidly expands into the pores of
299
the fibrous membrane and wets the membrane surface to achieve oil-water separation
300
[40].
301 302
Fig. 6 Diagrammatic presentation of the oil adsorption mechanism
303
The prepared PPA membrane was subjected to an oil adsorption performance test.
304
Three oils (crude oil, diesel oil and peanut oil) were used as the oil models. The oil
305
adsorption capacities of PPA with the ratio of PCL/PLLA 1:2 at different contact time
306
were shown in Fig. 5d. As shown in Fig. 5d, PPA adsorbed the oil fast in the first 30s.
307
Then the adsorption capacity increased slowly. The adsorption of diesel oil almost
308
reached saturation after 150s, while the adsorption of peanut oil and crude oil reached
309
saturation after 210s. The maximum adsorption capacity for crude oil, peanut oil and
310
diesel was 21.75 g/g, 14.46 g/g and 12.51 g/g, respectively. The oil adsorption
311
capacity was in the order of crude oil > peanut oil > diesel oil. The difference of
312
adsorption capacity might be due to the differences in oil density and viscosity [41].
313
The density and viscosity of the oils were all in the order of crude oil > peanut oil >
314
diesel oil. When same volume oil was adsorbed, the oil with higher density would
315
have higher adsorption capacity. The density of the oil varied not so much as the oil
316
adsorption capacity. Therefore, other factor affected the adsorption in a higher degree.
317
The viscosity plays an important role on adsorption [42]. The oil with high viscosity
318
diffuses slower and needs longer time to reach saturation. Furthermore, it is also hard
319
to diffuse out the membrane, then more oil is kept inside the membrane and results in
320
higher adsorption capacity. Therefore, the oil adsorption expressed above adsorption
321
behavior.
322
The reusability of PPA membrane was investigated through the procedure of
323
adsorption-desorption for 10 times. Fig.5e showed the results of reusability of PPA
324
membrane. The oil adsorption capacity decreased with the reusability cycles. During
325
oil adsorption, the oil would occupy the vacant pores and diffuse inside the fibers.
326
With the increase of the reusability cycle, more oil was kept inside the fibers and then
327
resulted in adsorption capacity decreased. After 6 cycles of re-adsorption, the oil
328
capacity kept almost stable, which indicated that each desorption could remove same
329
amount of oil after 6 cycles. The oil could be removed might be on the surface or in
330
the pores. The left oil was that diffused inside the fibers and kept there. The SEM
331
images of PPA at different stage of reusability during crude oil adsorption were shown
332
in Fig. 7a-7d. As shown in Fig. 7a, PPA before oil adsorption had obvious
333
three-dimension network structure. The fiber images were clear and smooth. After one
334
cycle of reuse, the morphology of the fiber changed a little （Fig. 7b）. Some fibers
335
swelled and aggregated with each other, whereas the pores were still clear. After six
336
cycles of reuse, swell and aggregation of the fibers became more obvious and the pore
337
size between the fibers decreased obvious（Fig. 7c）. After ten cycles of reuse, the
338
fibers could not be observed （Fig. 7d）. After ten cycles of reuse, the adsorption
339
capacity to crude oil, peanut oil and diesel oil was 10.44 g/g, 6.44 g/g, and 5.67 g/g,
340
respectively, about 50% of the first cycle. The results showed that the PPA membrane
341
could be reused many times after simple desorption. Similar morphology changes
342
were observed during reusability measurement for diesel oil and peanut oil adsorption.
343
The corresponding SEM images were shown in supplementary materials (Fig. S1).
344 345
Fig. 7 SEM images of PPA before and after crude oil adsorption (a: before oil
346
adsorption; b: reused once time; c: reused 6 times; d: reused 10 times) and water
347
adsorption (e: before water adsorption; f: after water adsorption).
348
The adsorption selectivity of PPA to crude oil and water was carried out to
349
investigate its possibility of separating oil from an oil-water mixture. The SEM
350
images of PPA before and after water adsorption were shown in Fig.7e and 7f. The
351
adsorption capacity of PPA to water was 0.8 g/g, which was much lower than that of
352
adsorption to oil (about 21 g/g). The morphology of PPA showed that the fibers did
353
not swell and the pores between the fibers remained after water adsorption. The
354
results indicate that the selectivity of oil/water adsorption of PPA membrane is good
355
(about 26: 1) and PPA is potential as adsorbent for separating oil from a mixture of
356
oil/water.
357
3.6. Adsorption isotherm
358
The adsorption data were analyzed with two adsorption isotherm models, namely
359
Langmuir and Freundlich [43, 44]. The corresponding parameters obtained for the
360
three oils were shown in Table 3. The adsorption isotherms of PPA for crude oil,
361
peanut oil and diesel oil were presented in Fig. 8. Both Langmuir and Freundlich
362
adsorption isotherms of PPA exhibited an approximately linear relationship. However,
363
the fitting results by the Freundlich mode (R2=0.9690-0.9903) were better than by
364
Langmuir model (R2=0.9364-0.9851). PPA membrane has three-dimensional
365
micro/nanofiber and a lot of pores between the crossed fibers. Since the material is
366
highly hydrophobic, the oil can be directly adsorbed onto the surface of the fiber and
367
trapped inside the pores. The isotherm results showed that the oil adsorption of PPA
368
was asymmetric or multilayer adsorption, which was consistent with reported oil
369
adsorption in fibers. [45, 46].
370 371 372
Fig. 8 Isotherm plots for three oil adsorption on PPA at 25℃. Table 3. Parameters of Langmuir and Freundlich model constants and correlation coefficients for adsorption of oil with PPA
373 Oil type Crude oil
Langmuir kl(L/g) 0.0573
Qm /(g/g) 25.92
Freundlich R
2
0.9364
kf(L/g) 2.917
n 2.1004
R2 0.9690
Peanut oil Diesel oil
0.0847 0.0610
15.77 13.31
0.9725 0.9851
2.571 1.511
2.4601 2.0735
0.9903 0.9889
374
375
4. Conclusions
376
PPA composite micro/naofibrous membranes were successfully prepared by SBS
377
method using a commercially available airbrush. The morphology, wettability,
378
mechanical strength, and oil adsorption capacity of the membrane could be adjusted
379
by varying the mass ratio of PCL/PLLA and spinning conditions. PPA could achieve
380
higher oil adsorption ability than the raw materials. The adsorption capacity could be
381
up to 24.65 g/g. The oil adsorption capacity of PPA gradually decreased with the
382
increase of reuse cycle, while 50% capacity could be kept after 10 cycles of reuse.
383
PPA had good oil/water adsorption selectivity, with an adsorption capacity ratio of
384
26:1. The research reveals that biodegradable PCL/PLLA composite membrane
385
prepared by SBS is potential in oil/water separation and oil recovery.
386
Acknowledgements
387
The authors appreciate the financial support from the Key Research and Development
388
Plan of Hainan Province (ZDYF2018232), the Project of Scientific Research Platform
389
construction of Hainan University (ZY2019HN09), and the Key projects of College
390
Students' innovation and Entrepreneurship of Hainan University (201910589491).The
391
authors also thank the financial support from the Key Laboratory of Water Pollution
392
Treatment and Resource Reuse of Hainan Province.
393
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Highlights Polycaprolactone/Poly(L-lactic acid) composite micro/nanofibrous membranes (PPA) were prepared through solution blow spinning. The water contact angle of PPA membrane was up to 155°. The oil adsorption capacity of PPA to crude oil was up to 24.56 g/g. The oil/water adsorption selectivity of PPA was 26:1. The oil adsorption capacity could remain about 50% after 10 cycles of reuse.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0254-0584(19)31153-8
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122338
Reference:
MAC 122338
To appear in:
Materials Chemistry and Physics
Received Date: 4 July 2019 Revised Date:
1 October 2019
Accepted Date: 17 October 2019
Please cite this article as: R. Li, Z. Li, R. Yang, X. Yin, J. Lv, L. Zhu, R. Yang, Polycaprolactone/ poly(L-lactic acid) composite micro/nanofibrous membrane prepared through solution blow spinning for oil adsorption, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122338. 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.
Graphical abstract
1
Polycaprolactone/Poly(L-lactic acid) composite
2
micro/nanofibrous membrane prepared through solution
3
blow spinning for oil adsorption
4 5 6
Rongguo Li, Zhiming Li, Ruochen Yang, Xueqiong Yin*, Ju Lv, Li Zhu*, Ruiting Yang
7 8
(Hainan Provincial Fine Chemical Engineering Research Center, Hainan University,
9
Haikou, Hainan, 570228, P.R. China.)
10
* Corresponding Author: [email protected] (Xueqiong Yin); [email protected] (Li
11
Zhu)
12
13
Abstract:
14
Polycaprolactone/poly (L-lactic acid) composite micro/nanofibrous membranes
15
(PPA) were prepared through solution blow spinning (SBS), using biodegradable poly
16
(L-lactic acid) (PLLA) and polycaprolactone (PCL) as the raw materials. PPA were
17
characterized with FTIR, SEM, XPS, TGA, and XRD. The effects of SBS parameters
18
on the morphology, porosity, density, mechanical property and oil adsorption
19
properties of PPA were investigated. PPA with different mass ratio of PCL/PLLA had
20
different fiber diameter and porosity. PPA with PCL/PLLA mass ratio 1:4 expressed
21
higher oil adsorption capacity than the raw materials, being 24.56 g/g, 14.54 g/g and
22
13.28 g/g to crude oil, peanut oil and diesel oil, respectively. The oil adsorption
23
capacity could remain about 50% after 10 cycles of reuse. PPA had good
24
hydrophobicity (water contact angle up to 155o and oil/water adsorption selectivity
25
26:1). The obtained PPA membrane is potential as adsorbent to separate oil from
26
oil/water mixture.
27
Keywords:
28
Polycaprolactone; Solution blow spinning; Oil adsorption
29
Introduction:
Composite
micro/nanofibrous
membrane;
Poly(L-lactic
acid);
30
Oil-water mixture is a common environmental pollutant, which widely exists in
31
petrochemical industry, manufacturing industry, transportation, everyday life, and
32
other circumstances. The oily water poses a great threat to the ecological environment
33
and human health [1, 2]. Therefore, cleaning up oil contaminants in water has become
34
an urgent issue [3]. The difficultness of treating oily wastewater is separating the oil
35
and water efficiently. Up to now, many methods have been developed to treat
36
oil-water mixture, such as chemical condensation [4], situ combustion [3],
37
gravitational sedimentation [5], air flotation [6], adsorption, etc. Comparing with
38
other methods, adsorption has the advantages of high efficiency, low cost, no need of
39
unique equipment, easy to handle, etc.
40
Micro/nanofibers are ultrafine fiber materials having a diameter between 100
41
nanometers
and
100
microns
[7].
The
membranes
being
composed
of
42
micro/nanofibers (called micro/nanofibrous membranes) have the structural features
43
of large specific surface area, high porosity, adjustable structure (such as morphology,
44
porosity, wettability, chemical components), etc.. Fibrous membranes are increasingly
45
used in the treatment of oily wastewater and oil spill, mainly through adsorption or
46
filtration [8].
47
solution blow spinning (SBS) [10], gas jet spinning [11], and solution centrifugal jet
48
spinning [12], etc. have been used to produce fibrous membrane. SBS was proposed
49
by Medeiros et al. [13] in 2009 and has become an alternative cost-effective
50
technology for the preparation of micro/nanofibrous membranes. SBS utilizes a
51
high-pressured gas stream with high velocity to draw dissolved polymer into
52
micro/nanofibers. Compared with other methods, SBS has the advantages of low
53
energy consumption, high safety, no requirement of equipment with high voltage or
54
high temperature, easy to scale up, etc. [14]. SBS is promising in preparing
55
micro/nanofibrous membrane. Zhang has prepared micro/nanofibrous polystyrene
Methods including melt blown spinning [8], electrospinning [9],
56
through SBS, which showed much higher oil adsorption capacity than the commercial
57
oil adsorbent polypropylene due to the high hydrophobicity, highly porosity and small
58
fibers [15]. SBS is also appropriate to prepare composite micro/nanofibrous
59
membrane, which is made of two or more polymers with different physical and
60
chemical properties and expresses different characteristics from the original polymers
61
[8].
62
Poly (L-lactic acid) (PLLA) is a polyester compound, which has low polarity,
63
biodegradability and good mechanical properties [16]. PLA fibrous oil-adsorbing
64
materials have been successfully prepared through electrospinning and melt blown
65
spinning. SBS has also been applied onto PLA to prepare nonwoven membrane. [17,
66
18] However, there had no reports on PLA oil adsorption material prepared through
67
SBS. Polycaprolactone (PCL) is a semicrystalline biodegradable polyester, obtained
68
from ε-caprolactone through a ring opening reaction. PCL has good property of
69
film-forming. Therefore, PCL has been used to prepare microfiltration membranes
70
and ultrafiltration membranes, which could be applied in various fields, such as daily
71
life, food, industry, and sewage treatment [19, 20]. PCL is often used as an additive in
72
resins preparation to improve their processing properties. PCL has not been reported
73
as an oil adsorbing material. The use of PCL as an oil adsorbing material or as a
74
toughening agent for other oil adsorbing materials could give a positive impact on the
75
development of oil adsorbing materials.
76
Crude oil, diesel and peanut oil are typical oil often existing in the petroleum
77
industry, transportation and kitchens, respectively. Petroleum industry, transportation,
78
kitchens are the main places to produce oily wastewater. Therefore, crude oil, diesel
79
and peanut oil were chosen as the targeted oil for adsorption experiment. [9, 21-22] In
80
this study, composite micro/nanofibrous membranes for oil adsorption were prepared
81
through SBS using PCL and PLLA as the raw materials. The effects of SBS
82
parameters on the morphology, porosity, and density of the composite membranes
83
were investigated. Adsorption performance and recyclability of the membranes on
84
crude oil, diesel and peanut oil were also measured. The schematic diagram of SBS
85
for preparation of PPA was presented in Fig. 1.
86
Fig. 1 schematic diagram of SBS for preparation of PPA
87
88
2. Experimental
89
2.1 Materials
90
Polycaprolactone (PCL, Mw=90 kDa) was purchased from Haifei Plastic
91
Chemical Co. Ltd. Poly (L-lactic acid) (PLLA, Mw=6.2 kDa) was purchased from
92
Chunjing Plastic Materials Co., Ltd.. Dichloromethane (DCM) was purchased from
93
Guangzhou Jinhua Chemical Reagent Co. Ltd (China, Guangzhou). Three oils,
94
including peanut oil (relative density 0.911g/cm3, viscosity 0.055Pa·S-1), crude oil
95
(relative density 0.95g/cm3, viscosity 0.277Pa·S-1) and diesel oil (relative density
96
0.84g/cm3, viscosity 0.001Pa·S-1), were purchased from local market. All reagents
97
were used without further purification.
98
2.2 Preparation of PCL/PLLA composite membrane (PPA)
99
Different mass ratios of PCL to PLLA (0:1, 1:0, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3 and
100
1:4) were simultaneously dissolved in 30 ml of DCM, and magnetically stirred. All
101
the spinning solution had a mass fraction of 7%. The solution was then poured into
102
the reservoir of a commercial airbrush (TAMIYA 7452, nozzle diameter: 0.5 mm).
103
After adjusting the gas pressure (0.3 MPa) and the distance (18 cm) from the airbrush
104
to the receiving plate, the trigger of the airbrush was pressed to start the spinning. The
105
prepared PPA membrane was collected on a plastic mesh and dried in a vacuum oven
106
at 50℃ for 5 hours before characterization and oil adsorption.
107
2.3 Characterization of PPA membrane
108
The PPA membrane was observed by a scanning electron microscopy (SEM,
109
S-3000N, Hitachi, Japan) and the fiber diameter in the SEM image was measured
110
using E-ruler software. The fourier transform infrared spectra (FTIR) of raw PCL, raw
111
PLLA and PPA membranes were measured by KBr method using a Paragon 1000
112
Fourier transform infrared spectrometer. The scanning range was in the range of
113
4000-400 cm-1. The wettability of the materials was characterized by measuring the
114
water contact angle (SL200B). The static water contact angle was measured 10 times
115
for each sample and the average data were taken. The structure and chemical
116
composition of raw PCL, raw PLLA and PPA membrane were analyzed by X-ray
117
photoelectron spectroscopy (XPS). The thermogravimetric analysis was carried out on
118
a thermogravimetric analyzer under the protection of nitrogen with the heating rate of
119
10oC/min, in the range of 30~600 oC. Tensile testing of PPA membranes were carried
120
out on a Electro-mechanical Universal Testing Machines (Model WDW-1 1KN) at a
121
cross head speed of 20 mm/min and a gauge length of 20 mm. Five specimens were
122
tested for each membrane [23].
123
The PPA membrane cut into squares (1 cm×1 cm) was weighed and the thickness
124
was measured with a vernier caliper. Each set of data was measured 5 times and
125
averaged. The volume was calculated according to the obtained data. PPA membrane
126
density (ρ) was calculated by equation (1).
127 128
129
ρ=
m v
(1)
The porosity (P%) of the fiber membrane was calculated by equation (2). P (%) = (1 −
ρ porous ) ×100% ρ solid
(2)
130
(where, ρsolid was the density of the cast film with the same PCL/PLLA mass ratio
131
with PPA; ρporous was the density of the PPA membrane)
132
2.4 Oil adsorption performance of PPA membrane
133
PPA cut into a certain size was placed in a beaker containing oil (peanut oil,
134
crude oil, or diesel oil) for adsorption test. After a period of time, the oil adsorbed PPA
135
was taken out with tweezers and put on a copper mesh to naturally drop off the excess
136
liquid on the surface of the membrane for 60 seconds. Then the oil-adsorbed PPA was
137
weighed quickly. The oil adsorption capacity (OAC) of PPA was calculated with
138
equation (3).
OAC ( g / g ) = (
139
Wi − 1) Wo
(3)
140
(Wo is the mass of the PPA membrane before oil adsorption; Wi is the mass of the PPA
141
membrane after oil adsorption)
142
The reusability adsorption experiment was carried out as follows: a certain
143
quality of PPA was put into the oil container. After the oil adsorption finished, the
144
membrane was removed and weighed after oil dropping naturally for 60 s. The oil
145
adsorption capacity was calculated with Eq(3). Then PPA membrane was extruded to
146
remove most of the adsorbed oil, and further soaked in n-butanol for about 5 hours.
147
Then membrane taken out from n-butanol was dried and used for next adsorption.
148
Above adsorption-desorption process was repeated for 10 times.
149
2.5. Adsorption isotherm experiment
150
The adsorption isotherm mechanism was investigated by fitting the Langmuir
151
equation (Eq(4)) and Freundlich equation (Eq(5)), respectively. During the
152
investigation, 45 mg of PPA was added to 50 mL oil aqueous solution with a rotation
153
speed of 300 r/min at 298K (25°C). The concentration of the oil (crude oil, peanut oil,
154
diesel oil) was in range of 4-80 g/L. [24].
155 156
=
+ =
(4)
+
(5)
157
Where, Ce is the equilibrium concentration of oil in aqueous solution; qe is the
158
adsorption capacity of adsorbent at equilibrium; Qm is the saturated oil adsorption
159
capacity of PPA; Kl is the Langmuir constant, representing adsorption heat in the
160
adsorption process of adsorbent; Kf and n are the constants of Freundlich isotherm
161
model at a given temperature.
162
3 Results and Discussion
163
3.1 Preparation of PCL/PLLA composite membrane (PPA)
164
Different mass ratios of PCL to PLLA were chosen to make a spinning solution
165
with a mass fraction of 7%. PPA was prepared through SBS using a common
166
commercial airbrush, and the specific SBS conditions were shown in Table 1. The
167
SEM images and fiber diameter distribution of the obtained samples were shown in
168
Fig. 2.
169
As shown in Table 1, the density and porosity of pure PCL and pure PLLA
170
membrane were 0.1064 g/cm3, 79.58% and 0.264 g/cm3, 80.09%, respectively. When
171
PCL and PLLA were blended before SBS, the obtained PPA membrane with different
172
PCL/PLLA mass ratio expressed different structure characteristics. The fiber diameter
173
and density decreased with the increase of PLLA amount, while the porosity increased.
174
The fiber diameter of pure PLLA membrane was 570 nm, whereas the diameter of
175
pure PCL membrane was 1380 nm, which indicating PLLA was easier to be spun
176
under the same SBS conditions. Therefore, with the increase of PLLA amount, the
177
composite polymer was more easily spun to fibers, resulting in smaller diameter, and
178
therefore higher porosity, low density and lower mechanical strength. As shown in
179
Table 1, pure PLLA membrane had the lowest tensile strength (0.240MPa), while pure
180
PCL membrane had the highest tensile strength (1.155 MPa) [25]. The tensile strength
181
of the PPA film increased with the proportion of PCL increasing, indicating the
182
presence of PCL increased the mechanical strength of PLLA. The water contact angle
183
first increased to 155o when the mass ratio increased to 1:1, being superhydrophobic,
184
further decreased with the increase of PLLA amount. The oil adsorption capacity of
185
the composite PPA also increased with the increase of PLLA amount. And PPA with
186
the ratio of 1:4 had best oil adsorption ability, higher than that of the raw materials.
187
For the same oil, the oil adsorption capacity is affected by the adsorbent. The fibers
188
diameter, porosity, and pore size would have impacts on the adsorption. With the
189
increase of PLLA, the porosity increased, whereas fiber diameters decreased.
190
Therefore, the accessibility of oil to the fibers and the pores increased, resulting in
191
higher oil adsorption capacity [16, 26-27]. In order to carry out the oil adsorption
192
experiments easily, PPA with PCL/PLLA ratio of 1:2 (having good hydrophobicity,
193
small diameter and operable mechanical strength) was selected for further oil
194
adsorption experiments.
195
Table 1. Spinning conditions and physiochemical characters of PPA membranes oil adsorption capacity PCL/ PLLA
Diam. (nm)
mass ratio
196
Porosit y (%)
Density 3
(g/cm )
Contact angle (°)
Tensile
(g/g)
strength (MPa)
Peanut
Crude
Diesel
oil
oil
oil
0:1
570±206
80.09
0.264
144.2±1.6
0.240
14.54
20.56
13.28
1:0
1380±406
79.58
0.1064
138.6±0.2
1.155
9.03
13.39
8.02
4:1
1140±704
78.72
0.2409
131.0±0.7
0.803
9.97
12.06
5.53
3:1
1130±509
81.33
0.2259
136.3±0.9
0.775
12.01
14.85
7.66
2:1
1090±410
82.04
0.2117
140.2±1.4
0.728
12.95
15.01
9.97
13.72
16.08
10.44
1:1
720±315
82.4
0.2105
155.0±0.2
0.627
1:2
780±269
86.26
0.1667
144.1±2.2
0.558
14.66
21.75
12.81
1:3
630±321
88.44
0.1605
134.7±1.1
0.538
17.49
22.16
14.58
1:4
520±254
88.93
0.1413
142.3±0.7
0.481
19.91
24.65
16.42
197 198
Fig. 2 SEM images and diameter distribution of PCL, PLLA and PPA with different
199
mass ratio of PCL and PLLA ((a)4:1、(b)3:1、(c)2:1、(d)1:1、(e)1:2、(f)1:3、(g)1:4).
200
To further understand the effects of spinning conditions (mass ratio of PCL and
201
PS, gas pressure and receiving distance) on the structure of PPA, an orthogonal test of
202
three factors and three levels of L9 (34) was carried out. The average diameter of
203
micro/nanofibers and the porosity of PPA were used to determine the optimal SBS
204
conditions. The levels and results of the orthogonal test were shown in supplementary
205
materials (Table S1-S3). By comparing the R1 and R2 data of the orthogonal test table,
206
the influence degree of each factor on the average diameter of the fiber and the
207
porosity was A (mass ratio) > B (spinning distance) > C (pressure).
208
3.2 Structural Characterization of PPA Membranes
209
3.2.1 FTIR results
210
In the FTIR spectrum of the raw PLLA (Fig. 3(a)), the peak at 1760 cm-1 was the
211
stretching vibration of C=O. The peaks at 2945 cm-1 and 1400 cm-1 were the
212
stretching and bending vibrations of -CH3, respectively. The stretching vibration and
213
rocking vibration peaks of –C-H appeared at 3000 cm-1 and 1300 cm-1, respectively.
214
The stretching vibration peak of C-O-C in PLLA was located at 1100 cm-1~1200 cm-1
215
[28]. In the spectrum of raw PCL, the stretching vibration and the rocking vibration
216
peak of CH2 appeared at 2957 cm-1 and 1400~1350 cm-1, respectively. The peak of
217
C=O was located at 1734 cm-1. And the anti-symmetric stretching vibration and the
218
symmetric stretching vibration of C-O-C appeared at 1250 cm-1 and 1150 cm-1,
219
respectively [29]. After SBS, the characteristic peaks of raw PCL and PLLA could be
220
observed in the spectrum of PPA. The peaks of C=O appeared at1760 and 1734 cm-1,
221
respectively, indicating PPA containing the components of PCL and PLLA.
222
3.2.2 XRD spectrum analysis
223
Pure PCL and pure PLLA were also solution blow spun to membranes. The XRD
224
patterns of PCL membrane, PLLA membrane and PPA membrane were shown in Fig.
225
3(b). All the samples showed peaks at 30°, 36°, 39°, 43°, 47° and 49°. The
226
crystallinity of PCL, PLLA, and PPA was 69.64%, 39.04% and 43.07%, respectively.
227
The crystallinity of PPA was between PCL and PLLA, indicating that adding PCL
228
enhanced the crystallinity of PLLA and therefore the mechanical strength was
229
improved (Table 1.).
230
3.2.3 Thermogravimetric analysis
231
Fig.3c and Fig.3d showed the thermogravimetric analysis (TG) and the
232
derivative thermogravimetry (DTG) curves of PCL, PLLA and PPA. It showed that
233
there was no weight loss below 100oC, indicating no moisture existing in PPA
234
membrane [30]. Besides the moisture loss below 100oC, both of raw PCL and raw
235
PLLA had only one pyrolysis temperature at range of 380-420oC and 320-360oC [31].
236
After spinning, there were two weight loss of PPA which occurred at 280~320℃ and
237
380~420oC, respectively. The results indicated that the highest weigh loss temperature
238
had no obvious change while some decomposition happened at a lower temperature
239
than PLLA. During SBS, the dissolved PCL and PLLA molecules interacted with
240
each other. The intermolecular interactions caused polymer chains rearrangement and
241
resulted in weakening of the intramolecular forces of partial bonds, such as CH3-C of
242
PLLA [32]. Therefore, the decomposition temperature decreased.
243 244
Fig. 3 FTIR spectra (a), XRD patterns (b), TGA (c) and DTG (d) of the raw PCL, raw
PLLA and PPA.
245 246
3.2.4 XPS spectral analysis
247
The XPS results of C1s of PCL, PLLA and PPA were shown in Fig. 4 and Table
248
2. Three samples all contained only three elements (C, H, and O). Since the three
249
samples contained only three kinds of carbon state (C-H（C-C） 、C-O and C=O), the
250
XPS pattern of PCL had three peaks at around 284.79ev, 286.4ev, 288.8eV,
251
corresponding to C-H（C-C）, C-O and C=O, respectively [33]. The binding energy of
252
C-H（C-C）and C=O in PLLA and PPA also appeared at 284.79 eV and 288.8 eV,
253
whereas the binding energy of C-O varied a little. The binding energy of C-O in
254
PLLA and PPA appeared at 286.78 eV and 286.83 eV. The difference between PCL
255
and PLLA was due to the long alkane group attaching to C-O in PCL decreased the
256
binding energy of C-O [34]. The C-O binding energy difference of PPA might be due
257
to the deviation from multi-peak fitting. According to the area of the peaks, the
258
contents of C-H（C-C）、C-O, C=O in PCL and PLLA were 69.74%, 15.55%, 14.71%
259
and 37.21% 37.21%, 28.58%, respectively, while 53.47%, 22.35%, 24.18% in PPA.
260
The contents of C-H/C-C, C-O and C=O in PPA was between those of PCL and PLLA.
261
The results indicated that PPA was the composite membrane of PCL and PLLA. The
262
unchanged location of the peaks after spinning expressed that no chemical structure
263
change occurred during SBS.
Table 2. Attributes of XPS peaks of raw PCL, raw PLLA and PPA.
264 Sample
PCL
PLLA
C-H/C-C
C-O
C=O
C-H/C-C
C-O
Area (%)
69.74
15.55
14.71
37.21
37.21
Binding
284.79
286.4
288.8
288.79
286.78
PPA C=O
C-H/C-C
C-O
C=O
28.58
53.47
22.35
24.18
288.8
284.79
286.83
288.8
energy (eV)
265
266 267 268
Fig. 4 XPS patterns of the raw PCL, raw PLLA and PPA 3.2.5 Wettability of PPA membrane
269
The wettability of a material plays an important role in oil adsorption and
270
oil-water separation process. The wettability of PCL/PLLA cast film and PPA was
271
characterized through measuring the static water contact angle (WCA), which was
272
shown in Fig.5. As shown in Fig. 5, the WCA of PCL/PLLA cast film with the ratio of
273
PCL/PLLA 1:2 was 68°(Fig. 5a), whereas that of PPA was 144.1° (Fig. 5b). The
274
images of PPA in contact with water and oils (crude oil, diesel oil and peanut oil) were
275
shown in Fig. 5c. The water droplet on PPA surface was almost spherical, while the
276
oils all spread on the surface widely.
277
Compared with PCL/PLLA cast film, PPA membrane had higher hydrophobicity
278
and lipophilicity. As shown in Table 1, the WCA of PPA varied with the ratio of
279
PCL/PLLA. The hydrophobicity is affected by the membrane morphology and
280
chemical structure. The spun membrane had higher porosity than the cast film, which
281
resulted in more hydrophobic air enclosed inside the membrane and therefore higher
282
hydrophobicity [35, 36]. Regarding all the spun PPA membrane, the WCA of PPA
283
increased with the increase of PLLA content and reached the highest 155° when the
284
ratio of PCL/PLLA being 1:1. With the increase of PLLA content, the porosity of PPA
285
membrane increased which would lead higher hydrophobicity [27, 37]. On the other
286
hand, the increase of less hydrophobic PLLA in PPA would lead less hydrophobicity.
287
Therefore, the WCA of PPA reached highest at PCL/PLLA ratio of 1:1.
288 289
Fig. 5 Water contact angle of the PCL/PLLA cast film (a) and PPA (b), images of
290
PPA contacting with water and oils (c), oil adsorption capacity (d) and reusability (e)
291
of PPA.
292
3.3 Oil adsorption performance measurement
293
The aliphatic chains of PLLA and PCL endow PPA membrane strong
294
lipophilicity and hydrophobicity. The oil adsorption mechanism is shown in Fig. 6.
295
The oleophilic groups, such as methyl segments of PLLA and CH2 of PCL can capture
296
oil molecules [38]. Moreover, the micro/nano structure and high surface area have
297
enhancing effects on the lipophilic-hydrophobicity of the micro/nanofibrous
298
membrane [39]. Due to the capillary action, the oil rapidly expands into the pores of
299
the fibrous membrane and wets the membrane surface to achieve oil-water separation
300
[40].
301 302
Fig. 6 Diagrammatic presentation of the oil adsorption mechanism
303
The prepared PPA membrane was subjected to an oil adsorption performance test.
304
Three oils (crude oil, diesel oil and peanut oil) were used as the oil models. The oil
305
adsorption capacities of PPA with the ratio of PCL/PLLA 1:2 at different contact time
306
were shown in Fig. 5d. As shown in Fig. 5d, PPA adsorbed the oil fast in the first 30s.
307
Then the adsorption capacity increased slowly. The adsorption of diesel oil almost
308
reached saturation after 150s, while the adsorption of peanut oil and crude oil reached
309
saturation after 210s. The maximum adsorption capacity for crude oil, peanut oil and
310
diesel was 21.75 g/g, 14.46 g/g and 12.51 g/g, respectively. The oil adsorption
311
capacity was in the order of crude oil > peanut oil > diesel oil. The difference of
312
adsorption capacity might be due to the differences in oil density and viscosity [41].
313
The density and viscosity of the oils were all in the order of crude oil > peanut oil >
314
diesel oil. When same volume oil was adsorbed, the oil with higher density would
315
have higher adsorption capacity. The density of the oil varied not so much as the oil
316
adsorption capacity. Therefore, other factor affected the adsorption in a higher degree.
317
The viscosity plays an important role on adsorption [42]. The oil with high viscosity
318
diffuses slower and needs longer time to reach saturation. Furthermore, it is also hard
319
to diffuse out the membrane, then more oil is kept inside the membrane and results in
320
higher adsorption capacity. Therefore, the oil adsorption expressed above adsorption
321
behavior.
322
The reusability of PPA membrane was investigated through the procedure of
323
adsorption-desorption for 10 times. Fig.5e showed the results of reusability of PPA
324
membrane. The oil adsorption capacity decreased with the reusability cycles. During
325
oil adsorption, the oil would occupy the vacant pores and diffuse inside the fibers.
326
With the increase of the reusability cycle, more oil was kept inside the fibers and then
327
resulted in adsorption capacity decreased. After 6 cycles of re-adsorption, the oil
328
capacity kept almost stable, which indicated that each desorption could remove same
329
amount of oil after 6 cycles. The oil could be removed might be on the surface or in
330
the pores. The left oil was that diffused inside the fibers and kept there. The SEM
331
images of PPA at different stage of reusability during crude oil adsorption were shown
332
in Fig. 7a-7d. As shown in Fig. 7a, PPA before oil adsorption had obvious
333
three-dimension network structure. The fiber images were clear and smooth. After one
334
cycle of reuse, the morphology of the fiber changed a little （Fig. 7b）. Some fibers
335
swelled and aggregated with each other, whereas the pores were still clear. After six
336
cycles of reuse, swell and aggregation of the fibers became more obvious and the pore
337
size between the fibers decreased obvious（Fig. 7c）. After ten cycles of reuse, the
338
fibers could not be observed （Fig. 7d）. After ten cycles of reuse, the adsorption
339
capacity to crude oil, peanut oil and diesel oil was 10.44 g/g, 6.44 g/g, and 5.67 g/g,
340
respectively, about 50% of the first cycle. The results showed that the PPA membrane
341
could be reused many times after simple desorption. Similar morphology changes
342
were observed during reusability measurement for diesel oil and peanut oil adsorption.
343
The corresponding SEM images were shown in supplementary materials (Fig. S1).
344 345
Fig. 7 SEM images of PPA before and after crude oil adsorption (a: before oil
346
adsorption; b: reused once time; c: reused 6 times; d: reused 10 times) and water
347
adsorption (e: before water adsorption; f: after water adsorption).
348
The adsorption selectivity of PPA to crude oil and water was carried out to
349
investigate its possibility of separating oil from an oil-water mixture. The SEM
350
images of PPA before and after water adsorption were shown in Fig.7e and 7f. The
351
adsorption capacity of PPA to water was 0.8 g/g, which was much lower than that of
352
adsorption to oil (about 21 g/g). The morphology of PPA showed that the fibers did
353
not swell and the pores between the fibers remained after water adsorption. The
354
results indicate that the selectivity of oil/water adsorption of PPA membrane is good
355
(about 26: 1) and PPA is potential as adsorbent for separating oil from a mixture of
356
oil/water.
357
3.6. Adsorption isotherm
358
The adsorption data were analyzed with two adsorption isotherm models, namely
359
Langmuir and Freundlich [43, 44]. The corresponding parameters obtained for the
360
three oils were shown in Table 3. The adsorption isotherms of PPA for crude oil,
361
peanut oil and diesel oil were presented in Fig. 8. Both Langmuir and Freundlich
362
adsorption isotherms of PPA exhibited an approximately linear relationship. However,
363
the fitting results by the Freundlich mode (R2=0.9690-0.9903) were better than by
364
Langmuir model (R2=0.9364-0.9851). PPA membrane has three-dimensional
365
micro/nanofiber and a lot of pores between the crossed fibers. Since the material is
366
highly hydrophobic, the oil can be directly adsorbed onto the surface of the fiber and
367
trapped inside the pores. The isotherm results showed that the oil adsorption of PPA
368
was asymmetric or multilayer adsorption, which was consistent with reported oil
369
adsorption in fibers. [45, 46].
370 371 372
Fig. 8 Isotherm plots for three oil adsorption on PPA at 25℃. Table 3. Parameters of Langmuir and Freundlich model constants and correlation coefficients for adsorption of oil with PPA
373 Oil type Crude oil
Langmuir kl(L/g) 0.0573
Qm /(g/g) 25.92
Freundlich R
2
0.9364
kf(L/g) 2.917
n 2.1004
R2 0.9690
Peanut oil Diesel oil
0.0847 0.0610
15.77 13.31
0.9725 0.9851
2.571 1.511
2.4601 2.0735
0.9903 0.9889
374
375
4. Conclusions
376
PPA composite micro/naofibrous membranes were successfully prepared by SBS
377
method using a commercially available airbrush. The morphology, wettability,
378
mechanical strength, and oil adsorption capacity of the membrane could be adjusted
379
by varying the mass ratio of PCL/PLLA and spinning conditions. PPA could achieve
380
higher oil adsorption ability than the raw materials. The adsorption capacity could be
381
up to 24.65 g/g. The oil adsorption capacity of PPA gradually decreased with the
382
increase of reuse cycle, while 50% capacity could be kept after 10 cycles of reuse.
383
PPA had good oil/water adsorption selectivity, with an adsorption capacity ratio of
384
26:1. The research reveals that biodegradable PCL/PLLA composite membrane
385
prepared by SBS is potential in oil/water separation and oil recovery.
386
Acknowledgements
387
The authors appreciate the financial support from the Key Research and Development
388
Plan of Hainan Province (ZDYF2018232), the Project of Scientific Research Platform
389
construction of Hainan University (ZY2019HN09), and the Key projects of College
390
Students' innovation and Entrepreneurship of Hainan University (201910589491).The
391
authors also thank the financial support from the Key Laboratory of Water Pollution
392
Treatment and Resource Reuse of Hainan Province.
393
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Highlights Polycaprolactone/Poly(L-lactic acid) composite micro/nanofibrous membranes (PPA) were prepared through solution blow spinning. The water contact angle of PPA membrane was up to 155°. The oil adsorption capacity of PPA to crude oil was up to 24.56 g/g. The oil/water adsorption selectivity of PPA was 26:1. The oil adsorption capacity could remain about 50% after 10 cycles of reuse.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.