- Email: [email protected]
Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer
Accepted Manuscript Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer Tao Zhang, Michael S. Silverstein PII:
S0032-3861(19)30130-2
DOI:
https://doi.org/10.1016/j.polymer.2019.02.010
Reference:
JPOL 21249
To appear in:
Polymer
Received Date: 21 November 2018 Revised Date:
1 February 2019
Accepted Date: 3 February 2019
Please cite this article as: Zhang T, Silverstein MS, Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer
aDepartment
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Tao Zhang,*a,b Michael S. Silverstein*a of Materials Science and Engineering, Technion – Israel Institute of
Technology, Haifa, 32000, Israel.
of Textile and Clothing Engineering, Soochow University, Suzhou
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bCollege
*Corresponding Authors E-mail:
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215123, China.
[email protected] (T. Zhang)
[email protected] (M. S. Silverstein)
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Keywords: emulsion templating; hydrogels; block copolymers.
1
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ABSTRACT PolyHIPEs (PHs) are porous polymers that are typically templated within surfactant-stabilized high internal phase emulsions (HIPEs). Hydrogel PHs
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(HG-PHs) have exhibited potential as absorbents, adsorbents, release systems, and tissue engineering scaffolds. Recent work has shown that HG-PHs with highly temperature sensitive water uptakes could be produced using a reactive
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block copolymer (BC) as the sole surfactant, monomer, and crosslinker. It is not
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clear, however, whether these properties could be transferred to a HG-PH based on conventional hydrogel monomers and synthesized within a HIPE stabilized using a reactive BC. This work investigates HG-PHs based on acrylamide (AAm) synthesized within a HIPE stabilized by F-127-DMA, a reactive crosslinking BC
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based on methacrylate endcapped Pluronic F-127 (a commercial PEO-PPO-PEO triblock copolymer with poly(ethylene oxide) (PEO) endblocks and a poly(propylene oxide) (PPO) midblock). All the F-127-DMA-containing HG-PHs
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exhibited highly interconnected porous structures, low densities, and robust
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compressive behaviors with no chalkiness. The combination of F-127-DMA crosslinking and emulsion-templating produced amplified and accelerated water uptakes, up to 161 g/g, associated with hydrogel-swelling driven void expansion, and enhanced the temperature sensitivity, with the uptake decreasing by ~50 % between 5 and 60 °C, compared to reference HG-PHs and to conventional hydrogels.
2
ACCEPTED MANUSCRIPT 1. Introduction PolyHIPEs (PHs) are macroporous polymers synthesized within the continuous phases of high internal phase emulsions (HIPEs), emulsions with
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over 74 vol % dispersed internal phase [1-4]. HIPEs are usually stabilized using non-ionic surfactants. HIPEs stabilized by amphiphilic particles, Pickering HIPEs, have been successfully formed using inorganic nanoparticles [5,6], organic
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nanoparticles [7,8], and star polymers [9,10]. The stabilization strategy has been
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demonstrated to be an integral factor in determining the morphology and the properties of the resulting PHs [11-13]. Block copolymers (BCs), copolymers exhibiting at least two chemically different blocks connected by covalent linkages, have also been used to stabilize HIPEs.4,25-28 Amphiphilic BCs, BCs
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containing both hydrophobic and hydrophilic blocks, are advantageous as surfactants since their structures can be tuned for specific applications [14,15]. The most common amphiphilic BCs used for HIPE stabilization are the
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PEO-PPO-PEO triblock copolymers (a poly(propylene oxide) (PPO) midblock and
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poly(ethylene oxide) (PEO) endblocks) [16,17]. The PEO-PPO-PEO BCs are inert during radical polymerization and can usually be removed through extraction following PH synthesis. The effects of the crosslinking strategy upon the structure and properties of PHs can be extraordinary. Oligomeric crosslinking has been used in place of lower molecular weight crosslinking comonomers to enhance the mechanical properties [18,19] and to introduce degradability [20-22]. Using the stabilizing 3
ACCEPTED MANUSCRIPT nanoparticles in Pickering HIPEs as crosslinking centers [23,24], instead of using conventional crosslinking comonomers, has produced effects that vary from enabling shape memory behavior [25] to internal phase encapsulation through
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the formation of closed-cell structures [26]. Hydrogel polyHIPEs (HG-PHs), PHs that are usually templated within oil-in-water (o/w) HIPEs, often exhibit enhanced mechanical properties [16,17],
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amplified and accelerated water uptake, and enhanced environmental sensitivity
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as compared to conventional hydrogels synthesized from the same monomers [27-30]. HG-PHs have drawn a considerable amount of interest for a diverse range of applications that include absorption [16,28], adsorption [31], controlled release [32], tissue engineering [33,34], and enhancing the efficiency of using
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water for fighting fires [35]. The water uptake and the mechanical behavior are among the fundamental considerations that play decisive roles in the adaptation
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of hydrogels for such applications [36].
Recently, reactive BCs have been developed as surfactants for HIPE
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stabilization. The BC surfactants were found on the PH void surfaces, since they are spontaneously absorbed at the oil-water interface and become anchored to the void surface, forming a coating [14]. Reactive BCs exhibiting a polymerization controlling agent or a chain transfer agent (CTA) have also been utilized to stabilize HIPEs [37-39]. These reactive BC surfactants can be covalently incorporated into the PH’s macromolecular structure via initiation or chain-transfer reactions. PHs from reactive BC-stabilized HIPEs can overcome 4
ACCEPTED MANUSCRIPT the drawbacks associated with PHs from HIPEs stabilized using traditional non-reactive surfactants, which must be removed since they could leach out during use. These drawbacks include the additional processing and the
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associated expense of surfactant removal, the environmental challenge in disposing of the relatively large amounts of surfactant, and the possibility that any residual surfactant may leach out during use [40]. The effects of reactive
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block copolymer stabilizers on the water uptake and the mechanical properties
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have seldom been studied. Such effects will be particularly significant when the reactive surfactant also acts as a crosslinking comonomer.
A series of unique polyHIPEs consisting solely of a reactive triblock copolymer, F-127 dimethacrylate (F-127-DMA), synthesized from a commercial
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PEO-PPO-PEO triblock copolymer (Pluronic F-127) by methacrylate endcapping of the two terminal –OH groups, was recently described by the authors [29].
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F-127-DMA acted as the HIPE stabilizer, the sole monomer, and the crosslinking monomer. The resulting microphase-separated structure, crystallinity, and
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temperature-responsive uptake of the PH reflected the behavior of the PEO-PPO-PEO. The PH synthesized with F-127-DMA as the sole monomer possessed a unique structure and advantageous properties. It would be advantageous to transfer such properties to a HG-PH synthesized from a conventional hydrogel monomer by using F-127-DMA as a crosslinking comonomer, replacing N,N’-methylenebisacrylamide (MBAAm), the conventional crosslinking comonomer. 5
ACCEPTED MANUSCRIPT This article describes the effects of F-127-DMA crosslinking on HG-PHs based on acrylamide (AAm) synthesized within F-127-DMA-stabilized HIPEs, as described in Scheme 1. F-127-DMA endowed the resulting highly interconnected
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macroporous AAm-based HG-PHs with thermosensitive water uptakes and enhanced mechanical properties compared to reference HG-PHs where MBAAm was used as a crosslinking comonomer (Scheme 1). MBAAm, in spite of having a
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molecular weight significantly lower than that of F-127-DMA, was chosen for this
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comparison since it is, by far, the dominant crosslinker used for such hydrogels.
Scheme 1. Scheme showing two types of HIPEs used for generating emulsion-templated highly porous hydrogels based on AAm. The HIPEs were formed by adding cyclohexane (between 75 and 85% of the HIPE) to the aqueous solution phase while mixing. The top HIPE contained MBAAm, a typical crosslinking comonomer, while the bottom HIPE contained a reactive surfactant 6
ACCEPTED MANUSCRIPT (F-127-DMA, a methacrylate-endcapped PEO-PPO-PEO block copolymer) as a crosslinking comonomer. Following polymerization, purification, and drying the mechanical properties in uniaxial compression and the water-uptake were
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evaluated. The MBAAm-crosslinked hydrogel fractured in a brittle manner at low strains and was only able to absorb a limited amount of water, while the F-127-DMA crosslinked hydrogel was mechanically robust (it did not fail at
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strains up to 70%) and was able to absorb significant quantities of water.
2. Experimental 2.1. Materials
AAm, MBAAm, and N,N,N',N'-tetramethylethylenediamine (TEMED) (all
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from Sigma) and ammonium persulfate (APS) (from Fluka) were analytical grade and used as received. The other reagents were analytical grade (methanol, cyclohexane, methacryloyl chloride, triethylamine (TEA), dichloromethane
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(DCM), diethyl ether). Deionized water was used throughout. The synthesis of
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F-127-DMA was described in a previous work [29]. Briefly, methacryloyl chloride was reacted with the terminal hydroxyl groups of F-127 in the presence of TEA (in DCM). The resulting F-127-DMA was precipitated in diethyl ether, producing a 91 % yield. 2.2. PolyHIPE synthesis: FA-X-Y, FAM-80-Y, and AM-80-0 The HIPE recipes listed in Table 1 are for the FA-X-Y series based solely on AAm and F-127-DMA, with X representing the mass fraction of the dispersed 7
ACCEPTED MANUSCRIPT phase (75, 80, or 85 wt %) and with Y representing the mass fraction of F-127-DMA in all the monomers (13, 20, or 26 wt %). The HIPE recipes listed in Table 2 are for the FAM-80-Y series based on AAm, F-127-DMA, and MBAAm,
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with the 80 denoting 80 wt % dispersed phase and with Y representing the mass fraction of F-127-DMA in all the monomers (13, 20, or 26 wt %). Table 2 also lists the recipe for AM-80-0, which is based solely on AAm and MBAAm (no
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F-127-DMA) in an F-127-stabilized HIPE containing 80 wt % internal phase.
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Table 1 and 2 also list the mass fraction of F-127-DMA in all the monomers (MF-127-DMA) and the mass fraction of MBAAm in all the monomers (MMBAAm). The same procedure was used for all the syntheses (the FA-X-Y, the FAM-80-Y, and AM-80-0). The components of the aqueous phase (Tables 1 and 2)
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were dissolved in water. Cyclohexane was then added dropwise into the aqueous phase under constant stirring with an overhead stirrer. Following the addition of
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cyclohexane, the stirring speed was reduced and TEMED was added to the HIPE. The polymerization was then conducted in a convection oven at 40 °C for 8 h.
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The resulting PHs were cut into cubes (about 1 cm × 1 cm × 1 cm) and the water, cyclohexane, and residual monomers were removed using Soxhlet extraction with methanol for 24 h. The HG-PHs were then dried in a vacuum oven at room temperature for 24 h. The AAm and MBAAm contents in the HIPE were kept constant and the variation in the F-127-DMA content was at the expense of the water. The moles of crosslinking comonomer molecules per mass polymer (Xm, listed in Tables 1 8
ACCEPTED MANUSCRIPT and 2) was calculated by dividing the total number of moles of the crosslinking comonomers (F-127-DMA and/or MBAAm) by the total mass of all the monomers (AAm, F-127-DMA, and/or MBAAm,). Xm increases with increasing
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F-127-DMA content for the FA-X-Y since the contribution of the additional crosslinks is more significant than the contribution of the additional mass. However, Xm decreases with increasing F-127-DMA content for the FA-80-Y since
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the number of MBAAm crosslinks is dominant and the contribution of the
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F-127-DMA to the mass is more significant than its contribution to the number of
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crosslinks. AM-80-0 has the highest Xm, 59.0 × 10-5 mol/g (Table 2).
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ACCEPTED MANUSCRIPT Table 1 The FA-X-Y recipes. PHa
FA-80-13
FA-80-20
FA-80-26
FA-75-20
FA-85-20
18.69
11.21
1.31
0.79
5.00
3.00
0.13
0.07
25.13
15.07
74.74
84.86
15.40
14.95
14.50
F-127-DMA
0.60
1.05
1.50
AAm
4.00
4.00
4.00
APS
0.10
0.10
0.10
Total
20.10
20.10
20.10
Cyclohexane
79.80
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Organic, dispersed phase (wt %)
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Water
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Aqueous, continuous phase (wt %)
79.80
79.80
TEMED (catalyst added following HIPE formation) (wt %)
MF-127-DMA (wt %)
13.04
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Xm (×10-5 mol/g)
1.04
0.10
0.13
0.07
20.79
27.27
20.76
20.84
1.65
2.27
1.65
1.65
represents the internal phase content (wt %) and Y represents the amount of
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aX
0.10
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0.10
F-127-DMA in all the monomers (wt %)
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ACCEPTED MANUSCRIPT Table 2 The FAM-80-Y and AM-80-0 recipes. PHa
FAM-80-13
FAM-80-20
FAM-80-26
AM-80-0
15.00
14.55
14.1
14.55
/
/
/
1.05
F-127-DMA
0.60
1.05
1.50
/
AAm
4.00
4.00
4.00
4.00
MBAAm
0.40
0.40
0.40
0.40
APS
0.10
Total
20.10
Dispersed, organic phase (wt %) Cyclohexane
78.80
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F-127
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Continuous, aqueous phase (wt %)
0.10
0.10
0.10
20.10
20.10
20.10
78.80
78.80
78.80
0.10
0.10
0.10
0.10
12.00
19.27
25.42
0.00
8.00
7.34
6.78
9.09
52.9
49.2
46.0
59.0
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MF-127-DMA (wt %)
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TEMED (catalyst added after HIPE formation) (wt %)
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MMBAAm (wt %)
Xm (×10-5 mol/g)
aY
represents the F-127-DMA content in all the monomers (wt %)
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ACCEPTED MANUSCRIPT 2.3. Reference hydrogel synthesis: FA-0-Y and FAM-0-Y The FA-0-Y and the FAM-0-Y, where Y represents the mass fraction of the F-127-DMA in the corresponding monomers (13, 20, or 26 wt %), are two series
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of conventional hydrogels used as references for the FA-X-Y and the FAM-80-Y, respectively. The FA-0-Y and the FAM-0-Y were prepared through the polymerization of the aqueous phase recipes with TEMED (Table 1 and Table 2,
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respectively, without adding the organic phase) in a convection oven at 40 °C for
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8 h. The resulting hydrogel was immersed in water (with a hydrogel to water ratio of over 1:30) for one week, where the water was changed every day. The FA-0-Y and the FAM-0-Y were dried in a vacuum oven at room temperature for 3 days.
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2.4. Yield, density, macroporous structure, and microphase separation The yields of the FA-X-Y, the FAM-80-Y, and AM-80-0 were determined using
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a mass balance following the Soxhlet extraction and drying, and the densities of the PHs (ρPH) and of the FA-0-Y and the FAM-0-Y reference hydrogels (ρHG) were
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determined gravimetrically. The porous structures of the FA-X-Y, the FAM-80-Y, and AM-80-0 were described using scanning electron microscopy (SEM, FEI Quanta 200 at 10 kV). Cryogenic fracture surfaces, obtained by immersing the HG-PHs in liquid nitrogen, were coated with a thin gold-palladium layer. The average interconnecting hole diameters (dh) and the average void diameters (dv) were determined from the SEM micrographs using 100 measurements each. The average void diameters were corrected for the random nature of the section by 12
ACCEPTED MANUSCRIPT multiplying by 2/(31/2) [41]. The Fourier transform infrared (FTIR) spectroscopy of AM-80-0, FAM-80-20, and FA-80-20 were taken in transmission (Bruker Vertex 70 FTIR, between 600 and 4000 cm-1 at a resolution of 4 cm-1). The PHs
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were ground with KBr and pressed into pellets for measurement. The spectra are presented and discussed in Fig. S1 in the Supplementary Information (SI). Small-angle X-ray scattering (SAXS) was performed under vacuum using the
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same SAXS system as that reported in the previous work (Molecular Metrology
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SAXS with an incident wavelength of 0.1542 nm) [29]. 2.5. Compressive stress-strain behavior
Compressive stress-strain tests (Instron 3345) were performed on dry HG-PH cubes (about 1.0 cm × 1.0 cm × 1.0 cm) at 10 % min-1 (about 0.1 cm min-1)
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and room temperature to a maximum strain of 70 % (an equipment-related limit). The compressive modulus of the dry HG-PH, ED, was determined from the
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linear slope of the stress-strain curve at low strains, with the average modulus taken from at least three tests. The levels of the resistance to densification in the
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dry HG-PHs were compared through the stress at 70% strain, σ70-D. If the sample failed at lower strains, the failure was described using the failure strain, εF-D, and the failure stress, σF-D. 2.6. Water uptake The equilibrium water uptakes of the FA-X-Y, the FAM-80-Y, the FA-0-Y, and AM-80-0 were determined gravimetrically at room temperature. Typically, a dry 13
ACCEPTED MANUSCRIPT sample cube (about 1.0 cm ×1.0 cm × 1.0 cm) with a known mass (MD) was placed in a vial with water for 24 hours to reach a fully swollen mass (Msw). The total water uptake (WPH-T), calculated from Equation 1, was an average of three
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results. The water uptakes in the FA-80-Y were also determined for different temperatures (from 5 to 60 °C in 5 °C increments). The temperature was controlled using a Thermo Neslab RTE-110 bath (Neslab Instruments). (1)
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WPH-T = (Msw – MD) / MD
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Water uptake associated with PH walls (WHG-T), water uptake associated with PH voids (WPH-V) and water uptake from hydrogel-swelling-driven void expansion (WPH-VE) were determined according to the method reported previously [16,27]. In brief, WHG-T was determined from the water uptake of the
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FA-0-Y, WPH-V was calculated from the PH density and the wall density, and
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WPH-VE was calculated by subtracting WHG-T and WPH-V from WPH-T.
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3. Results and discussion 3.1. Synthesis
Stable FA-X-Y HG-PH monoliths were successfully synthesized without the
need for a conventional crosslinking comonomer. The FA-X-Y exhibited relatively high polymerization yields, around 90 % (Table 3), with the yield increasing slightly with increasing F-127-DMA content and with decreasing dispersed phase content. On the other hand, a stable HG-PH monolith could not be formed in a 14
ACCEPTED MANUSCRIPT control HIPE stabilized using the non-reactive F-127 instead of the reactive F-127-DMA (there was no crosslinking comonomer). These results demonstrate that F-127-DMA-based crosslinking is essential for the synthesis of a stable
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HG-PH in the absence of a conventional crosslinking comonomer. The FAM-80-Y and AM-80-0, both of which contained a conventional crosslinker (MBAAm), exhibited similar polymerization yields of around 89 % (Table 4). The similarity
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of the yields indicates that the presence of F-127-DMA and/or MBAAm did not
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affect the yield. The polymerization yields of FA-0-Y were about 95%. The incorporation of the surfactant into the HG-PH’s macromolecular network through copolymerization overcomes the challenges associated with the conventional non-reactive surfactants (the additional processing, the cost
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and environmental impact of surfactant removal, and the possibility that residual surfactant would leach out). The polymerizable HIPE surfactants reported
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previously (based on low-molecular-weight surfactants [40,42] or on block copolymers [37-39]) were not designed to crosslink the PHs. The F-127-DMA
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crosslinking, however, also provides added value, as described below. The typical PH structures with relatively thick walls that result from the HIPE-templated polymerization of F-127-DMA alone in previous work [29] demonstrate that the polymerization is not restricted to the interface. Therefore, the crosslinks are not expected to be located solely on the void’s inner surface.
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ACCEPTED MANUSCRIPT Table 3 The properties of the FA-X-Y. PHa
FA-80-13
FA-75-20
FA-85-20
91
88
89
92
dv (μm)
51.4
86.0
43.7
dh (μm)
14.6
8.5
6.4
ρPH (g cm-3)
0.082
0.065
0.068
0.059
0.039
ρHG (g cm-3)
0.840
0.800
0.750
0.800
0.800
ED (MPa)
8.03
1.53
2.21
3.47
1.50
σ70-D (MPa)
0.88
0.47
0.34
0.82
0.39
WPH-T (g/g)
161
102
93
73
110
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87
33.6
15.3
6.8
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Yield (wt %)
FA-80-20 FA-80-26
represents the internal phase content (wt %) and Y represents the amount of
3.2. Density
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the F-127-DMA in all the monomers (wt %)
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The densities of the FA-X-Y were relatively low (Table 3), varying from 0.039 to 0.082 g cm-3, values that were similar to those expected from the monomer contents in the corresponding HIPE recipes (from 3.79 to 6.31 wt %). The density decreased with increasing dispersed phase content (higher void volume), as expected. Both ρPH and ρHG decreased with increasing F-127-DMA content since the crosslinking enhances the mechanical properties, the resistance to deformation, and thus the resistance to shrinkage and partial 16
ACCEPTED MANUSCRIPT collapse during polymerization, purification, and drying. These relatively low densities are quite different from those observed for the HG-PHs crosslinked using conventional crosslinking comonomers, whose densities are usually
four times the expected density [17,27,43].
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significantly higher than expected from the HIPE’s monomer contents and can be
The densities of the FAM-80-Y ranged from 0.076 to 0.096 g cm-3 and the
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density of AM-80-0 was 0.089 g cm-3 (Table 4). Similar to the FA-X-Y, the density
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decreased with increasing F-127-DMA contents since the crosslinking enhanced the structural stability. The densities of the FAM-80-Y and AM-80-0 were slightly higher than expected from the HIPE’s monomer contents, reflecting the tighter, denser network formed by the low molecular weight crosslinker. This tighter,
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denser network is also seen in the ρHG of around 0.93 g cm-3 for the FAM-0-Y in Table 4 which are much higher than the ρHG for the FA-0-Y in Table 3. The low
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molecular weight crosslinker seems to exert a stronger contractive force upon
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the polymers during the polymerization, purification, and drying processes.
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ACCEPTED MANUSCRIPT Table 4 The properties of the FAM-80-Y and AM-80-0. PHa
FAM-80-13 FAM-80-20 FAM-80-26 88
89
89
dv (μm)
53.9
46.3
38.8
55.8
dh (μm)
6.9
8.6
7.1
9.1
ρPH (g cm-3)
0.096
0.076
0.076
0.089
ρHG (g cm-3)
0.930
0.940
0.930
0.950
ED (MPa)
17.1
12.9
11.6
24.2
σ70-D (MPa)
1.88
εF-D (%)
/
σF-D (MPa)
/
WPH-T (g/g)
20
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1.33
1.07
/
/
/
6.3
/
/
1.17
21
21
21
represents the F-127-DMA content in all the monomers (wt %).
3.3. Structure
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aY
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88
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Yield (wt %)
AM-80-0
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The FA-X-Y (in Fig. 1), as well as the FAM-80-Y and AM-80-0 (in Fig. 2), all
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possessed highly interconnected macroporous structures. There are no significant differences in the interconnecting hole sizes and shapes in Fig. 1 beyond the typical variations that can be seen within the same sample. The average void diameters ranged from 34 to 89 μm for the FA-X-Y, ranged from 39 to 54 μm for the FAM-80-Y, and was 56 μm for AM-80-0. These average void diameters are significantly larger than the typical values, ranging from 1 to 20 μm, found in HG-PHs synthesized within o/w HIPEs that were stabilized using 18
ACCEPTED MANUSCRIPT conventional surfactants [16,17], indicating that F-127 and F-127-DMA are not as effective as the conventional, low molecular weight surfactants. The void size distributions for the FA-X-Y and the FAM-80-Y (Figs. S2 and S3, respectively, in
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the SI) are similar to those of typical HG-PHs. The average interconnecting hole diameters ranged from 6.4 to 15.3 μm for the FA-X-Y, ranged from 6.9 to 8.6 μm for the FAM-80-Y, and was 9.1 µm for AM-80-0. The ratios of the average
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interconnecting hole diameter to the average void diameter, ranging from 0.1 to
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0.3, are similar to those observed for typical PHs from surfactant-stabilized water-in-oil HIPEs [44]. The mass fractions of the dispersed phase and the mass fractions of F-127-DMA did not have significant effects on the porous structures of the PHs.
midblock
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As seen for the HG-PHs based on F-127-DMA alone, the F-127-DMA’s PPO undergoes
phase
separation
in
the
PH
walls
and
forms
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microphase-separated domains (as seen from the SAXS spectrum from FA-75-20 in Fig. S4, SI). The average distance between the neighboring microdomains in
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FA-75-20 is 50.1 nm, significantly larger than the 12.9 nm seen in the HG-PHs where F-127-DMA is the only monomer [29]. The larger spacing in FA-75-20 reflects the high AAm content, which pushes the microphase-separated domains apart.
19
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Fig. 1. Porous structures (SEM) of: (a) FA-80-13; (b) FA-80-20; (c) FA-80-26;
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(d) FA-75-20; (e) FA-85-20. Note the differences in the scales.
20
Fig.
2.
Porous
structures
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(SEM)
of:
(a) FAM-80-13;
(b) FAM-80-20;
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(c) FAM-80-26; (d) AM-80-0.
3.4. Enhanced mechanical behavior
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The mechanical behavior of the dry HG-PHs was investigated using
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compressive stress-strain tests. Typical compressive stress-strain curves from the dry FAM-80-Y and the dry AM-80-0 are shown in Fig. 3a. AM-80-0 was relatively stiff and brittle, breaking at 6.3 % strain, reflecting the relatively high Tg of PAAm. The FAM-80-Y, on the other hand, were robust and did not fail at strains up to 70 %, exhibiting the linear, plateau, and densification regions seen in the types of stress-strain curves that are often associated with PHs. The enhanced robustness of FAM-80-Y results from the macromolecular flexibility 21
ACCEPTED MANUSCRIPT introduced by the F-127-DMA. While AM-80-0 broke into pieces upon failure, the FAM-80-Y maintained their square cross-section throughout the compression (Fig. 3a, inset). Of the FAM-80-Y, FAM-80-13 has the highest ED and σ70-D,
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reflecting its somewhat higher density. The modulus of FAM-80-13, which is about 40% higher than those of FAM-80-20 and FAM-80-26, corresponds well with the density of FAM-80-13, which is about 26 % higher than the other
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FAM-80-Y.
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The brittleness of AM-80-0 and the flexibility of the FAM-80-Y are also reflected in the significantly higher friability of AM-80-0. The friabilities were compared by drawing lines with the PHs on a piece of cardboard. AM-80-0 left a clear “chalky” trace, while the FAM-80-Y did not (shown in Fig. S5 for AM-80-0
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and FAM-80-26, SI). The reduction in friability can be ascribed to the F-127-DMA, which enhances the flexibility of the macromolecular structure [19].
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The compressive stress-stain curves of the dry FA-80-Y in Fig. 3b are also similar to those of typical PHs, with a linear region, a plateau region, and a
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densification region. The FA-80-Y were also robust and did not fail at up to 70 % strain. For the FA-X-20, the density does not seem to have a significant effect on ED or σ70-D. For FA-80-13, both ED and σ70-D were larger than those of the other FA-80-Y, reflecting the higher density that resulted from the more extensive shrinkage at the relatively low crosslinking monomer content (an Xm of 1.04×10-5 mol/g).
22
ACCEPTED MANUSCRIPT The ED (Fig. 3c) and σ70-D (Fig. 3d) of the FA-80-Y are significantly lower than those of the FAM-80-Y (Tables 3 and 4), reflecting the larger number of crosslinking comonomers per mass polymer in the FAM-80-Y, the significantly
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higher Xm. The MBAAm crosslinking tends to cause greater shrinkage during drying, producing somewhat higher densities, and leading to the higher ED and
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σ70-D.
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Fig. 3. Mechanical behavior of the dry PHs: (a) compressive stress-strain curves of AM-80-0 and the FAM-80-Y, where the insets (A) and (B) are photographs of a
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AM-80-0 sample and a typical FAM-80-Y sample (FAM-80-26), respectively, following the removal of the compressive stress; (b) compressive stress-strain
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curves of the FA-80-Y; (c) variation of ED with the F-127-DMA content in FAM-80-Y and FA-80-Y; (d) variation of σ70-D with the F-127-DMA content in FAM-80-Y and FA-80-Y.
3.5. Enhanced water uptake The water uptakes in the FAM-80-Y and in AM-80-0 were similar, around 20 g/g (Table 4). The relatively low water uptakes in the FAM-80-Y and in AM-80-0 23
ACCEPTED MANUSCRIPT reflect the limits on swelling imposed by the relatively high degree of MBAAm crosslinking and the Xm of ~50 ×10-5 mol/g (Table 2). The FA-X-Y, on the other hand, exhibited relatively high water uptakes that ranged from 73 to 161 g/g at
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room temperature (Fig. 4a). The uptake increased with increasing internal phase content (X), as expected from the increase in the void content. The uptake also increased with a decrease in the F-127-DMA content (Y) as expected from the
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decrease in the crosslink density, and the resulting increase in macromolecular
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mobility, that enables more extensive swelling. The extent of crosslinking in the FA-X-Y was relatively low, ~1 ×10-5 mol/g (Table 1). The significant differences in the Xm for similar masses of crosslinking commoners reflect the relatively high molecular weight of F-127-DMA. The relatively low crosslink density of the
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FA-X-Y enables more extensive swelling, and hence, more extensive water uptakes.
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The water uptake within HG-PHs has been described as including the uptake associated with the hydrogel walls (WHG-T), the uptake associated with the void
volume
(WPH-V),
and
the
uptake
associated
with
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original
hydrogel-swelling-driven void expansion (WPH-VE) [16,27]. The calculation of the FA-X-Y water uptake components provides insight into the uptake mechanism. As expected, WHG-T, which is around 14 g/g, increases somewhat with decreasing crosslinking comonomer content for similar internal phase contents (Fig. 4a) and WPH-V, which is also around 15 g/g, increases somewhat with increasing internal phase content and with decreasing density for similar internal phase contents 24
ACCEPTED MANUSCRIPT (Fig. 4b). The water uptake, however, is completely dominated by hydrogel-driven void expansion, which increases significantly with decreasing crosslinking comonomer content (Fig. 4a) and with increasing internal phase
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content (Fig. 4b). WPH-VE varies from ~65 g/g (around 70% of the total uptake) to ~130 g/g (around 80% of the total uptake). It is the low degree of crosslinking and the flexibility of the triblock copolymer crosslinking comonomer that enable
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the extensive swelling in the FA-X-Y. The water uptakes of the FAM-80-Y and
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AM-80-0 are relatively low, and therefore, the assignment of the uptake to WHG-T, WPH-V, and WPH-VE are not presented. Similar
to
F-127,
the
water
solubility
of
F-127-DMA
is
temperature-dependent [45]. The water uptake in FA-80-Y underwent a
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significant reduction (40 to 50 %) on increasing the temperature from 20 to 60 °C (Fig. 4c). Interestingly, there was an uptake plateau at lower temperatures
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and a critical temperature at which the decrease in the uptake begins. The critical temperature decreased with increasing F-127-DMA content, reflecting
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the increase in the overall hydrophobicity of the HG-PH. F-127-DMA crosslinking can, therefore, impart a temperature-dependent water uptake to HG-PHs based on conventional hydrogel monomers. FA-0-13 and FA-80-13 are formed from the same hydrogel, with the uptake of FA-0-13 corresponding to the WHG-T in FA-80-13. The significantly lower water uptake in FA-0-13 does not exhibit a significant temperature sensitivity (Fig. 4c). Similar to the emulsion templated, responsive hydrogels reported previously [28,29], emulsion templating not only 25
ACCEPTED MANUSCRIPT amplifies the water uptake, but also enhances the environmental sensitivity, amplifying the temperature response. The water uptake in FAM-80-13, which is also significantly lower than that in FA-80-13, is similar to the FA-80-13 WPH-V
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(Fig. 4c) and reflects the relatively temperature-insensitive uptake in the porous structure. The lack of swelling, and thus void expansion, in FAM-80-13 results from the lack of macromolecular mobility that is imposed by the relatively high
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degree of MBAAm crosslinking.
26
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Fig. 4. (a) The FA-80-Y (Xm: 1.04, 1.65, and 2.27) water uptakes (WHG-T, WPH-V, and WPH-VE); (b) the FA-X-20 water uptakes (WHG-T, WPH-V, and WPH-VE); (c) temperature variation in the water uptakes in the FA-80-Y, FAM-80-13, and FA-0-13 ; (d) log-log variation with time of the water uptakes in FAM-80-20, FAM-80-20, AM-80-20, and FA-0-20. 27
ACCEPTED MANUSCRIPT AM-80-0 (without F-127-DMA) exhibited a rapid water uptake, with its equilibrium uptake attained in less than one minute for a 1cm × 1cm × 1cm cube (Fig. 4d). This rapid uptake in the relatively hydrophilic hydrogel can be
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associated with the capillary action associated with the micrometer-scale porosity [17]. F-127-DMA crosslinking had a significant effect on the rate of water uptake. FA-80-20 and FAM-80-20, on the other hand, exhibited
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significantly slower rates of uptake and their equilibrium uptakes (similar for
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FAM-80-20 and the rapid-uptake AM-80-0) were only attained after an hour (Fig. 4d). This reduction in uptake reflects the more hydrophobic, PPO-rich nature of the void surfaces in F-127-DMA crosslinked HG-PHs. FA-0-20 exhibited a relatively low uptake rate and, after 24 h, did not even reach its equilibrium
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uptake (~14.5 g/g after 3 days) (Fig. 4d). The significantly higher and more rapid uptake in FA-80-20, compared to FA-0-20, results from the emulsion-templated structure which both amplifies and accelerates the water uptake. The
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combination of F-127-DMA crosslinking and emulsion-templating, therefore,
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amplified and accelerated the uptake and enhanced temperature sensitivity (compared to FAM-80-20 and FA-0-20) and produced an increase in hydrophobicity (compared to AM-80-0).
4. Conclusions HG-PHs based on conventional hydrogel monomers can be synthesized in o/w HIPEs containing F-127-DMA as the sole surfactant and as the sole 28
ACCEPTED MANUSCRIPT crosslinker, avoiding the surfactant removal processes needed in conventional surfactant-stabilized PHs. The F-127-DMA crosslinked HG-PHs (the FA-X-Y and the reference FAM-80-Y) exhibited highly interconnected porous structures, with
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the average void diameters ranging from 33.6 to 53.9 μm, exhibiting relatively low densities, ranging from 0.039 to 0.096 g cm-3, and exhibited microphase-separated
PPO
domains
within
the
walls.
The
dry
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F-127-DMA-containing HG-PHs exhibited robust compressive behaviors, with
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the modulus and the stress at 70% strain increasing with decreasing F-127-DMA content and with the presence of F-127-DMA preventing chalkiness. The
combination
of
F-127-DMA
crosslinking
(only)
and
emulsion-templating produced amplified water uptakes, ranging from 73 to 161
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g/g at room temperature, compared to the uptakes of ~20 g/g for the FAM-80-Y and for the FA-0-Y. From 70 to 80 % of the uptakes were associated with
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hydrogel-swelling void expansion, which increased with increasing porosity and with decreasing F-127-DMA content. The combination of F-127-DMA
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crosslinking (only) and emulsion-templating also enhanced the temperature sensitivity of the hydrogels compared to the FAM-80-Y and the FA-0-Y, with the FA-X-Y water uptakes decreasing by ~50 % between 5 and 60 °C. The critical temperature decreased from ~20 to ~30 oC with increasing F-127-DMA content, reflecting the decrease in hydrophobicity (PPO content). While the emulsion templating accelerated the rate of uptake, compared to the FA-0-Y, through
29
ACCEPTED MANUSCRIPT capillary action, the presence of the more hydrophobic PPO on the surface reduced the rate of uptake compared to AM-80-0.
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Supporting Information Figures S1-S5: FTIR spectra (for AM-80-0, FAM-80-20, and FA-80-20); void diameter distributions (for the FA-X-Y, for the FAM-80-Y with AM-80-0); the
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SAXS spectrum from FA-75-20 showing 50 nm spacing for the F-127-DMA’s
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microphase separated PPO midblocks; “chalk” lines drawn using the friable AM-80-0 and the non-friable FAM-80-26.
Acknowledgements
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The authors gratefully acknowledge the partial support of the Israel Science Foundation (294/12 and 519/16) and the Israel Ministry of Science (880011). T. Zhang was partially supported at the Technion by an Israel Council for Higher
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Education Fellowship. The authors thank Dr. Rafael Khalfin for his kind help with
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the SAXS characterizations.
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ACCEPTED MANUSCRIPT
HIGHLIGHTS Reactive block copolymer (R-BC): methacrylate endcapped PEO-PPO-PEO
•
R-BC: stabilize oil-in-water high internal phase emulsions with acrylamide (AAm)
•
Synthesized: R-BC-crosslinked, highly porous, emulsion-templated, AAm-based
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•
hydrogels
Mechanical properties: robust compressive behaviors with no chalkiness
•
Water uptake: amplified and accelerated with enhanced temperature sensitivity
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•
S0032-3861(19)30130-2
DOI:
https://doi.org/10.1016/j.polymer.2019.02.010
Reference:
JPOL 21249
To appear in:
Polymer
Received Date: 21 November 2018 Revised Date:
1 February 2019
Accepted Date: 3 February 2019
Please cite this article as: Zhang T, Silverstein MS, Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer, Polymer (2019), doi: https:// doi.org/10.1016/j.polymer.2019.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Robust, highly porous hydrogels templated within emulsions stabilized using a reactive, crosslinking triblock copolymer
aDepartment
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Tao Zhang,*a,b Michael S. Silverstein*a of Materials Science and Engineering, Technion – Israel Institute of
Technology, Haifa, 32000, Israel.
of Textile and Clothing Engineering, Soochow University, Suzhou
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bCollege
*Corresponding Authors E-mail:
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215123, China.
[email protected] (T. Zhang)
[email protected] (M. S. Silverstein)
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Keywords: emulsion templating; hydrogels; block copolymers.
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ABSTRACT PolyHIPEs (PHs) are porous polymers that are typically templated within surfactant-stabilized high internal phase emulsions (HIPEs). Hydrogel PHs
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(HG-PHs) have exhibited potential as absorbents, adsorbents, release systems, and tissue engineering scaffolds. Recent work has shown that HG-PHs with highly temperature sensitive water uptakes could be produced using a reactive
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block copolymer (BC) as the sole surfactant, monomer, and crosslinker. It is not
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clear, however, whether these properties could be transferred to a HG-PH based on conventional hydrogel monomers and synthesized within a HIPE stabilized using a reactive BC. This work investigates HG-PHs based on acrylamide (AAm) synthesized within a HIPE stabilized by F-127-DMA, a reactive crosslinking BC
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based on methacrylate endcapped Pluronic F-127 (a commercial PEO-PPO-PEO triblock copolymer with poly(ethylene oxide) (PEO) endblocks and a poly(propylene oxide) (PPO) midblock). All the F-127-DMA-containing HG-PHs
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exhibited highly interconnected porous structures, low densities, and robust
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compressive behaviors with no chalkiness. The combination of F-127-DMA crosslinking and emulsion-templating produced amplified and accelerated water uptakes, up to 161 g/g, associated with hydrogel-swelling driven void expansion, and enhanced the temperature sensitivity, with the uptake decreasing by ~50 % between 5 and 60 °C, compared to reference HG-PHs and to conventional hydrogels.
2
ACCEPTED MANUSCRIPT 1. Introduction PolyHIPEs (PHs) are macroporous polymers synthesized within the continuous phases of high internal phase emulsions (HIPEs), emulsions with
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over 74 vol % dispersed internal phase [1-4]. HIPEs are usually stabilized using non-ionic surfactants. HIPEs stabilized by amphiphilic particles, Pickering HIPEs, have been successfully formed using inorganic nanoparticles [5,6], organic
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nanoparticles [7,8], and star polymers [9,10]. The stabilization strategy has been
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demonstrated to be an integral factor in determining the morphology and the properties of the resulting PHs [11-13]. Block copolymers (BCs), copolymers exhibiting at least two chemically different blocks connected by covalent linkages, have also been used to stabilize HIPEs.4,25-28 Amphiphilic BCs, BCs
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containing both hydrophobic and hydrophilic blocks, are advantageous as surfactants since their structures can be tuned for specific applications [14,15]. The most common amphiphilic BCs used for HIPE stabilization are the
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PEO-PPO-PEO triblock copolymers (a poly(propylene oxide) (PPO) midblock and
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poly(ethylene oxide) (PEO) endblocks) [16,17]. The PEO-PPO-PEO BCs are inert during radical polymerization and can usually be removed through extraction following PH synthesis. The effects of the crosslinking strategy upon the structure and properties of PHs can be extraordinary. Oligomeric crosslinking has been used in place of lower molecular weight crosslinking comonomers to enhance the mechanical properties [18,19] and to introduce degradability [20-22]. Using the stabilizing 3
ACCEPTED MANUSCRIPT nanoparticles in Pickering HIPEs as crosslinking centers [23,24], instead of using conventional crosslinking comonomers, has produced effects that vary from enabling shape memory behavior [25] to internal phase encapsulation through
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the formation of closed-cell structures [26]. Hydrogel polyHIPEs (HG-PHs), PHs that are usually templated within oil-in-water (o/w) HIPEs, often exhibit enhanced mechanical properties [16,17],
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amplified and accelerated water uptake, and enhanced environmental sensitivity
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as compared to conventional hydrogels synthesized from the same monomers [27-30]. HG-PHs have drawn a considerable amount of interest for a diverse range of applications that include absorption [16,28], adsorption [31], controlled release [32], tissue engineering [33,34], and enhancing the efficiency of using
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water for fighting fires [35]. The water uptake and the mechanical behavior are among the fundamental considerations that play decisive roles in the adaptation
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of hydrogels for such applications [36].
Recently, reactive BCs have been developed as surfactants for HIPE
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stabilization. The BC surfactants were found on the PH void surfaces, since they are spontaneously absorbed at the oil-water interface and become anchored to the void surface, forming a coating [14]. Reactive BCs exhibiting a polymerization controlling agent or a chain transfer agent (CTA) have also been utilized to stabilize HIPEs [37-39]. These reactive BC surfactants can be covalently incorporated into the PH’s macromolecular structure via initiation or chain-transfer reactions. PHs from reactive BC-stabilized HIPEs can overcome 4
ACCEPTED MANUSCRIPT the drawbacks associated with PHs from HIPEs stabilized using traditional non-reactive surfactants, which must be removed since they could leach out during use. These drawbacks include the additional processing and the
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associated expense of surfactant removal, the environmental challenge in disposing of the relatively large amounts of surfactant, and the possibility that any residual surfactant may leach out during use [40]. The effects of reactive
SC
block copolymer stabilizers on the water uptake and the mechanical properties
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have seldom been studied. Such effects will be particularly significant when the reactive surfactant also acts as a crosslinking comonomer.
A series of unique polyHIPEs consisting solely of a reactive triblock copolymer, F-127 dimethacrylate (F-127-DMA), synthesized from a commercial
TE D
PEO-PPO-PEO triblock copolymer (Pluronic F-127) by methacrylate endcapping of the two terminal –OH groups, was recently described by the authors [29].
EP
F-127-DMA acted as the HIPE stabilizer, the sole monomer, and the crosslinking monomer. The resulting microphase-separated structure, crystallinity, and
AC C
temperature-responsive uptake of the PH reflected the behavior of the PEO-PPO-PEO. The PH synthesized with F-127-DMA as the sole monomer possessed a unique structure and advantageous properties. It would be advantageous to transfer such properties to a HG-PH synthesized from a conventional hydrogel monomer by using F-127-DMA as a crosslinking comonomer, replacing N,N’-methylenebisacrylamide (MBAAm), the conventional crosslinking comonomer. 5
ACCEPTED MANUSCRIPT This article describes the effects of F-127-DMA crosslinking on HG-PHs based on acrylamide (AAm) synthesized within F-127-DMA-stabilized HIPEs, as described in Scheme 1. F-127-DMA endowed the resulting highly interconnected
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macroporous AAm-based HG-PHs with thermosensitive water uptakes and enhanced mechanical properties compared to reference HG-PHs where MBAAm was used as a crosslinking comonomer (Scheme 1). MBAAm, in spite of having a
SC
molecular weight significantly lower than that of F-127-DMA, was chosen for this
AC C
EP
TE D
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comparison since it is, by far, the dominant crosslinker used for such hydrogels.
Scheme 1. Scheme showing two types of HIPEs used for generating emulsion-templated highly porous hydrogels based on AAm. The HIPEs were formed by adding cyclohexane (between 75 and 85% of the HIPE) to the aqueous solution phase while mixing. The top HIPE contained MBAAm, a typical crosslinking comonomer, while the bottom HIPE contained a reactive surfactant 6
ACCEPTED MANUSCRIPT (F-127-DMA, a methacrylate-endcapped PEO-PPO-PEO block copolymer) as a crosslinking comonomer. Following polymerization, purification, and drying the mechanical properties in uniaxial compression and the water-uptake were
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evaluated. The MBAAm-crosslinked hydrogel fractured in a brittle manner at low strains and was only able to absorb a limited amount of water, while the F-127-DMA crosslinked hydrogel was mechanically robust (it did not fail at
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SC
strains up to 70%) and was able to absorb significant quantities of water.
2. Experimental 2.1. Materials
AAm, MBAAm, and N,N,N',N'-tetramethylethylenediamine (TEMED) (all
TE D
from Sigma) and ammonium persulfate (APS) (from Fluka) were analytical grade and used as received. The other reagents were analytical grade (methanol, cyclohexane, methacryloyl chloride, triethylamine (TEA), dichloromethane
EP
(DCM), diethyl ether). Deionized water was used throughout. The synthesis of
AC C
F-127-DMA was described in a previous work [29]. Briefly, methacryloyl chloride was reacted with the terminal hydroxyl groups of F-127 in the presence of TEA (in DCM). The resulting F-127-DMA was precipitated in diethyl ether, producing a 91 % yield. 2.2. PolyHIPE synthesis: FA-X-Y, FAM-80-Y, and AM-80-0 The HIPE recipes listed in Table 1 are for the FA-X-Y series based solely on AAm and F-127-DMA, with X representing the mass fraction of the dispersed 7
ACCEPTED MANUSCRIPT phase (75, 80, or 85 wt %) and with Y representing the mass fraction of F-127-DMA in all the monomers (13, 20, or 26 wt %). The HIPE recipes listed in Table 2 are for the FAM-80-Y series based on AAm, F-127-DMA, and MBAAm,
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with the 80 denoting 80 wt % dispersed phase and with Y representing the mass fraction of F-127-DMA in all the monomers (13, 20, or 26 wt %). Table 2 also lists the recipe for AM-80-0, which is based solely on AAm and MBAAm (no
SC
F-127-DMA) in an F-127-stabilized HIPE containing 80 wt % internal phase.
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Table 1 and 2 also list the mass fraction of F-127-DMA in all the monomers (MF-127-DMA) and the mass fraction of MBAAm in all the monomers (MMBAAm). The same procedure was used for all the syntheses (the FA-X-Y, the FAM-80-Y, and AM-80-0). The components of the aqueous phase (Tables 1 and 2)
TE D
were dissolved in water. Cyclohexane was then added dropwise into the aqueous phase under constant stirring with an overhead stirrer. Following the addition of
EP
cyclohexane, the stirring speed was reduced and TEMED was added to the HIPE. The polymerization was then conducted in a convection oven at 40 °C for 8 h.
AC C
The resulting PHs were cut into cubes (about 1 cm × 1 cm × 1 cm) and the water, cyclohexane, and residual monomers were removed using Soxhlet extraction with methanol for 24 h. The HG-PHs were then dried in a vacuum oven at room temperature for 24 h. The AAm and MBAAm contents in the HIPE were kept constant and the variation in the F-127-DMA content was at the expense of the water. The moles of crosslinking comonomer molecules per mass polymer (Xm, listed in Tables 1 8
ACCEPTED MANUSCRIPT and 2) was calculated by dividing the total number of moles of the crosslinking comonomers (F-127-DMA and/or MBAAm) by the total mass of all the monomers (AAm, F-127-DMA, and/or MBAAm,). Xm increases with increasing
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F-127-DMA content for the FA-X-Y since the contribution of the additional crosslinks is more significant than the contribution of the additional mass. However, Xm decreases with increasing F-127-DMA content for the FA-80-Y since
SC
the number of MBAAm crosslinks is dominant and the contribution of the
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F-127-DMA to the mass is more significant than its contribution to the number of
AC C
EP
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crosslinks. AM-80-0 has the highest Xm, 59.0 × 10-5 mol/g (Table 2).
9
ACCEPTED MANUSCRIPT Table 1 The FA-X-Y recipes. PHa
FA-80-13
FA-80-20
FA-80-26
FA-75-20
FA-85-20
18.69
11.21
1.31
0.79
5.00
3.00
0.13
0.07
25.13
15.07
74.74
84.86
15.40
14.95
14.50
F-127-DMA
0.60
1.05
1.50
AAm
4.00
4.00
4.00
APS
0.10
0.10
0.10
Total
20.10
20.10
20.10
Cyclohexane
79.80
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Organic, dispersed phase (wt %)
SC
Water
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Aqueous, continuous phase (wt %)
79.80
79.80
TEMED (catalyst added following HIPE formation) (wt %)
MF-127-DMA (wt %)
13.04
EP
Xm (×10-5 mol/g)
1.04
0.10
0.13
0.07
20.79
27.27
20.76
20.84
1.65
2.27
1.65
1.65
represents the internal phase content (wt %) and Y represents the amount of
AC C
aX
0.10
TE D
0.10
F-127-DMA in all the monomers (wt %)
10
ACCEPTED MANUSCRIPT Table 2 The FAM-80-Y and AM-80-0 recipes. PHa
FAM-80-13
FAM-80-20
FAM-80-26
AM-80-0
15.00
14.55
14.1
14.55
/
/
/
1.05
F-127-DMA
0.60
1.05
1.50
/
AAm
4.00
4.00
4.00
4.00
MBAAm
0.40
0.40
0.40
0.40
APS
0.10
Total
20.10
Dispersed, organic phase (wt %) Cyclohexane
78.80
SC
F-127
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H2O
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Continuous, aqueous phase (wt %)
0.10
0.10
0.10
20.10
20.10
20.10
78.80
78.80
78.80
0.10
0.10
0.10
0.10
12.00
19.27
25.42
0.00
8.00
7.34
6.78
9.09
52.9
49.2
46.0
59.0
EP
MF-127-DMA (wt %)
TE D
TEMED (catalyst added after HIPE formation) (wt %)
AC C
MMBAAm (wt %)
Xm (×10-5 mol/g)
aY
represents the F-127-DMA content in all the monomers (wt %)
11
ACCEPTED MANUSCRIPT 2.3. Reference hydrogel synthesis: FA-0-Y and FAM-0-Y The FA-0-Y and the FAM-0-Y, where Y represents the mass fraction of the F-127-DMA in the corresponding monomers (13, 20, or 26 wt %), are two series
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of conventional hydrogels used as references for the FA-X-Y and the FAM-80-Y, respectively. The FA-0-Y and the FAM-0-Y were prepared through the polymerization of the aqueous phase recipes with TEMED (Table 1 and Table 2,
SC
respectively, without adding the organic phase) in a convection oven at 40 °C for
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8 h. The resulting hydrogel was immersed in water (with a hydrogel to water ratio of over 1:30) for one week, where the water was changed every day. The FA-0-Y and the FAM-0-Y were dried in a vacuum oven at room temperature for 3 days.
TE D
2.4. Yield, density, macroporous structure, and microphase separation The yields of the FA-X-Y, the FAM-80-Y, and AM-80-0 were determined using
EP
a mass balance following the Soxhlet extraction and drying, and the densities of the PHs (ρPH) and of the FA-0-Y and the FAM-0-Y reference hydrogels (ρHG) were
AC C
determined gravimetrically. The porous structures of the FA-X-Y, the FAM-80-Y, and AM-80-0 were described using scanning electron microscopy (SEM, FEI Quanta 200 at 10 kV). Cryogenic fracture surfaces, obtained by immersing the HG-PHs in liquid nitrogen, were coated with a thin gold-palladium layer. The average interconnecting hole diameters (dh) and the average void diameters (dv) were determined from the SEM micrographs using 100 measurements each. The average void diameters were corrected for the random nature of the section by 12
ACCEPTED MANUSCRIPT multiplying by 2/(31/2) [41]. The Fourier transform infrared (FTIR) spectroscopy of AM-80-0, FAM-80-20, and FA-80-20 were taken in transmission (Bruker Vertex 70 FTIR, between 600 and 4000 cm-1 at a resolution of 4 cm-1). The PHs
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were ground with KBr and pressed into pellets for measurement. The spectra are presented and discussed in Fig. S1 in the Supplementary Information (SI). Small-angle X-ray scattering (SAXS) was performed under vacuum using the
SC
same SAXS system as that reported in the previous work (Molecular Metrology
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SAXS with an incident wavelength of 0.1542 nm) [29]. 2.5. Compressive stress-strain behavior
Compressive stress-strain tests (Instron 3345) were performed on dry HG-PH cubes (about 1.0 cm × 1.0 cm × 1.0 cm) at 10 % min-1 (about 0.1 cm min-1)
TE D
and room temperature to a maximum strain of 70 % (an equipment-related limit). The compressive modulus of the dry HG-PH, ED, was determined from the
EP
linear slope of the stress-strain curve at low strains, with the average modulus taken from at least three tests. The levels of the resistance to densification in the
AC C
dry HG-PHs were compared through the stress at 70% strain, σ70-D. If the sample failed at lower strains, the failure was described using the failure strain, εF-D, and the failure stress, σF-D. 2.6. Water uptake The equilibrium water uptakes of the FA-X-Y, the FAM-80-Y, the FA-0-Y, and AM-80-0 were determined gravimetrically at room temperature. Typically, a dry 13
ACCEPTED MANUSCRIPT sample cube (about 1.0 cm ×1.0 cm × 1.0 cm) with a known mass (MD) was placed in a vial with water for 24 hours to reach a fully swollen mass (Msw). The total water uptake (WPH-T), calculated from Equation 1, was an average of three
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results. The water uptakes in the FA-80-Y were also determined for different temperatures (from 5 to 60 °C in 5 °C increments). The temperature was controlled using a Thermo Neslab RTE-110 bath (Neslab Instruments). (1)
SC
WPH-T = (Msw – MD) / MD
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Water uptake associated with PH walls (WHG-T), water uptake associated with PH voids (WPH-V) and water uptake from hydrogel-swelling-driven void expansion (WPH-VE) were determined according to the method reported previously [16,27]. In brief, WHG-T was determined from the water uptake of the
TE D
FA-0-Y, WPH-V was calculated from the PH density and the wall density, and
EP
WPH-VE was calculated by subtracting WHG-T and WPH-V from WPH-T.
AC C
3. Results and discussion 3.1. Synthesis
Stable FA-X-Y HG-PH monoliths were successfully synthesized without the
need for a conventional crosslinking comonomer. The FA-X-Y exhibited relatively high polymerization yields, around 90 % (Table 3), with the yield increasing slightly with increasing F-127-DMA content and with decreasing dispersed phase content. On the other hand, a stable HG-PH monolith could not be formed in a 14
ACCEPTED MANUSCRIPT control HIPE stabilized using the non-reactive F-127 instead of the reactive F-127-DMA (there was no crosslinking comonomer). These results demonstrate that F-127-DMA-based crosslinking is essential for the synthesis of a stable
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HG-PH in the absence of a conventional crosslinking comonomer. The FAM-80-Y and AM-80-0, both of which contained a conventional crosslinker (MBAAm), exhibited similar polymerization yields of around 89 % (Table 4). The similarity
SC
of the yields indicates that the presence of F-127-DMA and/or MBAAm did not
M AN U
affect the yield. The polymerization yields of FA-0-Y were about 95%. The incorporation of the surfactant into the HG-PH’s macromolecular network through copolymerization overcomes the challenges associated with the conventional non-reactive surfactants (the additional processing, the cost
TE D
and environmental impact of surfactant removal, and the possibility that residual surfactant would leach out). The polymerizable HIPE surfactants reported
EP
previously (based on low-molecular-weight surfactants [40,42] or on block copolymers [37-39]) were not designed to crosslink the PHs. The F-127-DMA
AC C
crosslinking, however, also provides added value, as described below. The typical PH structures with relatively thick walls that result from the HIPE-templated polymerization of F-127-DMA alone in previous work [29] demonstrate that the polymerization is not restricted to the interface. Therefore, the crosslinks are not expected to be located solely on the void’s inner surface.
15
ACCEPTED MANUSCRIPT Table 3 The properties of the FA-X-Y. PHa
FA-80-13
FA-75-20
FA-85-20
91
88
89
92
dv (μm)
51.4
86.0
43.7
dh (μm)
14.6
8.5
6.4
ρPH (g cm-3)
0.082
0.065
0.068
0.059
0.039
ρHG (g cm-3)
0.840
0.800
0.750
0.800
0.800
ED (MPa)
8.03
1.53
2.21
3.47
1.50
σ70-D (MPa)
0.88
0.47
0.34
0.82
0.39
WPH-T (g/g)
161
102
93
73
110
aX
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87
33.6
15.3
6.8
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SC
88.8
TE D
Yield (wt %)
FA-80-20 FA-80-26
represents the internal phase content (wt %) and Y represents the amount of
3.2. Density
EP
the F-127-DMA in all the monomers (wt %)
AC C
The densities of the FA-X-Y were relatively low (Table 3), varying from 0.039 to 0.082 g cm-3, values that were similar to those expected from the monomer contents in the corresponding HIPE recipes (from 3.79 to 6.31 wt %). The density decreased with increasing dispersed phase content (higher void volume), as expected. Both ρPH and ρHG decreased with increasing F-127-DMA content since the crosslinking enhances the mechanical properties, the resistance to deformation, and thus the resistance to shrinkage and partial 16
ACCEPTED MANUSCRIPT collapse during polymerization, purification, and drying. These relatively low densities are quite different from those observed for the HG-PHs crosslinked using conventional crosslinking comonomers, whose densities are usually
four times the expected density [17,27,43].
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significantly higher than expected from the HIPE’s monomer contents and can be
The densities of the FAM-80-Y ranged from 0.076 to 0.096 g cm-3 and the
SC
density of AM-80-0 was 0.089 g cm-3 (Table 4). Similar to the FA-X-Y, the density
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decreased with increasing F-127-DMA contents since the crosslinking enhanced the structural stability. The densities of the FAM-80-Y and AM-80-0 were slightly higher than expected from the HIPE’s monomer contents, reflecting the tighter, denser network formed by the low molecular weight crosslinker. This tighter,
TE D
denser network is also seen in the ρHG of around 0.93 g cm-3 for the FAM-0-Y in Table 4 which are much higher than the ρHG for the FA-0-Y in Table 3. The low
EP
molecular weight crosslinker seems to exert a stronger contractive force upon
AC C
the polymers during the polymerization, purification, and drying processes.
17
ACCEPTED MANUSCRIPT Table 4 The properties of the FAM-80-Y and AM-80-0. PHa
FAM-80-13 FAM-80-20 FAM-80-26 88
89
89
dv (μm)
53.9
46.3
38.8
55.8
dh (μm)
6.9
8.6
7.1
9.1
ρPH (g cm-3)
0.096
0.076
0.076
0.089
ρHG (g cm-3)
0.930
0.940
0.930
0.950
ED (MPa)
17.1
12.9
11.6
24.2
σ70-D (MPa)
1.88
εF-D (%)
/
σF-D (MPa)
/
WPH-T (g/g)
20
SC
1.33
1.07
/
/
/
6.3
/
/
1.17
21
21
21
represents the F-127-DMA content in all the monomers (wt %).
3.3. Structure
TE D
aY
RI PT
88
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Yield (wt %)
AM-80-0
EP
The FA-X-Y (in Fig. 1), as well as the FAM-80-Y and AM-80-0 (in Fig. 2), all
AC C
possessed highly interconnected macroporous structures. There are no significant differences in the interconnecting hole sizes and shapes in Fig. 1 beyond the typical variations that can be seen within the same sample. The average void diameters ranged from 34 to 89 μm for the FA-X-Y, ranged from 39 to 54 μm for the FAM-80-Y, and was 56 μm for AM-80-0. These average void diameters are significantly larger than the typical values, ranging from 1 to 20 μm, found in HG-PHs synthesized within o/w HIPEs that were stabilized using 18
ACCEPTED MANUSCRIPT conventional surfactants [16,17], indicating that F-127 and F-127-DMA are not as effective as the conventional, low molecular weight surfactants. The void size distributions for the FA-X-Y and the FAM-80-Y (Figs. S2 and S3, respectively, in
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the SI) are similar to those of typical HG-PHs. The average interconnecting hole diameters ranged from 6.4 to 15.3 μm for the FA-X-Y, ranged from 6.9 to 8.6 μm for the FAM-80-Y, and was 9.1 µm for AM-80-0. The ratios of the average
SC
interconnecting hole diameter to the average void diameter, ranging from 0.1 to
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0.3, are similar to those observed for typical PHs from surfactant-stabilized water-in-oil HIPEs [44]. The mass fractions of the dispersed phase and the mass fractions of F-127-DMA did not have significant effects on the porous structures of the PHs.
midblock
TE D
As seen for the HG-PHs based on F-127-DMA alone, the F-127-DMA’s PPO undergoes
phase
separation
in
the
PH
walls
and
forms
EP
microphase-separated domains (as seen from the SAXS spectrum from FA-75-20 in Fig. S4, SI). The average distance between the neighboring microdomains in
AC C
FA-75-20 is 50.1 nm, significantly larger than the 12.9 nm seen in the HG-PHs where F-127-DMA is the only monomer [29]. The larger spacing in FA-75-20 reflects the high AAm content, which pushes the microphase-separated domains apart.
19
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SC
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ACCEPTED MANUSCRIPT
Fig. 1. Porous structures (SEM) of: (a) FA-80-13; (b) FA-80-20; (c) FA-80-26;
AC C
EP
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(d) FA-75-20; (e) FA-85-20. Note the differences in the scales.
20
Fig.
2.
Porous
structures
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SC
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ACCEPTED MANUSCRIPT
(SEM)
of:
(a) FAM-80-13;
(b) FAM-80-20;
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(c) FAM-80-26; (d) AM-80-0.
3.4. Enhanced mechanical behavior
EP
The mechanical behavior of the dry HG-PHs was investigated using
AC C
compressive stress-strain tests. Typical compressive stress-strain curves from the dry FAM-80-Y and the dry AM-80-0 are shown in Fig. 3a. AM-80-0 was relatively stiff and brittle, breaking at 6.3 % strain, reflecting the relatively high Tg of PAAm. The FAM-80-Y, on the other hand, were robust and did not fail at strains up to 70 %, exhibiting the linear, plateau, and densification regions seen in the types of stress-strain curves that are often associated with PHs. The enhanced robustness of FAM-80-Y results from the macromolecular flexibility 21
ACCEPTED MANUSCRIPT introduced by the F-127-DMA. While AM-80-0 broke into pieces upon failure, the FAM-80-Y maintained their square cross-section throughout the compression (Fig. 3a, inset). Of the FAM-80-Y, FAM-80-13 has the highest ED and σ70-D,
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reflecting its somewhat higher density. The modulus of FAM-80-13, which is about 40% higher than those of FAM-80-20 and FAM-80-26, corresponds well with the density of FAM-80-13, which is about 26 % higher than the other
SC
FAM-80-Y.
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The brittleness of AM-80-0 and the flexibility of the FAM-80-Y are also reflected in the significantly higher friability of AM-80-0. The friabilities were compared by drawing lines with the PHs on a piece of cardboard. AM-80-0 left a clear “chalky” trace, while the FAM-80-Y did not (shown in Fig. S5 for AM-80-0
TE D
and FAM-80-26, SI). The reduction in friability can be ascribed to the F-127-DMA, which enhances the flexibility of the macromolecular structure [19].
EP
The compressive stress-stain curves of the dry FA-80-Y in Fig. 3b are also similar to those of typical PHs, with a linear region, a plateau region, and a
AC C
densification region. The FA-80-Y were also robust and did not fail at up to 70 % strain. For the FA-X-20, the density does not seem to have a significant effect on ED or σ70-D. For FA-80-13, both ED and σ70-D were larger than those of the other FA-80-Y, reflecting the higher density that resulted from the more extensive shrinkage at the relatively low crosslinking monomer content (an Xm of 1.04×10-5 mol/g).
22
ACCEPTED MANUSCRIPT The ED (Fig. 3c) and σ70-D (Fig. 3d) of the FA-80-Y are significantly lower than those of the FAM-80-Y (Tables 3 and 4), reflecting the larger number of crosslinking comonomers per mass polymer in the FAM-80-Y, the significantly
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higher Xm. The MBAAm crosslinking tends to cause greater shrinkage during drying, producing somewhat higher densities, and leading to the higher ED and
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SC
σ70-D.
TE D
Fig. 3. Mechanical behavior of the dry PHs: (a) compressive stress-strain curves of AM-80-0 and the FAM-80-Y, where the insets (A) and (B) are photographs of a
EP
AM-80-0 sample and a typical FAM-80-Y sample (FAM-80-26), respectively, following the removal of the compressive stress; (b) compressive stress-strain
AC C
curves of the FA-80-Y; (c) variation of ED with the F-127-DMA content in FAM-80-Y and FA-80-Y; (d) variation of σ70-D with the F-127-DMA content in FAM-80-Y and FA-80-Y.
3.5. Enhanced water uptake The water uptakes in the FAM-80-Y and in AM-80-0 were similar, around 20 g/g (Table 4). The relatively low water uptakes in the FAM-80-Y and in AM-80-0 23
ACCEPTED MANUSCRIPT reflect the limits on swelling imposed by the relatively high degree of MBAAm crosslinking and the Xm of ~50 ×10-5 mol/g (Table 2). The FA-X-Y, on the other hand, exhibited relatively high water uptakes that ranged from 73 to 161 g/g at
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room temperature (Fig. 4a). The uptake increased with increasing internal phase content (X), as expected from the increase in the void content. The uptake also increased with a decrease in the F-127-DMA content (Y) as expected from the
SC
decrease in the crosslink density, and the resulting increase in macromolecular
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mobility, that enables more extensive swelling. The extent of crosslinking in the FA-X-Y was relatively low, ~1 ×10-5 mol/g (Table 1). The significant differences in the Xm for similar masses of crosslinking commoners reflect the relatively high molecular weight of F-127-DMA. The relatively low crosslink density of the
TE D
FA-X-Y enables more extensive swelling, and hence, more extensive water uptakes.
EP
The water uptake within HG-PHs has been described as including the uptake associated with the hydrogel walls (WHG-T), the uptake associated with the void
volume
(WPH-V),
and
the
uptake
associated
with
AC C
original
hydrogel-swelling-driven void expansion (WPH-VE) [16,27]. The calculation of the FA-X-Y water uptake components provides insight into the uptake mechanism. As expected, WHG-T, which is around 14 g/g, increases somewhat with decreasing crosslinking comonomer content for similar internal phase contents (Fig. 4a) and WPH-V, which is also around 15 g/g, increases somewhat with increasing internal phase content and with decreasing density for similar internal phase contents 24
ACCEPTED MANUSCRIPT (Fig. 4b). The water uptake, however, is completely dominated by hydrogel-driven void expansion, which increases significantly with decreasing crosslinking comonomer content (Fig. 4a) and with increasing internal phase
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content (Fig. 4b). WPH-VE varies from ~65 g/g (around 70% of the total uptake) to ~130 g/g (around 80% of the total uptake). It is the low degree of crosslinking and the flexibility of the triblock copolymer crosslinking comonomer that enable
SC
the extensive swelling in the FA-X-Y. The water uptakes of the FAM-80-Y and
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AM-80-0 are relatively low, and therefore, the assignment of the uptake to WHG-T, WPH-V, and WPH-VE are not presented. Similar
to
F-127,
the
water
solubility
of
F-127-DMA
is
temperature-dependent [45]. The water uptake in FA-80-Y underwent a
TE D
significant reduction (40 to 50 %) on increasing the temperature from 20 to 60 °C (Fig. 4c). Interestingly, there was an uptake plateau at lower temperatures
EP
and a critical temperature at which the decrease in the uptake begins. The critical temperature decreased with increasing F-127-DMA content, reflecting
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the increase in the overall hydrophobicity of the HG-PH. F-127-DMA crosslinking can, therefore, impart a temperature-dependent water uptake to HG-PHs based on conventional hydrogel monomers. FA-0-13 and FA-80-13 are formed from the same hydrogel, with the uptake of FA-0-13 corresponding to the WHG-T in FA-80-13. The significantly lower water uptake in FA-0-13 does not exhibit a significant temperature sensitivity (Fig. 4c). Similar to the emulsion templated, responsive hydrogels reported previously [28,29], emulsion templating not only 25
ACCEPTED MANUSCRIPT amplifies the water uptake, but also enhances the environmental sensitivity, amplifying the temperature response. The water uptake in FAM-80-13, which is also significantly lower than that in FA-80-13, is similar to the FA-80-13 WPH-V
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(Fig. 4c) and reflects the relatively temperature-insensitive uptake in the porous structure. The lack of swelling, and thus void expansion, in FAM-80-13 results from the lack of macromolecular mobility that is imposed by the relatively high
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degree of MBAAm crosslinking.
26
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Fig. 4. (a) The FA-80-Y (Xm: 1.04, 1.65, and 2.27) water uptakes (WHG-T, WPH-V, and WPH-VE); (b) the FA-X-20 water uptakes (WHG-T, WPH-V, and WPH-VE); (c) temperature variation in the water uptakes in the FA-80-Y, FAM-80-13, and FA-0-13 ; (d) log-log variation with time of the water uptakes in FAM-80-20, FAM-80-20, AM-80-20, and FA-0-20. 27
ACCEPTED MANUSCRIPT AM-80-0 (without F-127-DMA) exhibited a rapid water uptake, with its equilibrium uptake attained in less than one minute for a 1cm × 1cm × 1cm cube (Fig. 4d). This rapid uptake in the relatively hydrophilic hydrogel can be
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associated with the capillary action associated with the micrometer-scale porosity [17]. F-127-DMA crosslinking had a significant effect on the rate of water uptake. FA-80-20 and FAM-80-20, on the other hand, exhibited
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significantly slower rates of uptake and their equilibrium uptakes (similar for
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FAM-80-20 and the rapid-uptake AM-80-0) were only attained after an hour (Fig. 4d). This reduction in uptake reflects the more hydrophobic, PPO-rich nature of the void surfaces in F-127-DMA crosslinked HG-PHs. FA-0-20 exhibited a relatively low uptake rate and, after 24 h, did not even reach its equilibrium
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uptake (~14.5 g/g after 3 days) (Fig. 4d). The significantly higher and more rapid uptake in FA-80-20, compared to FA-0-20, results from the emulsion-templated structure which both amplifies and accelerates the water uptake. The
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combination of F-127-DMA crosslinking and emulsion-templating, therefore,
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amplified and accelerated the uptake and enhanced temperature sensitivity (compared to FAM-80-20 and FA-0-20) and produced an increase in hydrophobicity (compared to AM-80-0).
4. Conclusions HG-PHs based on conventional hydrogel monomers can be synthesized in o/w HIPEs containing F-127-DMA as the sole surfactant and as the sole 28
ACCEPTED MANUSCRIPT crosslinker, avoiding the surfactant removal processes needed in conventional surfactant-stabilized PHs. The F-127-DMA crosslinked HG-PHs (the FA-X-Y and the reference FAM-80-Y) exhibited highly interconnected porous structures, with
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the average void diameters ranging from 33.6 to 53.9 μm, exhibiting relatively low densities, ranging from 0.039 to 0.096 g cm-3, and exhibited microphase-separated
PPO
domains
within
the
walls.
The
dry
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F-127-DMA-containing HG-PHs exhibited robust compressive behaviors, with
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the modulus and the stress at 70% strain increasing with decreasing F-127-DMA content and with the presence of F-127-DMA preventing chalkiness. The
combination
of
F-127-DMA
crosslinking
(only)
and
emulsion-templating produced amplified water uptakes, ranging from 73 to 161
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g/g at room temperature, compared to the uptakes of ~20 g/g for the FAM-80-Y and for the FA-0-Y. From 70 to 80 % of the uptakes were associated with
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hydrogel-swelling void expansion, which increased with increasing porosity and with decreasing F-127-DMA content. The combination of F-127-DMA
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crosslinking (only) and emulsion-templating also enhanced the temperature sensitivity of the hydrogels compared to the FAM-80-Y and the FA-0-Y, with the FA-X-Y water uptakes decreasing by ~50 % between 5 and 60 °C. The critical temperature decreased from ~20 to ~30 oC with increasing F-127-DMA content, reflecting the decrease in hydrophobicity (PPO content). While the emulsion templating accelerated the rate of uptake, compared to the FA-0-Y, through
29
ACCEPTED MANUSCRIPT capillary action, the presence of the more hydrophobic PPO on the surface reduced the rate of uptake compared to AM-80-0.
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Supporting Information Figures S1-S5: FTIR spectra (for AM-80-0, FAM-80-20, and FA-80-20); void diameter distributions (for the FA-X-Y, for the FAM-80-Y with AM-80-0); the
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SAXS spectrum from FA-75-20 showing 50 nm spacing for the F-127-DMA’s
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microphase separated PPO midblocks; “chalk” lines drawn using the friable AM-80-0 and the non-friable FAM-80-26.
Acknowledgements
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The authors gratefully acknowledge the partial support of the Israel Science Foundation (294/12 and 519/16) and the Israel Ministry of Science (880011). T. Zhang was partially supported at the Technion by an Israel Council for Higher
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Education Fellowship. The authors thank Dr. Rafael Khalfin for his kind help with
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the SAXS characterizations.
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HIGHLIGHTS Reactive block copolymer (R-BC): methacrylate endcapped PEO-PPO-PEO
•
R-BC: stabilize oil-in-water high internal phase emulsions with acrylamide (AAm)
•
Synthesized: R-BC-crosslinked, highly porous, emulsion-templated, AAm-based
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•
hydrogels
Mechanical properties: robust compressive behaviors with no chalkiness
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Water uptake: amplified and accelerated with enhanced temperature sensitivity
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•