Microphase-Separated Macroporous Polymers from an Emulsion

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Microphase-Separated Macroporous Polymers from an EmulsionTemplated Reactive Triblock Copolymer Tao Zhang* and Michael S. Silverstein* Department of Materials Science and Engineering, Technion−Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *

ABSTRACT: A polyHIPE (PH) is a macroporous polymer synthesized within a high internal phase emulsion (HIPE). The high-porosity, highly interconnected, open-cell structures and the PH-swelling-driven void expansion mechanism have made PH systems of interest for liquid absorption. Usually, only hydrophobic liquids or only hydrophilic liquids are considered for absorption applications. Here, PHs that are based solely on a reactive triblock copolymer (consisting of methacrylate-terminated poly(ethylene oxide) (PEO) end blocks and a poly(propylene oxide) midblock) were synthesized within oil-in-water HIPEs that are bereft of conventional monomers, cross-linking comonomers, and surfactants. The PH walls consisted of microphaseseparated structures with crystalline PEO domains, a microstructure that is difficult to achieve in PHs. These PHs absorbed both hydrophobic liquids and hydrophilic liquids, with the PH-swelling-driven void expansion amplifying the absorption. For water absorption, the PH structure enhanced the temperature sensitivity (decreasing absorption with increasing temperature) and accelerated the response rate.

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linkages.29 Amphiphilic BCs, containing both hydrophilic and hydrophobic blocks, are advantageous for self-assembly applications and act as surfactants.30 Amphiphilic BCs have been used to stabilize HIPEs,5,31−33 with the most common BCs being PEO−PPO−PEO triblock copolymers consisting of poly(ethylene oxide) (PEO) end blocks and a poly(propylene oxide) (PPO) midblock.5,19 These BCs do not react during radical polymerization and can be removed by extraction following PH synthesis. Very recently, reactive BCs have been developed for HIPE stabilization. Such surfactants are incorporated into the PH’s macromolecular structure, and since they are located at the oil− water interface, they are found on the void surfaces.31 These BCs, anchored on the surface, produced a three-dimensional modification of the PH surface. 31 Reactive BCs that incorporated a chain transfer agent (CTA) or a polymerization controlling agent were also used for HIPE stabilization.34−36 These reactive BC surfactants were covalently incorporated into the PH’s macromolecular structure via chain-transfer or initiation reactions. PHs with covalently bound reactive BC surfactants can overcome some of the drawbacks that are associated with PHs synthesized within HIPEs that are stabilized using traditional surfactants. The drawbacks of using traditional surfactants include the additional processing steps needed for their removal, the high costs associated with the surfactants and with their removal, the environmental challenges posed by removing large amounts of surfactant, and

INTRODUCTION A polyHIPE (PH) is a macroporous polymer synthesized within the continuous external phase of a high internal phase emulsion (HIPE), an emulsion with over 74 vol % dispersed internal phase.1−3 PHs have drawn considerable attention from both academia and industry, with potential applications in absorption,4−6 adsorption,5,7 controlled release,8 tissue engineering,9,10 chromatography,11 and fire retardancy.12 For all these applications, the uptake of liquids is an integral component. Enhancing the hydrophilicity of hydrophobic PHs, synthesized in water-in-oil (w/o) HIPEs, can be achieved through surface modification13 or through the generation of more complex macromolecular structures such as bicontinuous hydrophilic−hydrophobic PHs.14,15 Conventional hydrogel PHs (HG-PHs), usually synthesized within oil-in-water (o/w) HIPEs, preferentially absorb hydrophilic liquids. Previous work on HG-PHs has demonstrated that both the extent of crosslinking and the macromolecular structure can affect the water uptake.16 The water uptake in HG-PHs based on an anionic comonomer has been shown to increase with increasing pH.17 Recent advances in the synthesis of PHs within o/w HIPEs include the synthesis of complex macromolecular structures including double networks,18 doubly cross-linked networks,19 and polymers from deep eutectics.20 HIPEs are traditionally stabilized using nonionic surfactants. Recent work shows that HIPEs can also be stabilized using inorganic nanoparticles (NPs),21−23 organic NPs,24,25 and miktoarm stars,26 and these are termed Pickering HIPEs. Moreover, the stabilization strategy has been shown to be an integral factor in determining the properties of the PHs.27,28 Block copolymers (BCs) consist of at least two chemically different polymer blocks that are connected by covalent © XXXX American Chemical Society

Received: January 29, 2018 Revised: May 1, 2018

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Scheme 1. (a) A Scheme Illustrating the Synthesis of a PH through the Polymerization of a Reactive BC in an O/W Emulsion; (b) a Typical Porous Structure (SEM); (c) a Comparison of a Dry PH Sample (PF-85) with PH Samples That Underwent Equilibrium Swelling in a Liquid (Water, Ethanol, Toluene, or Dichloromethane (DCM))

127 (25.2 g, 2 mmol) and triethylamine (2.0 mL, 12 mmol) were dissolved in DCM (200 mL) and cooled to 0 °C in an ice water bath. Methacryloyl chloride (1.4 mL, 12 mmol) in DCM (50 mL) was added dropwise to the F-127 solution under stirring. Stirring was continued for 12 h at room temperature after adding the methacryloyl chloride. The reaction mixture was subsequently washed with H2O (75 mL), an NaHCO3 aqueous solution (5%, 50 mL), and H2O (75 mL). After drying with anhydrous Na2SO4, the DCM solution was concentrated to 75 mL by evaporation and then precipitated into an excess of diethyl ether to obtain F-127-DMA. Finally, the F-127-DMA was dried under vacuum at room temperature. The solubility of F-127DMA was evaluated by producing 15 wt % solutions in water, toluene, or DCM. Synthesis of PF-X. The HIPE recipes are listed in Table 1. The resultant PHs are denoted PF-X, where X represents the volume

the complications posed by the leaching of residual surfactant during use.37 Previous work on BC-based, emulsion-templated porous polymers showed that emulsion-templated porous polymers could be fabricated from a BC alone.38−40 These porous polymers, however, were relatively fragile, as indicated by the partial collapse of the monolithic, emulsion-templated, porous structures. The fragility of these porous polymers can be ascribed to the absence of covalent bonds between the BC chains. Introducing polymerizable groups to such amphiphilic BCs and polymerizing these groups to cross-link the BCstabilized HIPEs may enable the formation of mechanically robust PHs from the BC alone. This novel approach to PH synthesis can be used to produce PHs with amphiphilic uptakes that could be advantageous for absorption applications. Here, a series of BC-based PHs were synthesized and their amphiphilic uptakes were investigated. The PHs were synthesized within o/w HIPEs stabilized with a reactive triblock copolymer, F-127 dimethacrylate (F-127-DMA) (Scheme 1a), that was based on Pluronic F-127, a PEO− PPO−PEO BC with hydroxyl end groups. The resulting PHs exhibited highly semicrystalline, microphase-separated, polymer walls; highly interconnected, macroporous structures (scanning electron micrograph (SEM) in Scheme 1b); and amphiphilic uptakes (Scheme 1c). The amphiphilic uptakes, tunable by varying the fraction of the dispersed phase, were amplified, and the rates of uptake were accelerated, through emulsion templating. In addition, the temperature sensitivity of the water absorption was enhanced through emulsion templating.

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Table 1. PF-X Recipes external, aqueous phase (vol %) F-127-DMA H2O APS total internal, organic phase (vol %) cyclohexane TEMED (catalyst added after HIPE formation) (vol %)

PF-75

PF-80

PF-85

3.75 21.25 0.04 25.04

3.00 17.00 0.03 20.03

2.25 12.75 0.03 15.03

74.89

79.91

84.91

0.07

0.06

0.06

fraction of the dispersed phase. The typical procedures for the preparation of the PF-X from 50 mL of the corresponding HIPE are described below. Cyclohexane was added dropwise to an aqueous solution of APS and F-127-DMA, under constant stirring with an overhead stirrer at 400 rpm. To enhance HIPE homogeneity, the stirring was continued for another 2 min after the addition of all the cyclohexane. The stirring speed was then reduced to 80 rpm, the TEMED catalyst was added, and the stirring was continued for an additional 3 min. The exposure of these o/w HIPEs to air during polymerization was limited by carefully placing a second, almost identical, o/w HIPE above the PF-X HIPE (the nonpolymerizable F-127 was substituted for the F-127-DMA). The polymerization of F-127-DMA was carried out in a convection oven at 40 °C for 16 h. Following polymerization, the nonpolymerizable HIPE above the resulting PF-X was removed.

EXPERIMENTAL SECTION

Synthesis. Materials. Pluronic F-127, triethylamine (TEA), methacryloyl chloride, NaHCO 3 , and N,N,N′,N’-tetramethylethylenediamine (TEMED) were purchased from Sigma-Aldrich, ammonium persulfate (APS) was purchased from Fluka, and all the chemicals were used as received. The other reagents such as ethanol, diethyl ether, cyclohexane, toluene, and DCM were from Biolab and were analytical grade. Deionized water was used throughout all the experiments. Synthesis of F-127-DMA. F-127-DMA was synthesized from F-127 by an end-capping reaction with methacryloyl chloride in DCM according to a method reported previously.41 Typically, Pluronic FB

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Macromolecules The PHs were frozen in a −25 °C freezer for 3 h, were cut into cubes (about 1 cm × 1 cm × 1 cm), and were purified using Soxhlet extraction (first with ethanol and then with diethyl ether, each for 24 h). Following extraction, the PHs were placed into a beaker, were frozen (−25 °C for 3 h), and were then dried in a vacuum oven at room temperature for 24 h (the beaker was wrapped in a paper towel to reduce the heat transfer rate). Synthesis of R-15. A reference polymer, R-15, was prepared by polymerizing an aqueous solution containing 15 wt % F-127-DMA and 0.15 wt % APS (identical to the aqueous phase contents of the HIPEs) in a convection oven at 40 °C for 16 h. The resulting reference hydrogel was immersed in water (the hydrogel:water ratio was 1:50) for 4 days, changing the water every day. R-15 was then dried in a vacuum oven at room temperature for 24 h to obtain a monolith with a density of 0.80 g cm−3. Characterization. Gel Contents, Density, and Macroporous Structure. The gel contents of the PF-X, based on the F-127-DMA content in the HIPE, were determined using a mass balance following Soxhlet extraction, and the densities were determined gravimetrically. The porous structures of the PF-X were observed using SEM at 10 kV (FEI Quanta 200). Cryogenic fracture surfaces, obtained by immersing the PF-X in liquid nitrogen before fracture, were coated with a thin gold−palladium layer. The average void diameters and the average interconnecting hole diameters were determined by measuring at least 100 of each. The average of 100 void diameters was multiplied by 2/ (31/2) to correct for the statistical nature of the section.42 Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were performed under vacuum using a SAXS/WAXS system (Molecular Metrology Instrument) equipped with a sealed microfocus tube (MicroMax-200+S, with an incident wavelength of 0.1542 nm). Calorimetric measurements were carried out using differential scanning calorimetry (DSC) in nitrogen (Mettler DSC-821e). About 10 mg of F-127, F-127-DMA or the PF-X was put in a DSC pan. The samples were quenched to −75 °C and subsequently heated to 100 °C to detect the melting temperature of the as-synthesized polymers (first heating). After a 5 min isotherm at 100 °C, the samples were cooled to −75 °C (first cooling). The heating and cooling were carried out at 10 °C min−1. The melting temperatures and the crystallization temperatures were determined from the maximum of the endothermic peak and the minimum of the exothermic peak, respectively. The crystallinities of the PEO blocks (Xc‑PEO) within F-127, F-127-DMA, and PF-X were determined from the peak area, dividing by both the fraction of PEO, f PEO (calculated from the molecular structure), and by the ΔHm of perfectly crystalline PEO (205 J gPEO−1).43 According to their chemical structures, the PEO fraction within F-127 is 70 wt %, while the PEO fraction within F-127-DMA is 69 wt %. Liquid Uptake. The ambient temperature equilibrium liquid uptakes in the PF-X and R-15 were determined gravimetrically. Typically, a sample cube (about 0.5 cm × 0.5 cm × 0.5 cm) with a known dry mass (Wd) was placed in a vial containing a liquid (deionized water, ethanol, toluene, or DCM) until fully swollen (up to 24 h), and then the mass of the swollen sample (Wsw) was determined. The equilibrium mass uptake (Um∞), calculated using eq 1, was an average of at least three cubes. The room temperature mass uptake as a function of time (Um(t)) was normalized by Um∞ to yield Nm(t) (eq 2).

Um∞ = (Wsw − Wd)/Wd

similar to that described above. The water temperature was controlled using a water bath (Thermo Neslab RTE-110) and verified using a thermometer. The normalized mass-based water uptake as a function of time in cold water or in hot water was used to compare the response rates of the PF-X and R-15. The first step was swelling PF-X and R-15 cubes to equilibrium in water at 10 °C and measuring the “first step” uptake (U10 fs). The cubes were then immersed in water at 45 °C to determine the variation of the normalized uptake during heating to 45 °C (N45(t)) (eq 4) from the uptake measured on immersion in water at 45 °C (U45(t)).

N45(t ) = U45(t )/U10 fs

(4)

The normalized mass-based water uptake on cooling to 10 °C as a function of time, N10(t) in eq 5, was determined after the cubes reached equilibrium in water at 45 °C. The cubes were then immersed in water at 10 °C to determining N10(t) from the uptake measured during the experiment (U10(t)).

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N10(t ) = U10(t )/U10 fs

(5)

RESULTS AND DISCUSSION F-127-DMA: Structure and Properties. The reactive triblock copolymer F-127-DMA was successfully synthesized by end-capping the hydroxyl end groups of Pluronic F-127. The chemical structure of F-127-DMA, verified by 1H NMR (Figure S1), contained over 91% methacrylation. The solutions of 15 wt % F-127-DMA in water, toluene, or dichloromethane were all transparent. F-127-DMA was able to successfully stabilize cyclohexane-in-water HIPEs containing 75−85 vol % cyclohexane. These HIPEs exhibited relatively high stability, with no phase separation observed over 2 weeks. Concentrations of F127-DMA in water that were larger than 15 wt % led to gel formation during the initial stage of cyclohexane addition, with the cyclohexane-swollen F-127-DMA micelles forming an entangled network.45 PF-X: Porous Structure and Density. The PF-X PHs, with F-127-DMA as the only monomer, were synthesized successfully. The gel contents were around 91 wt %, indicating that the PHs were highly cross-linked, as expected from a polymerized dimethacrylate. The densities of the PF-X varied from 0.152 to 0.217 g cm−3 (Figure 1), decreasing with increasing dispersed phase content. The PF-X densities are significantly higher than those estimated from the HIPE recipes, reflecting some shrinkage during drying. Conceptually,

(1)

Nm(t ) = Um(t )/Um∞

(2) −1

The equilibrium volumetric uptake (Uv∞, in mL g ), used to produce a more representative comparison of liquids with different densities, was calculated using eq 3 and the known density of the swelling liquid (ρl) (1.00 g cm−3 for water, 0.816 g cm−3 for ethanol, 0.866 g cm−3 for toluene, and 1.235 g cm−3 for DCM).44 Uv∞ = Um∞/ρl

(3)

Water Uptakes at Various Temperatures. The water uptakes of the PF-X and the R-15 were also determined as a function of temperature, from 5 to 50 °C in increments of 5 °C, using a method

Figure 1. Average void diameters, average interconnecting hole diameters, and densities of the PF-X as a function of the HIPE’s dispersed phase content. C

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Figure 2. Porous structures (SEM) of (a) PF-75, (b) PF-80, and (c) PF-85.

the PF-X are very different from PHs synthesized within HIPEs that contain surfactants, monomers, and cross-linking comonomers.1 Here, F-127-DMA served as the surfactant, as the monomer, and as the cross-linking comonomer. All the PF-X exhibited highly interconnected macroporous structures (Figure 2). The average void diameters ranged from 8.0 to 18.0 μm, and the average interconnecting hole diameters ranged from 1.8 to 5.8 μm (Figure 2) (the void diameter distributions are shown in Figure S2). The average void diameters, the breadths of the distributions (the full widths at half-maximum), and the average interconnecting hole diameters all increased with increasing dispersed phase content. The ratio of the average interconnecting hole diameter to the average void diameter ranged from 0.2 to 0.5, typical of surfactantstabilized PHs.46,47 Microphase Separation and Crystallinity. The SAXS results from the PF-X in Figure 3a all exhibit a single broad

Figure 4. DSC thermograms of F-127, F-127-DMA, and the PF-X: (a) first heating and (b) first cooling.

blocks in the as-synthesized PF-X exhibited melting points (Tm) ranging from 48.9 to 53.2 °C. The melting peaks were broad in comparison with those of F-127 and F-127-DMA, which exhibited Tm of 62.8 and 59.0 °C, respectively. The PF-X PEO blocks exhibited crystallization peaks between 20.0 and 29.1 °C that were lower and broader than those of F-127 and F127-DMA, with peaks at 33.5 and 31.6 °C, respectively. The melting temperatures and heats of fusion (ΔHm) from the DSC thermograms for F-127, as synthesized F-127-DMA, and as-synthesized PF-X are summarized in Table 2. The PEO Table 2. Tm, ΔHm, and Xc‑PEO of the PEO Blocks in F-127, As-Synthesized F-127-DMA, and As-Synthesized PF-X Figure 3. PF-X: (a) SAXS profiles and (b) WAXS profiles. F-127 F-127DMA PF-75 PF-80 PF-85

peak that indicates the presence of microphase-separated PEO blocks and PPO blocks. The average distance between neighboring microdomains, based on the first-order scattering peak, was 12.9 nm. Interestingly, the microphase separation within the PH walls was not affected by the internal phase content. The relatively broad SAXS peaks indicate that the microphase-separated structures within the PH walls were not ordered. The crystalline nature of the microphase-separated structures within the PH walls is indicated by the narrow WAXS peaks in Figure 3b. The PF-X crystallinity can be ascribed to the PEO blocks, and the similarities in the spectra indicate that the crystal structure was unaffected by the internal phase content. The crystalline nature of the PEO blocks was confirmed through thermal analysis. The first heatings of F-127, F-127DMA, and the as-synthesized PF-X are presented in Figure 4a, and the corresponding first coolings are in Figure 4b. The PEO

Tm (°C)

ΔHm (J gsample−1)

f PEO

ΔHm (J gPEO−1)

Xc‑PEO (wt %)

60.6 57.1

−120.7 −109.9

0.70 0.69

−172.4 −159.3

84.1 77.7

48.2 50.0 53.7

−63.7 −73.2 −71.2

0.69 0.69 0.69

−92.3 −106.1 −103.2

45.0 51.7 50.3

crystallinities in the PF-X, which varied from 45.0 to 51.7 wt %, were significantly smaller than those in F-127 and F-127-DMA, 84.1 and 77.7 wt %, respectively. These trends were identical for Tc and ΔHc (Table S1). The reduction and broadening of the melting and crystallization peaks, and the reductions in Tm and crystallinity, reflect the limitations on macromolecular mobility imposed by the reaction of the chain ends and the formation of cross-linked networks. Amphiphilic Uptake. The PF-X were able to absorb both hydrophilic solvents (water and ethanol) and hydrophobic solvents (toluene, DCM), as seen for the volumetric uptakes in D

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Figure 5. (a) Equilibrium volumetric uptakes of the various liquids in the PF-X and in R-15. (b) Normalized uptakes of the various liquids in PF-85 as a function of time.

Figure 6. (a) Temperature response of the water uptakes in the PF-X and in R-15. (b) Water uptake cycles in PF-85: cooling from 50 to 5 °C and then heating from 5 to 50 °C.

Figure 5a. The uptakes in the PF-X increased with the internal phase content, with PF-85 exhibiting the highest uptakes (46.0, 42.4, 57.8, and 97.1 mL g−1 of water, ethanol, toluene, and dichloromethane, respectively, with the corresponding mass uptakes shown in Figure S3). The uptakes in the PF-X were at least 4 times higher than the uptakes in the corresponding R-15 (7.8, 6.7, 11.8, and 18.9 mL g−1 of water, ethanol, toluene, and dichloromethane, respectively). The very large water uptakes in HG-PHs are usually ascribed to the uptakes within the PH walls, within the original voids, and within the volume generated by hydrogel-swelling-driven void expansion.5,12,17 Similarly, the enhanced uptakes of both hydrophilic and hydrophobic liquids in the PF-X can be ascribed to the uptake in the original voids, in the walls, and in the volume generated by PH-swelling-driven void expansion. The ability to absorb relatively large amounts of both hydrophilic and hydrophobic liquids makes the PF-X very different from conventional HGPHs, which tend to preferentially absorb hydrophilic liquids.4,17 The unique absorptions seen here for the PF-X reflect the presence of both the more hydrophilic PEO blocks and the more hydrophobic PPO blocks.

The normalized uptakes of water, ethanol, toluene, and DCM in Figure 5b (log−log plots of N(t)) are quite rapid, with PF-85 reaching its equilibrium uptakes in about 2 h. The exact time at which the N(t) began to increase rapidly was strongly dependent upon the nature of the liquid. The rapid increases in N(t) for the hydrophobic solvents (toluene, DCM) began at around 15 min, and those of the hydrophilic solvents (water, ethanol) began at around 30 min. The higher uptakes in the hydrophobic solvents in Figure 5a reflect the relatively high room temperature solubilities of the polymers in both toluene and DCM, whose solubility parameters are quite similar to those of PEO and PPO. The crystalline PEO domains act as physical cross-links, limiting the macromolecular mobility and swelling. The hydrophobic solvents can dissolve the crystalline domains more rapidly, enhancing the macromolecular mobility and, thus, the rates of uptake, which are about twice those of the hydrophilic solvents (Figure 5b). Interestingly, a transient maximum water uptake was reached before the equilibrium value, a phenomenon that has been observed in other porous hydrogels.48 The uptakes, from around 5 min to a time that corresponds to 60% of the E

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in water at 10 °C from equilibrium at 45 °C (Figure 7b). PF-80 and PF-85 reached their new equilibrium water uptakes in 20 and 10 min, respectively. In comparison, after 1 h, PF-75 and R15 only reached 90 and 80% of their equilibrium uptakes, respectively. The new equilibrium uptakes in Figure 7b, that were reached after cooling in water at 10 °C (from the previous equilibrium at 45 °C), were similar to the “first step” uptakes at 10 °C (Figure 7a). This temperature cyclability is similar to that seen in Figure 6b. A comparison of the meaning behind the curves in Figure 7b for PF-75 and R-15 is enlightening. The equilibrium absorptions at 10 °C for PF-75 and R-15 (Figure 6a) are 53.0 and 11.8 g g−1, respectively. The normalized uptake following 60 min at 45 °C (60 min in Figure 7a and 0 min in Figure 7b) are 37.5% (19.9 g g−1) and 60.5% (7.1 g g−1) for PF75 and R-15, respectively. Therefore, the actual uptake in PF-75 at 45 °C was almost thrice that in R-15 (in contrast to the lower normalized uptake). Following 60 min at 45 °C (Figure 7b), the uptake in PF-75 was 48.5 g g−1 (an average rate of 0.477 g (g min)−1) and the uptake was continuing to increase, while the uptake in R-15 seemed to be reaching an asymptote at 9.7 g g−1 (an average rate of 0.043 g (g min)−1, less than 10% of the average rate for PF-75).

equilibrium uptake, can be described using a semiempirical power-law equation N(t) = ktn, where k is a constant and n is the power-law exponent. For a thin, rectangular sample, n = 0.5 represents Fickian diffusion, n = 1 represents case II diffusion, and anything between is termed anomalous diffusion.49 Based on the maximum slopes in Figure 5b, which are around 0.6, the transport mechanism within the PF-85 was anomalous (but strongly diffusive). Amplified Temperature Response. The water uptake in PF-X is strongly temperature dependent, as seen in Figure 6a, where the PF-X uptake at 50 °C is only 33% of that at 5 °C. The water uptake decreased moderately from 5 to 15 °C, decreased much more significantly from 15 to 25 °C, and then decreased moderately from 25 to 50 °C. This temperatureresponsive behavior is seen for all the PF-X. Both the water uptake at 5 °C and the water release on heating to 50 °C increased with increasing internal phase content. The temperature-responsive water uptakes in the PF-X originate in the temperature response of F-127.41 R-15 exhibited a temperature response, with the uptake at 50 °C being 50% of that at 5 °C. The significantly higher temperature sensitivity of the PF-X indicates that emulsion templating amplifies the inherent F-127 temperature response. The PF-X represent a novel type of highly thermoresponsive macroporous hydrogel with unique macromolecular structures whose properties, such as the responsive temperature range, are interestingly similar to those of conventional N-isopropylacrylamide-based hydrogels and could be used as an alternative system.50−52 The temperature-responsive water uptakes in the PF-X exhibited a high degree of cyclability, with relatively constant PF-85 water uptakes during six heating−cooling cycles in Figure 6b (around 95.8 g g−1 at 5 °C and 28.1 g g−1 at 50 °C). Accelerated Response Rate. On heating in water at 45 °C from equilibrium at 10 °C, the swollen PF-X reached their new equilibrium water uptakes within 5−15 min (Figure 7a), with

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CONCLUSIONS The emulsion-templated PF-X were successfully synthesized within o/w HIPEs that were stabilized using the reactive F-127DMA with no additional stabilizers, monomers, or cross-linking comonomers. The high-porosity PF-X, with densities ranging from 0.15 to 0.22 g cm−3, depending on the internal phase content, exhibited highly interconnected macroporous structures with average void diameters ranging from 8 to 18 μm and average interconnecting hole diameters ranging from 1.8 to 5.8 μm. The average void diameter, the breadth of the void diameter distribution, and the average hole diameter all increased with increasing internal phase content. The PEO and PPO in the PF-X walls underwent microphase separation, and the average 12.9 nm distance between neighboring microdomains was independent of the internal phase content. The PEO blocks in the microphase-separated structure were crystalline, with crystallinities of around 50%, melting points of around 50 °C, and crystallization temperatures of around 25 °C. The uptakes of both hydrophilic and hydrophobic liquids by the PF-X, which increased with increasing internal phase content, were significantly higher than the uptakes within the corresponding reference (polymerized F-127-DMA), reflecting the integral contribution of the PH-swelling-driven void expansion mechanism to the absorption behavior. Overall, the absorption of DCM was the highest (at around 97 mL g−1) with the absorption decreasing in the following order: toluene, water, and ethanol. Interestingly, the normalized absorption rates for the hydrophobic solvents were about twice those for the hydrophilic solvents, reflecting the greater overall solubility of the blocks in hydrophobic solvents. The water uptakes in the PF-X were strongly temperature-responsive, with the uptakes decreasing with increasing temperature and varying reproducibly over several cycles. For the PH with the highest internal phase content, the uptake varied from around 96 g g−1 at 5 °C to around 28 g g−1 at 50 °C. PH-swelling-driven void expansion not only amplified the extent of absorption, it also enhanced the sensitivity to temperature and accelerated the response rate to variations in temperature.

Figure 7. Water uptakes in the PF-X and in R-15 (normalized by the “first step” equilibrium uptake at 10 °C): (a) heating in water at 45 °C from an equilibrium at 10 °C; (b) cooling in water at 10 °C from an equilibrium at 45 °C.

the response times decreasing with increasing internal phase contents (lower polymer/water ratios). In comparison, it took R-15 over 1 h to reach its new equilibrium uptake. This difference in the time to reach equilibrium clearly demonstrates that the relatively low polymer/water ratio inherent in the emulsion-templated structure serves to accelerate the response rate. Accelerated response rates were also found during cooling F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00213. Figures S1−S3: 1H NMR spectrum of F-127-DMA, the PF-X void diameter distributions, and the mass-based comparison of solvent uptakes; Table S1: the Tc, ΔHc, and Xc‑PEO of the PEO blocks from the “first cooling” DSC thermograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.Z.). *E-mail [email protected] (M.S.S.). ORCID

Michael S. Silverstein: 0000-0002-9377-4608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 Education Fellowship. The authors thank Dr. Rafael Khalfin for his help with the SAXS and WAXS characterizations.

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