Nanoporous Polymer Networks Templated by Gemini Surfactant

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Article Cite This: Chem. Mater. 2018, 30, 185−196

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Nanoporous Polymer Networks Templated by Gemini Surfactant Lyotropic Liquid Crystals James Jennings,†,∥ Brian Green,‡ Tyler J. Mann,§ C. Allan Guymon,*,‡ and Mahesh K. Mahanthappa*,†,§ †

Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States Department of Chemical & Biochemical Engineering, University of Iowa, Iowa City, Iowa 52242-1527, United States § Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡

S Supporting Information *

ABSTRACT: Nanoporous polymers with periodic, ordered structures have attracted significant interest for their potential applications as drug delivery vehicles, biomaterials, separations membranes, and materials for energy storage. Inducing polymer nanostructure through lyotropic liquid crystal-templated (LLCtemplated) cross-linking photopolymerizations offers a promising means for morphological control at smaller length scales, which are difficult to access by other established strategies. We report the synthesis of a gemini dicarboxylate surfactant that self-assembles in water to form various aqueous LLC mesophases over a broad range of amphiphile concentrations, with an especially strong propensity to form the coveted bicontinuous double gyroid (GI) network mesophase. Aqueous GI LLCs surprisingly persist upon incorporation of as much as 10−37 wt % hexane-1,6-diol dimethacrylate (HDDMA) into the hydrophobic domains of these supramolecular surfactant assemblies, and cross-linking photopolymerization of the HDDMA unexpectedly proceeds with retention of this intricate LLC nanostructure. The nanoporous nature of the resulting templated polymers remains after surfactant removal by solvent extraction, as manifested by increased swelling ratios in water and 2-propanol as compared to isotropic materials of similar compositions. The exquisite level of control over polymer network porosity provided by templating within GI phases furnishes a promising new route toward nanostructured hydrophobic polymers.

1. INTRODUCTION Materials with well-defined, functional, and percolating nanopores are highly sought after as heterogeneous catalysts and selective separations membranes,1 therapeutic drug delivery vehicles,2 and tissue scaffolds for regenerative medicine.3 While mesoporous silicates, aluminates, zeolites, and metal−organic frameworks (MOFs) are useful media with pores of tunable shapes and sizes ranging from ∼0.4 to 5 nm in diameter,4,5 their brittle mechanical properties render them difficult to process into useful membrane formats for selective gas separations, selective ion transport in fuel cells and batteries, and water purification.6,7 However, nanoporous polymeric materials offer exciting opportunities to address these important needs for new materials, with the added benefits of their scalable processability into thin film geometries with finely tailored morphologies and tunable mechanical properties. One popular strategy for the design and synthesis of nanoporous polymeric materials with variable degrees of periodic long-range order relies on the self-assembly of block polymers.8 Controlled/living polymerizations afford access to a variety of functional multiblock polymers, that thermodynamically self-assemble into well-defined morphologies (e.g., © 2017 American Chemical Society

hexagonally packed cylinders, periodic and disorganized networks, and lamellae) at the length scales of their constituent polymer chains (∼5−100 nm).9 By incorporating degradable segments into such multiblock polymers, various groups have realized the synthesis of nanoporous polymer monoliths10 and thin films with potential applications as selective separations membranes11,12 and nanolithography templates for advanced microelectronic device manufacture.13−16 In such cases, the pore dimensions of these materials are thermodynamically limited by the monomer chemistries in the initial block polymer precursor.17 This strategy is further limited by the narrow composition phase windows associated with bicontinuous block polymer network phases with three-dimensionally percolating domain structures, which lead to porous polymers with the highest surface areas and nanoporosities.18 Finally, the chemical and physical properties of materials arising from this strategy depend sensitively on the constituent monomers. Received: October 4, 2017 Revised: December 1, 2017 Published: December 22, 2017 185

DOI: 10.1021/acs.chemmater.7b04183 Chem. Mater. 2018, 30, 185−196

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physical properties.30,35 To the best of our knowledge, no reports have successfully realized the direct LLC-templated synthesis of nanoporous cross-linked networks of hydrophobic monomers within the coveted Q phase mesostructures, which are expected to exhibit the highest surface areas with pores that percolate in three dimensions. In this paper, we describe a new surfactant molecule that enables the direct LLC templating of cross-linked nanoporous polymers with Q phase mesostructures with 3D percolating pore structures. Our strategy builds on insights from recent experimental studies of the LLC phase behaviors of small molecule gemini dicarboxylate amphiphiles, which form aqueous LLCs with normal bicontinuous QI morphologies over wide amphiphile concentration windows. 36,37 We demonstrate that these aqueous LLCs may be swelled with the hydrophobic hexane-1,6,-diol dimethacrylate (HDDMA) monomer to yield Lα, HI, and QI mesophases. Remarkably, photoinitiated polymerizations of these difunctional monomers that are localized within the hydrophobic domains of these LLCs yield cross-linked polymers with QI network mesostructures, as assessed by small- and wide-angle X-ray scattering (SWAXS). Subsequent removal of the ionic surfactant template by solvent extraction yields nanoporous polymers, the swelling behaviors and mechanical properties of which are examined in detail. As compared to control polymers synthesized in isotropic media, these studies reveal that this new gemini dicarboxylate surfactant template affords fine control over the final polymer nanostructures that give rise to their unique bulk properties.

An alternative approach for synthesizing nanoporous polymeric materials relies on the concentration-dependent self-assembly of amphiphilic molecules in selective solvents (e.g., water) to form spatially periodic noncovalent assemblies known as lyotropic liquid crystals (LLCs).2 Comprising hydrophobic tails covalently bonded to hydrophilic headgroups, small molecule surfactants self-assemble in water into a plethora of morphologies, such as periodic discontinuous micelle packings (I), hexagonally packed cylinders (H), bicontinuous network (Q) phases, and lamellae (Lα).19 These phases are classified as water-rich or “normal” (Type I) phases in which water comprises the matrix phase, or water-poor or “inverse” (Type II) phases. Various researchers have reported the successful syntheses of nanoporous polymers by the selfassembly of polymerizable surfactants into LLCs and their subsequent polymerizations to covalently fix these structures.20−22 While this approach enables access to polymers with periodic arrays of nanopores having diameters ∼0.7−3 nm, the demanding multistep syntheses of polymerizable surfactants that reliably form H, Q, or Lα phases that lead to nanoporous cross-linked materials tends to limit its practical utility.23,24 Furthermore, cross-linking surfactant amphiphiles often induces subtle changes in their intermolecular packings, which trigger polymerization-driven LLC phase transitions that reduce the measured porosities of the final polymer monoliths.23,25 LLC-templated polymerizations have emerged as a versatile and attractive alternative strategy for producing nanoporous polymeric materials from relatively inexpensive starting materials.26,27 This methodology relies on spatially localizing monomers within either the hydrophilic or the hydrophobic domains of a small molecule surfactant LLC and inducing their polymerization.28 Various successful syntheses of nanoporous hydrophilic polymers have been achieved by this strategy through the careful selection of Type I LLC templates that allowed for their loading with cross-linkable monomers and their subsequent photopolymerization to yield anisotropic structures.29−31 Surfactant templates, exemplified by commercially available alkyltrimethylammonium bromide and nonionic alkyl ethoxylate (CiEj or “Brij”) surfactants, enable the LLCtemplated, cross-linking polymerization of various hydrophilic monomers such as bis(acrylamides) and oligo(ethylene glycol) dimethacrylates potentially using a single template molecule.30 Rapid photopolymerization within these LLC templates crucially minimizes polymerization-induced phase separation between the growing polymer network and its template to yield high-porosity materials.32 Furthermore, polymerizations that proceed more rapidly than the time scale of surfactant LLC reorganization obviate polymerization-induced phase transitions, enabling syntheses of hydrophilic materials whose final structures more faithfully reflect those of the original mesostructured templates. These nanoporous materials may be used with the surfactant template remaining, or the template may be removed by subsequent solvent extractions. Hydrogels derived from normal II, HI, QI, and Lα templates, in which hydrophilic monomer polymerization is initiated in the aqueous matrix domains, exhibit significantly enhanced mechanical properties, surface areas, water diffusivities and swellabilities, permeabilities, and stimulus response as compared to their bulk cross-linked analogs.29,33,34 However, related reports of crosslinking polymerizations of hydrophobic monomers in normal HI and Lα LLC phases have yielded only modest successes toward the synthesis of nanoporous polymers with comparable

2. EXPERIMENTAL SECTION Materials. All materials were purchased from the Sigma-Aldrich Chemical Co. (Milwaukee, WI) and used as received unless stated otherwise. Diisopropylamine and hexamethylphosphoramide (HMPA) were distilled from CaH2 and stored under N2(g). Anaerobic and anhydrous THF was obtained by sparging with N2(g) for 30 min, prior to cycling through a column of activated molecular sieves in a Vacuum Atmospheres, Co., (Hawthorne, CA) solvent purification system for 12 h. n-Butyllithium (2.685 M in hexanes) was titrated against diphenylacetic acid in anhydrous and anaerobic THF. 1,3-Dibromopropane was purified by distillation and stored away from light to avoid spurious decomposition. (CH3)4NOH(aq) was titrated against a standardized 1 N HCl(aq) solution to yield [HO−] = 0.985 M. Deionized water (>18 MΩ resistance) was freshly obtained from a Thermo Scientific Barnstead NANOpure system. Molecular Characterization. 1H and 13C NMR spectra were obtained in CD3OD at 22 °C using a Bruker Avance 400 or Varian INOVA 500 spectrometer. All spectra were referenced to the residual protiated solvent peaks in the samples. Elemental analyses (C, H, and N) were performed by Atlantic Microlab, Inc. (Norcross, GA). Attenuated total reflectance (ATR) measurements utilized a Bruker Tensor 27 FTIR spectrometer equipped with a diamond ATR stage (Pike Technologies, Madison, WI) with a spectral resolution of 2 cm−1, and the data were processed using the Opus Software version 6.5 (Bruker Optik GmbH). Synthesis of Bis(tetramethylammonium) 1,22-hexacosadiene-10,14-dicarboxylate (TMA-83u). A 500 mL two-necked round-bottom flask equipped with magnetic stir bar was charged with THF (250 mL) and diisopropylamine (26.5 mL, 188 mmol) under nitrogen. The mixture was cooled to −30 °C, and n-butyllithium (60.0 mL of 2.685 M solution in hexanes, 161 mmol) was added dropwise via addition funnel. After 30 min, a solution of 10-undecenoic acid (15.0 g, 81.5 mmol) in THF (50 mL) was added dropwise. HMPA (14.0 mL, 80.5 mmol) was then added and the cooling bath removed, in order to allow the stirred reaction to warm to 22 °C. The reaction was stirred at 22 °C until it became homogeneous and clear. The 186

DOI: 10.1021/acs.chemmater.7b04183 Chem. Mater. 2018, 30, 185−196

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Chemistry of Materials mixture was subsequently cooled to −30 °C and 1,3-dibromopropane (4.08 mL, 40.2 mmol) added dropwise. The reaction flask was then warmed to 22 °C by allowing the cooling bath to slowly warm to ambient temperature overnight. The reaction was quenched with chilled 10% w/v HCl(aq) (150 mL), and the aqueous and organic layers were separated. The aqueous layer was extracted with Et2O (3 × 150 mL), and the combined organic fractions were washed sequentially with 10% w/v HCl(aq) (3 × 100 mL), water (150 mL), and saturated NaCl(aq) (150 mL). After drying over MgSO4(s), the organic layer was filtered, and all volatiles were removed in vacuo. The resulting solid was purified by recrystallization from EtOH and dried under vacuum [yield of 1,22-hexacosadiene-10,14-dicarboxylic acid: 9.98 g (60%)]. The purified gemini dicarboxylic acid (8.56 g, 21.0 mmol) was dissolved in CH3OH (75 mL), and (CH3)4NOH (42.6 mL of 0.985 M solution in H2O, 42.0 mmol) was added dropwise at 22 °C. The reaction mixture was allowed to stir overnight. After filtration through a filter paper (Whatman grade 2), CH3OH was removed in vacuo. The product was azeotropically “freeze-dried” by suspending it in C6H6, freezing the solution in liquid nitrogen, and subliming the C6H6 under high vacuum. After four freeze-drying cycles, a pale yellow solid was obtained in quantitative yield. The solid was stored under argon in a glovebox to prevent atmospheric moisture absorption. 1 H NMR (400 MHz, CD3OD): δ 3.21 (d, J = 1.0 Hz, 6H), 2.16 (dd, J = 8.5, 6.8 Hz, 1H), 1.61 (p, J = 7.1 Hz, 1H), 1.32 (q, J = 4.7, 3.2 Hz, 6H), 0.98−0.81 (m, 1H). 13C NMR (126 MHz, MeOD): δ 183.78 (CO), 138.82 (CC), 113.23 (CC), 54.45 (t, N(CH3)4), 49.41 (d, CH), 33.69 (CH2), 33.53 (CH2), 33.17 (CH2), 29.67 (CH2), 29.26 (CH2), 28.87 (CH2), 28.77 (CH2), 27.78 (d, CH2), 26.06 (CH2). Anal. Calcd for C33H66N2O4·3.8H2O: C, 63.59; H, 11.90; N, 4.49. Found: C, 63.46; H, 11.89; N, 4.48. Since the surfactant TMA-83u is hydrated by an average of 3.8 H2O molecules, all subsequent text references to the weight fraction of TMA-83u in a lyotropic liquid crystal composition refer specifically to this hydrated form. Preparation of Aqueous Lyotropic Liquid Crystals (LLCs). TMA-83u was weighed into a dry vial under an argon atmosphere in a glovebox. Outside of the glovebox, the desired amount of ultrapure water was added to the solid, and the mixture was homogenized by three cycles of high-speed centrifugation (at 4996g for 10 min) and hand mixing. The sample vials were sealed with Parafilm to avoid dehydration and the LLCs allowed to rest for at least 12 h prior to SAXS analysis. Preparation of Polymerizable LLC Assemblies. TMA-83u was weighed into an oven-dried amber vial under argon in a glovebox, prior to the addition of measured amounts of hexane-1,6-diol dimethacrylate (HDDMA, ≥90%) and the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%) in air. In all cases, DMPA comprised ∼1 wt % of the total polymerizable mixture. A homogeneous mixture of these three components was formed by their suspension in C6H6 as a cosolvent, freezing the solution in liquid nitrogen, and subliming the C6H6 under vacuum. This TMA-83u/HDDMA/DMPA mixture was then hydrated with a measured amount of ultrapure water, and it was homogenized by three cycles of high-speed centrifugation (at 4996g for 10 min) and hand mixing. Isotropic control polymer samples (ISO) under “isotropic” reaction conditions (in the absence of any supramolecular LLC structure) were prepared as above, except that CH3OH was used in place of H2O and/or TMA-83u in order to prevent LLC formation before or after polymerization. Representative LLC Photopolymerization. A polymerizable mixture (∼0.5 g) was sandwiched between two untreated glass microscope slides with a glass microscope slide spacer at each end to ensure a uniform film thickness of 1 mm. This sample thickness was chosen to minimize polymerization retardation by O2(g) inhibition.38 Samples were irradiated at 365 nm for 5 min, using a long-wave UV lamp (Blak−Ray B-100AP, Upland, CA) with an intensity of 10 mW/ cm2 at a distance of 30.5 cm. After polymerization, 8 mm diameter discs were cut from each polymer film using a biopsy punch. At least 3 polymer discs of each composition were prepared for subsequent solvent swelling and mechanical testing studies.

Small- and Wide-Angle X-ray Scattering (SWAXS). Synchrotron SWAXS analyses were conducted at the Sector 12-ID-B beamline of the Advanced Photon Source (Argonne National Laboratory, Argonne, IL) using a beam energy of 14.00 keV (λ = 0.8856 Å). Twodimensional SWAXS patterns were collected on a Pilatus 2 M detector with a sample-to-detector distance of either 2.000 or 2.028 m, calibrated with a silver behenate standard (d = 58.38 Å). LLC samples were sealed in hermetic, alodined aluminum DSC pans (TA Instruments, Eden Prairie, MN) and heated using a Linkam DSC hot-stage. Samples were equilibrated at each temperature for at least 5 min prior to X-ray exposure for 0.1−1 s. 2D-SAXS patterns thus obtained were azimuthally integrated using the DataSqueeze software package (http://www.datasqueezesoftware.com), in order to obtain one-dimensional intensity I(q) versus q (Å−1) scattering profiles. Ambient-temperature lab source SAXS analyses relied on a Bruker D8 Discover X-ray diffractometer equipped with a VANTEC-500 detector, located in the Materials Science Center at the University of WisconsinMadison. Cu Kα X-rays generated from a micro-X-ray source with Montel mirror were passed through a 0.5 mm diameter pinhole to collimate the beam. Samples were sandwiched between Kapton windows, mounted in sealed holders to minimize water loss, and exposed to X-rays for 5−10 min. 2D SAXS patterns were collected with a sample-to-detector distance of 22.3 cm as calibrated with a silver behenate standard. Temperature-dependent lab source SAXS data were collected using a Rigaku SMAX-3000 high-brilliance SAXS instrument. Cu Kα X-rays generated by a Micromax 002+ source were focused using a Max−Flux optic, and collimated using three 0.5 mm pinhole apertures. Sealed sample holders with Kapton windows were mounted in the vacuum chamber on a Linkam hot-stage, and they were allowed to equilibrate at each temperature for at least 5 min prior to data acquisition (typical exposure times were ∼5−10 min). 2D-SAXS patterns were recorded on a Gabriel X-ray detector (150 mm diameter area detector) with a sample-to-detector distance of 42.19 cm that was calibrated using a silver behenate standard sample. Surfactant Extraction from Nanoporous Polymers. The discs of each polymerized film were weighed prior to soaking in 20 mL of acetone, which was replaced every 24 h for 5 days in order to extract the surfactant from the nanopores. The acetone-swollen discs were then exhaustively dried under vacuum for 24 h, and their dry masses were determined. Comparison of the dry mass with the original amount of monomer used to prepare each disc provided an estimate of surfactant mass that was either physically or covalently trapped in the polymer. Solvent Swelling Studies. In order to determine the extent of solvent swelling by the dry polymers after surfactant removal, polymer discs were immersed into either water or 2-propanol at 22 °C and allowed to equilibrate for 72 h. The discs were then removed from the solvent bath, patted dry to remove excess solvent, and weighed. The solvent swelling ratio (W) was calculated according to the following equation:

W=

Ws − Wd Wd

where Ws is the mass of the solvent-swollen polymer disc and Wd is the mass of the dry polymer disc. Dynamic Mechanical Analysis. The compressive moduli of water-swollen and dry samples were measured using a DMA Q800 dynamic mechanical analyzer (TA Instruments, Eden Prairie, MN) equipped with a submersion compression clamp in static mode. Prior to measurement, the drive shaft position, clamp mass, clamp offset, and clamp compliance were calibrated per manufacturer protocols. The thickness and diameter of the polymer disks were measured, and the samples were placed on the compressive clamp of the instrument. The discs were compressed to approximately 90−95% of their original height, and the compressive modulus was determined by linearly fitting the slope of the resulting stress−strain (σ−ε) curve. 187

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3. RESULTS AND DISCUSSION Bicontinuous network (Q) phase structures are highly desirable for aqueous LLC-templated polymerizations because their three-dimensionally percolating pore structures yield high surface area and high-porosity materials. Commonly observed self-assembled, aqueous Q mesophases include the double gyroid (G), double diamond (D), and primitive (P) networks, which, respectively, comprise two spatially periodic and interpenetrating networks of either 3-, 4-, or 6-fold connectors embedded in a matrix component.39 The hydrophilic/hydrophobic domain interfaces of these periodic network structures significantly deviate from constant mean-curvature: They exhibit regions of both negative Gaussian (“saddle splay”) and zero (“flat”) curvature. Consequently, aqueous LLC Q phase stability sensitively depends on the chemical details of the surfactant tail and headgroup structures, which dictate their abilities to accommodate packing frustration upon hydration. Typical single-tail surfactants form stable Q phases over somewhat narrow composition phase windows that typically span ≤20 wt % at any given temperature.40−44 Reports by the groups of Gin23,24 and Mahanthappa36,37,45 have revealed that gemini (“twin tail−twin head”) surfactants46 exhibit an unusually strong propensity to undergo waterinduced self-assembly into normal double gyroid (GI) LLC phases (Figure 1). Gemini dicarboxylate amphiphiles, in which

Scheme 1. Synthesis of Gemini Surfactant TMA-83u and Structure of HDDMA

60% yield after recrystallization. Stoichiometric deprotonation of the bis(alkanoic acid) with 2 equiv of (CH3)4NOH in CH3OH and subsequent azeotropic drying yielded the TMA83u amphiphile as a solid hydrate containing ∼3.8 H2O molecules per formula unit. The (CH3)4N+ (TMA) counterion was specifically selected because of its high degree of dissociation from the carboxylate headgroups and its highly hydrophilic nature, which apparently enables facile formation of homogeneous LLCs at 22 °C (vide inf ra). Based on prior reports that demonstrated that surfactants bearing reactive functionalities enable LLC-templated polymerizations with enhanced structure retention upon polymerization,32 we focused on TMA-83u with potentially reactive terminal olefin functionalities as our initial candidate for LLC-templated polymerizations of HDDMA. We initially mapped the temperature and water concentration-dependent phase behavior of TMA-83u by interrogating the nanoscale morphologies of hydrated samples with water contents varying in 5 wt % increments (see Experimental Section for sample preparation details), using a combination of polarized light microscopy (PLM) and synchrotron small- and wide-angle X-ray scattering (SWAXS).19 We observed a progression of ordered LLC phases in the composition range 30−90 wt % TMA-83u in H2O, for which representative SWAXS patterns are given in Figure 2A (fully indexed patterns are provided in Figure S1). In accord with expectations based on previous reports,37 we found that aqueous LLCs comprising 55−85 wt % TMA-83u formed stiff, non-birefringent solids characteristic of a bicontinuous network phase LLC. SWAXS analyses revealed that these LLCs exhibit up to 17 peaks with the relative peak positions q/q* = √6, √8, √14, √16, √18, √20, √24, etc. of a double gyroid (G) morphology (space group 230 symmetry, Ia3̅d) over the temperature range T = 22−100 °C. Samples containing 90 wt % TMA-83u form a pure G phase LLC when T ≤ 80 °C, whereas Lα/G phase coexistence is observed at T = 90−100 °C (Figure 2A). The large number of sharp scattering maxima in these SWAXS patterns of pure double gyroid LLCs suggests exceptional longrange translational order in these samples, with typical unit cell dimensions a = 6.4−7.8 nm. LLCs formed in the concentration range 30−50 wt % TMA-83u are instead soft, birefringent solids that exhibit a stringy physical texture. SWAXS patterns for these low-surfactant-concentration samples typically exhibit only three sharp scattering maxima at q/q* = √1, √3, and √4, indicative of a hexagonally packed cylinders morphology with modest translational order. Further decreasing the TMA-83u concentration in these aqueous LLCs below 30 wt % results in the formation of freely flowing, isotropic solutions of disordered micelles. The water concentration-dependent

Figure 1. Water induces the self-assembly of anionic gemini dicarboxylate surfactants into normal (Type I) lyotropic liquid crystalline double gyroid (GI) phase, featuring interpenetrating and three-dimensionally percolating networks of three-fold connectors in a water matrix.

two alkanoate chains are tethered via their α-carbons through a hydrocarbyl linker, form aqueous LLC GI phases over exceptionally large amphiphile concentration windows (≥15 wt % wide) between T = 22 and 100 °C.36 Perroni et al. subsequently demonstrated that the GI phase composition windows of gemini dicarboxylate surfactants are substantially larger when the linker is either 3 or 5 carbons in length, as compared to those for surfactants with even parity linkers.37 Based on the molecular design rules discerned from these studies, we sought to exploit the exceptional thermodynamic stability of aqueous GI phase LLCs based on gemini dicarboxylate surfactants with odd parity linkers to directly template cross-linking polymerizations of spatially confined hydrophobic monomers toward nanoporous polymers with bicontinuous mesostructures. Based on previously reported syntheses of gemini dicarboxylate amphiphiles,36 we conveniently synthesized the gemini dicarboxylate TMA-83u in two synthetic steps from commercially available, biorenewable 10-undecenoic acid by the route depicted in Scheme 1. Specifically, reaction of 1,3-dibromopropane with 2 equiv of the lithium enolate of 10-undecenoic acid furnished 1,22-hexacosadiene-10,14-dicarboxylic acid in 188

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Figure 2. (A) Representative azimuthally integrated one-dimensional SAXS profiles for LLCs derived from the self-assembly of TMA-83u in ultrapure H2O, with peak markers indicating the expected positions associated with each supramolecular morphology. (B) The aqueous lyotropic liquid crystal phase diagram for TMA-83u displays a large double gyroid (GI) phase window spanning the range 55−90 wt % amphiphile in H2O, with hexagonally packed cylinders (HI) and isotropic fluid dispersions of disordered micelles (“Iso”) forming at higher water contents. “X” and “LC +X” refer to crystalline surfactant and LLC/crystal coexistence, which occur at low hydrations.

Figure 3. Representative intensity (I) versus scattering wave vector (q) SWAXS intensity profiles acquired at 22 °C for aqueous LLCs formed by ternary blends of TMA-83u, the hydrophobic monomer HDDMA, and H2O, for which (A) pure lamellar (LLC79,10,10), GI (LLC59,14,26), and HI (LLC29,25,45), as well as (B) coexisting GI + DI morphologies (LLC49,25,25 and LLC37,37,25) are observed. The markers on each plot specify expected positions of Bragg reflections characteristic of the H (▼), G (◆), D (◊) and Lα (■) LLC phases. The broad peak in panel (B) at q ∼ 4.2 nm−1 (∗) is an artifact that stems from the Kapton windows of the X-ray sample holder.

dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator at 22 °C. Samples were prepared by combining precise quantities of the surfactant TMA-83u, HDDMA, and DMPA in benzene as a cosolvent. These mixtures were subsequently frozen, and the benzene cosolvent was sublimed under high vacuum to yield finely dispersed mixtures. Addition of measured amounts of water to the latter dispersions, followed by three cycles of iterative high-speed centrifugation and hand mixing, yielded well-defined LLCs (see Experimental Section for details). Polymerizable mixtures thus formed are hereafter labeled LLCx,y,z, wherein x, y, and z denote the respective weight percentages of TMA-83u, HDDMA, and H2O. Note that x + y + z = 99 wt %, since each formulation contains 1 wt % DMPA as a polymerization photoinitiator. As a consequence of the large multicomponent phase space associated with these TMA-83u/HDDMA/H2O/DMPA mixtures, we focused our attention on mixtures containing either 10, 25, or 40 wt % HDDMA monomer with [TMA-83u] = 14−79 wt % and [H2O] = 10−46 wt %. SWAXS and PLM analyses of the LLCx,y,z polymerizable mixtures revealed the formation of two distinct lyotropic

lyotropic mesophase behavior of TMA-83 is summarized in the phase diagram given in Figure 2B. The observed phase progression, Lα → G → H → disordered micelles with increasing water content, implies that the nonlamellar phases exhibit interfacial curvature toward the hydrophobic domains, consistent with normal (Type I) phases that are hereafter denoted with the subscript “I”. Thus, the GI phase formed by TMA-83u comprises two interpenetrating, enantiomeric, hydrophobic networks of 3-fold connectors decorated with carboxylate functionalities situated in an aqueous matrix (Figure 1). Based on the remarkably large LLC GI phase window exhibited by TMA-83u, we were motivated to examine its ability to directly template the photoinitiated cross-linking polymerization of the hydrophobic monomer hexane-1,6-diol dimethacrylate (HDDMA) into a porous mesostructure. HDDMA was selected due to its commercial availability, high hydrophobicity, and low volatility that render it procedurally simple to handle. As a first step toward this ultimate goal, we executed detailed studies of the LLC phase behavior of TMA83u/HDDMA/H 2 O mixtures containing 1 wt % 2,2189

DOI: 10.1021/acs.chemmater.7b04183 Chem. Mater. 2018, 30, 185−196

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Chemistry of Materials mesophases when [HDDMA] = 10−14 wt %. Under these conditions with [H2O] ≤ 20 wt %, as in LLC79,10,10, the resulting LLCs are optically birefringent solids that exhibit a sharp primary SWAXS peak (q*) along with one weaker higher order reflection at 2q* (see Figure 3A; see Figure S2 for fully indexed patterns). These combined observations suggest formation of a Lα phase, which forms only at high temperatures in the binary TMA-83u/H2O phase diagram (Figure 2B). In the range 25 ≤ [H2O] < 40 wt % with [HDDMA] ≤ 14 wt %, we obtained stiff and optically non-birefringent LLCs samples such as LLC59,14,26 with intense SWAXS maxima located at q/ q* = √6, √8, and √14 indicative of an ordered G phase LLC (Figure 3A). Further increases in the water content of these polymerizable mixtures beyond [H2O] ≥ 40 wt % with the same [HDDMA] gave rise to low-viscosity, disordered micellar dispersions (see SWAXS data in the Supporting Information, Figure S3). The observed phase sequence again suggests that these LLCs are all normal phases. Upon increasing the monomer loading to [HDDMA] ≈ 25 wt %, we observed three distinct, spatially periodic LLC morphologies. At low [H2O] ≤ 20 wt %, we again find Lα phases. However, increasing the hydration to [H2O] = 25−40 wt % yielded stiff LLC samples with unexpected SWAXS signatures. While the optically non-birefringent nature of these samples suggested the formation of a cubic phase, the lab source SAXS peak pattern was inconsistent with the formation of a single cubic phase (Figure 3B and Figure S2). We note that the two most intense peaks located at q/q* = √6 and √8 (q* = 0.1632 Å−1) exhibit a ∼10:1 intensity ratio characteristic of the (211) and (220) reflections of a GI morphology. The remaining, less intense SWAXS peaks appear at q/q* = √2, √3, √4, and √6 (q* = 0.1449 Å−1) and apparently correspond to the (110), (111), (200), and (211) peaks of a double diamond LLC phase with Pn3m ̅ symmetry (space group 224). We thus assign the phases formed in this region of composition space to be GI + DI two-phase coexistence (vide inf ra); we note that conceptually related phase coexistence has been reported in inverse (Type II) LLCs in the presence of various additives.47,48 While its existence is not precluded for any reason,49 this phase assignment represents the first observation of a normal double diamond (DI) morphology (albeit impure) in surfactant LLCs to the best of our knowledge. Finally, increasing the hydration state of these ternary mixtures to [H2O] = 42−46 wt % drives the formation of optically birefringent and stringy LLCs, which exhibit easily discerned SAXS peaks at q/q* = √1, √3, √4, √7, and √9 symptomatic of a HI phase (P6/mm symmetry) as seen in Figure 3A. We summarize the phase behavior of all of the mixtures described thus far in the ternary LLC phase portrait at T = 22 °C shown in Figure 4. We generally observed that the attempted inclusion of ≥37 wt % HDDMA in these four component mixtures yielded macrophase separation into a clear liquid coexisting with second LLC or isotropic fluid phase. Isolation and subsequent 1 H NMR analyses of the clear liquid revealed that it was HDDMA, whereas the other phase formed was either an oilswollen LLC or disordered micelle dispersion. The identities of the coexisting LLC phases were determined by SWAXS and are included in the phase portrait in Figure 4. Note that LLC37,37,26 comprises a LLC phase with no sign of HDDMA macrophase separation, whereas LLC37.5,37.5,25 forms coexisting GI/DI LLCs evidenced the high-resolution SAXS pattern in Figure 3B with excess HDDMA (also see Figure S2). These results establish an

Figure 4. Partial ternary phase portrait at 22 °C for LLCs comprising TMA-83u, HDDMA, and H2O with 1 wt % DMPA photoinitiator present, indicating unexpectedly large GI and coexisting GI + DI phase windows, which are flanked by lamellae (Lα) and hexagonally packed cylinder (HI) phases. Red circles indicate compositions that were photopolymerized (PLLC samples) and analyzed by SAXS prior to surfactant removal by solvent extraction.

upper bound of ∼37 wt % HDDMA monomer that can be loaded into these aqueous TMA-83u LLCs. Thus, we conclude that 40 wt % HDDMA is just beyond the maximum capacity for oil sorption into the ordered LLC phase. We discuss the origins of this behavior below (vide inf ra). While we have thus far suggested that the aqueous LLCs formed by TMA-83u/HDDMA/DMPA are normal (Type I) phases based on their location in phase space relative to the Lα morphology, we sought to directly test this idea. By analogy to the fact that inverse (Type II) phases persist in excess water as a consequence of their hydrophobic matrices,50 normal (Type I) phases should persist in excess oil due to their hydrophilic majority phases. Also, the complementary addition of excess water to normal LLC phases should drive their dissolution into freely flowing disordered micelle dispersions. We found that adding excess water (10 wt equiv per 1 wt equiv LLC) to the putative GI, GI + DI, and HI phases resulted in their rapid dissolution to yield cloudy, low-viscosity aqueous dispersions. The corresponding addition of excess HDDMA monomer (10 wt equiv per 1 wt equiv LLC) to these same LLCs does not dissolve the LLC, even after vortex mixing for 3 min and ultrasonication for 30 min at 22 °C. Thus, we conclude that these HDDMA-loaded TMA-83u aqueous LLCs are indeed normal phases, comprising a hydrophilic matrix with hydrophobic cables in which the HDDMA is spatially confined. Therefore, the gemini dicarboxylate TMA-83u LLCs take up large amounts of the hydrophobic HDDMA monomer with only a few apparent changes to their morphological behaviors. Quantitative comparisons of the unit cell dimensions of the binary TMA-83u/H2O LLC phases and those loaded with HDDMA reveal further details of the impact of added HDDMA on the adopted LLC morphologies. In the binary TMA-83u/H2O LLCs, the unit cell sizes of both the HI and GI phases increase with water content as shown in Figure 5. SWAXS analyses show that the addition of HDDMA at concentrations [HDDMA] = 10 and 25 wt % leads to expansion of both the HI and GI unit cell parameters at a given [TMA-83u]:[H2O] as compared to LLCs formed in the absence of monomer. The unit cell parameters for each of the HI and GI phases increase monotonically with increasing 190

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gemini dicarboxylate phase behavior in the presence of hydrocarbon oils suggest that the exact surfactant structure and identity of its charge-compensating counterion sensitively guide their various responses to oil addition. Photopolymerization of HDDMA-Swollen Gemini Dicarboxylate Aqueous LLCs. Having successfully demonstrated the self-assembly of Lα, HI, DI + GI, and pure GI phase aqueous LLCs in TMA-83u/HDDMA/H2O/DMPA mixtures, we sought to evaluate the morphological consequences of photochemically initiated cross-linking polymerizations of HDDMA localized therein. Given the substantial width of the HDDMA-loaded GI phase LLC window, we anticipated that these formulations would result in porous polymers with useful properties arising from potential retention of the LLCtemplated mesostructure.52,53 Effectively, we hypothesized that H2O and TMA-83u would act as nanoscopically ordered porogens around which a cross-linked, hydrophobic HDDMA network could be templated. We specifically photopolymerized the five LLC mixtures listed in Table 1 at compositions demarcated by open red circles in the ternary phase diagram in Figure 4, in order to assess the efficacy of each LLC morphology to template HDDMA cross-linking. Note that these compositions were selected because they do not contain excess HDDMA, which would lead to polymers exhibiting macroscopic heterogeneities. Short exposures to 365 nm light at 22 °C resulted in transformation of the sample textures from viscous gels to solid, and insoluble cross-linked polymers. Using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, we observed almost complete consumption of the methacrylate moieties. These polymeric samples are labeled PLLCx,y,z, where x, y, and z denote the respective weight percentages of TMA-83u, HDDMA, and water in the prepolymer mixture. In spite of their different LLC morphologies prior to polymerization, SAXS analyses revealed that all but one of the polymerized mixtures transformed into a Q phase (Figure 6, Figure S4, and Table 1) with intense SAXS peaks at q/q* = √6 and √8, indicative of either a GI or DI morphology. Polymerization of the GI phase of prepolymer LLC59,14,26 leads to remarkable coarsening of the LLC grain size evidenced by the appearance of new, higher-order SAXS maxima located at q/q* = √20, √22, √24, and √26 in PLLC59,14,26 that enable definitive identification of a GI phase (Figure 6a). Note that HDDMA cross-linking within the LLC reduced the measured domain spacings (Table 1), evidenced by a shift in the principal scattering peak to a higher q-value. Similarly wellordered, polymeric GI phases arose from polymerizations of LLC49,25,25 and LLC29,25,45 which initially exhibit coexisting GI

Figure 5. Dependence of the unit cell dimension (a) from SWAXS on ternary LLC composition for samples containing 0 (open symbols), 10, and 25 wt % HDDMA demonstrating that the LLC unit cell sizes monotonically increase for each of the HI and GI phases upon addition of either H2O or HDDMA. Unit cell dimensions were calculated from a = 2π√6/|q| for the GI phases and a = (2/√3)(2π/|q|) for the HI phase, where |q| is the location of the first scattering peak.

[HDDMA]. A 10 wt % HDDMA loading in a TMA-83u GI phase drives a ∼10% increase in the cell parameter, while 25 wt % HDDMA in the same LLCs with comparable water contents leads to a ∼18% increase in the gyroid unit cell dimension. The impact of HDDMA loading on the HI phase is apparently similar. We ascribe this swelling of the cell parameter upon HDDMA addition to its localization primarily within the hydrophobic domains of these aqueous LLCs. The ability of the TMA-83u LLCs to accommodate up to ∼37% HDDMA is unexpected and larger than that solubilized by ordered LLCs based on other known anionic gemini dicarboxylate amphiphiles51 or cationic CTAB derivatives.25 The fact that the GI morphology is retained across such a wide phase window is also unanticipated, especially given that oil addition to aqueous gemini surfactant LLCs was previously reported to drive the formation of higher curvature spherical aggregates.51 By analogy to prior observations by Kunieda et al.,51 we speculate that macrophase separation of the monomer at [HDDMA] ≥ 40 wt % stems from the fact that further swelling of the hydrophobic domains would require an overall decrease in interfacial curvature. However, such decreases in interfacial curvature are electrostatically restricted by the highly dissociated (CH3)4N+/carboxylate counterion−headgroup pair that seeks to maintain a near constant interfacial area per surfactant. Thus, the hydrophobic domains only swell to a limited degree, and they eject excess oil upon reaching saturation. We note that the addition of HDDMA also decreases the water concentration range over which LLCs form as compared to TMA-83u in the absence of monomer, suggesting that HDDMA disrupts the delicate balance of electrostatic interactions between the ionic counterion−headgroup pairs that favor LLC formation. These differences in

Table 1. Morphological Behaviors of Ternary TMA-83u/HDDMA/H2O LLCs before and PLLCs after Photochemically Initiated HDDMA Cross-Linking Polymerization samplea

TMA-83u (wt %)

LLC79,10,10 LLC64,25,10 LLC59,14,26 LLC49,25,25 LLC29,25,45

79 64 59 49 29

HDDMA (wt %) H2O (wt %) 10 25 14 25 25

10 10 26 25 45

LLC morphologyb

LLC unit cell,b a (nm)

PLLC morphologyb

PLLC unit cell,b a (nm)

Lα Lα GI GI + DI HI

2.6 2.8 8.3 9.4 and 6.1 4.7

Lα + GI GI GI GI GI

2.5 and 6.5 6.5 6.7 7.0 6.8

a

Samples are identified as LLCx,y,z, where x, y, and z are the respective weight percentages of TMA-83u, HDDMA, and H2O. bDetermined by SWAXS at 22 °C. 191

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Figure 6. Representative one-dimensional synchrotron SWAXS intensity profiles before and after HDDMA cross-linking polymerization (prior to surfactant removal) of the ternary blends (A) LLC59,14,26 (GI → GI), (B) LLC29,25,45 (HI → GI), and (C) LLC79,10,10 (Lα → Lα + GI). The markers in each plot specify the calculated positions of Bragg reflections characteristic for each of the GI (◆), HI (▼), and Lα (■) LLC phases.

Table 2. Composition-Dependent Swelling Ratios and Compressive Moduli for PLLC Samples Synthesized by LLC-Templated Polymerizations after Surfactant Removal sample

TMA-83u (wt %)

HDDMA (wt %)

H2O (wt %)

LLC morphology

PLLC78,11,10 PLLC64,25,10 PLLC59,13,27 PLLC50,26,23 PLLC34,24,41 ISO51,25,0 ISO0,24,51

78 64 59 50 34 51 0

11 25 13 26 24 25 24

10 10 27 23 41 0 51

Lα Lα GI G I + DI HI isotropic isotropic

water swelling ratio (W) 2.2 1.8 5.4 2.7 3.1 0.9 0.6

± ± ± ± ± ± ±

0.2 0.2 0.4 0.3 0.4 0.1 0.2

2-propanol swelling ratio (W) 1.3 1.5 4.4 3.0 1.4 1.3 1.0

± ± ± ± ± ± ±

0.2 0.2 0.1 0.1 0.2 0.1 0.3

modulus (kPa) 70 120 40 80 60 150 160

± ± ± ± ± ± ±

20 20 10 20 20 20 30

retention. The hydration of the counterion−headgroup pair of a dicarboxylate gemini surfactant induces their dissociation, which leads to a strong intramolecular repulsion between the covalently linked headgroups. In order to mitigate of this intramolecular electrostatic repulsion, the headgroups twist away from one another and force the hydrophobic tails to splay apart. The resulting anisotropic molecular conformation, which is quite different from the conical shape associated with singletail amphiphiles, destabilizes the formation of the Lα phase in favor of a GI phase. The linker length parity further influences the HI phase stability: If the linker contains an odd number of atoms, maximal hydration of the surfactant headgroups requires the linkages to adopt gauche conformations that break the symmetry of the gemini surfactant conformation. Very recently published MD simulations demonstrate that these latter conformations alter the degree of counterion−headgroup association to destabilize the HI phase in favor of tighter intermolecular surfactant packing into a GI LLC.57 The balance of these subtle counterion−headgroup correlations enables the TMA-83u LLCs to accommodate large amounts of hydrophobic HDDMA monomer into the Lα, GI, and HI phases. Previously reported LLC phase behavior studies of ionic gemini surfactant/oil/water mixtures in which the surfactant counterions are highly dissociated suggest that the interfacial area per surfactant is nearly constant upon oil addition, when the oil segregates toward the middle of the hydrophobic domains.58 At intermediate hydrations, we expect that the hydrophobic HDDMA is strongly segregated within the hydrocarbon domains, and thus the interfacial surfactant packing is driven mainly by the counterion−headgroup correlations. Thus, the fact that GI and HI LLC templates yield polymerized GI phases (with smaller unit cell dimensions) probably stems from the

+ DI (Figure S5) and cylindrical (HI; Figure 6b) prepolymer morphologies, respectively. We ascribe the observed domain spacing decrease and the HI → GI phase transformation to the hydrophobic volume decrease upon HDDMA polymerization54 within the LLC templates (assuming minimal dehydration during the rapid photopolymerization at 22 °C). An alternative explanation for the phase transition from HI to GI would be macrophase separation of HDDMA during polymerization, in which case the SAXS peaks recorded in Figure 6 would correspond to those of an aqueous LLC alone rather than a LLC-templated polymer. The latter behavior is often observed when phase separation occurs in LLC -templated polymers, wherein the structure observed by SAXS reverts to that of the LLC system at the same [surfactant]:[water] ratio without any interactions with the monomer or polymer.55 However, if the HDDMA polymerized independently of the LLC template for LLC29,25,45, the resulting SAXS profiles would show only HI peaks associated with a binary LLC comprising 39 wt % TMA83u (see Figure 2). Therefore, we can rule out phase separation as the cause of the observed phase transition. Only in the case of LLC79,10,10 did we observe incomplete transformation of the initial Lα phase into Lα/GI phase coexistence upon polymerization (Figure 6c). The nearly exclusive formation of GI phases upon HDDMA polymerization (prior to surfactant removal) in spite of the different initial LLC morphologies is remarkable and unexpected. Recent atomistic molecular dynamics (MD) simulations of gemini carboxylate surfactants56 reveal that the conformations of these surfactants and the counterion− headgroup interactions are quite different from those of traditional single-tail amphiphiles, for which LLC-templated polymerizations rarely lead to GI phase formation nor 192

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Figure 7. Swelling ratios in (A) H2O and (B) 2-propanol for isotropic (△) and nanoporous PLLCs measured at 22 °C. Samples were prepared with either 10 (red) or 25 (blue) wt % HDDMA and polymerized in LLC templates with HI (●), GI or GI + DI (◆), or Lα (■) initial morphologies.

(isotropic) homopolymerizations.26 These differences in crosslink placement and network porosity at constant cross-link density may be discerned through solvent swelling studies, using either 2-propanol as a good solvent or H2O as a bad solvent for the polymer network. Defined as the ratio of the mass of solvent absorbed per unit mass of polymer, the solvent swelling ratio (W) serves as useful proxy for the porosities of the PLLC HDDMA networks listed in Table 2. The solvent swelling properties of the PLLC samples and the polymer produced under isotropic conditions were dramatically different. In water, the isotropic control polymers ISO51,25,0 and ISO0,24,51 displayed Wwater < 100% swelling, whereas the LLCtemplated polymers exhibited significantly enhanced water swellings as high as Wwater = 500% (Figure 7A). LLC templating also enabled the preparation of cross-linked polymers at HDDMA loadings as low as 12 wt %, compositions at which isotropic control experiments did not form solids due to low monomer conversions and poor mechanical stability. In 2propanol, isotropic polymers and PLLC78,11,10 and PLLC64,25,10 each presented W2‑propanol ∼ 150% swelling. However, PLLC60,12,27 and PLLC50,26,24, which derive from QI phase LLCs with percolating hydrophobic domains within a water matrix, displayed significantly enhanced swelling in isopropanol (W2‑propanol ∼ 300−400%). These results are consistent with LLC-templated hydrophilic polymers where the water swelling ratio increased up to 5-fold compared to an isotropic sample.34,60 Since water is a bad solvent for PHDDMA with a maximum water sorption of 0.58 wt %,54 the significantly enhanced water swelling observed in the LLC-templated polymers (Figure 7A) indicates a marked overall increase in pore volume and surface area relative to isotropic control samples. This increased porosity must arise from the nanopore structure arising from spatially restricted polymerization within the supramolecular network phase template that exhibits water-filled nanopores ∼3.4−3.8 nm in diameter. These data also demonstrate that decreasing the HDDMA content in the LLC template increases the final polymer porosity, consistent with the higher volume fractions of the surfactant and water in the templating reaction that synergistically act as structured porogens. Thus, these studies reveal that new materials with tunable and high porosities may be templated by dicarboxylate gemini surfactant LLCs with properties that depend sensitively on the initial surfactant, monomer, and water concentrations in the selfassembled mesophase. Complementary studies of PHDDMA network swelling in 2propanol reveal the significance of the initial LLC template morphology in governing the primary cross-linked network structure (Figure 7B). Even though all templated polymerization reactions yielded GI phases, only materials synthesized

decrease in hydrophobic volume upon HDDMA polymerization.54 However, this reasoning does not explain why the lamellar LLC79,10,10 transforms into a GI phase. Since a combination of intra- and intermolecular electrostatic interactions destabilizes the Lα phase in binary TMA-83u/H2O LLCs, and the addition of HDDMA fosters Lα phase formation, we surmise that HDDMA acts as a cosurfactant59 at low hydrations wherein the ester functionalities interdigitate between the headgroups to mitigate unfavorable Coulombic repulsions. We speculate that cross-linking polymerization of the methacrylates draws the esters into the interior of the hydrophobic domains, thus removing the ester cosurfactant moieties from the interface and inducing a curvature change (albeit incomplete) toward a GI mesophase. Physical Characterization of Nanoporous LLC-Templated Polymers. We comparatively evaluated the solvent swelling behaviors and compressive moduli of cross-linked HDDMA polymer networks (PLLC) against those synthesized in isotropic media (Table 2). The latter control samples were synthesized by the polymerization of HDDMA in methanol in (1) the presence of TMA-83u with no added H2O (ISO51,25,0), and (2) the absence of TMA-83u with water present (ISO0,24,51) to obviate template self-assembly. Prior to any physical property testing, we sought to remove the TMA-83u template from each polymer monolith to avoid the undue influence of the highly variable surfactant loadings (24−78 wt %) in each material. Surfactant removal was effected by solvent extraction with acetone over 5 days (see Experimental Section for details). Gravimetric analyses after exhaustive solvent extraction and drying revealed that ≤10 wt % of the initial TMA-83u loading was retained in each of the templated and isotropic polymers with the exception of PLLC34,24,42 which retained ∼80% of the original surfactant loading. The high level of surfactant retention in this last sample may stem from physical immobilization of the surfactant within the polymer matrix, although the possibility of copolymerization of the unactivated alkenes in TMA-83u with HDDMA due to their spatial organization within an HI phase cannot be entirely ruled out. PLM analyses of the polymers after surfactant extraction revealed the extinction of birefringence associated with the original ordered LLC morphologies. While ionic TMA-83u surfactant removal from these polymers allows collapse of the LLC structure that was stabilized by electrostatic interactions between the surfactant headgroups, we anticipated that the porosity of the nanotemplated PHDDMAs would remain and manifest in various physical attributes. LLC templating of cross-linked HDDMA within different ordered phases offers a means for systematically changing the underlying network structure and the placement of cross-links in 3D space, as compared to materials produced by bulk 193

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swellabilities in water and 2-propanol as compared to analogous polymer networks prepared in the disordered templates. This enhanced porosity reflects the nanoporous nature of the Q phase LLC template, in which water and surfactant comprise the majority phase. Spatial confinement of the HDDMA monomer at concentrations as low as 13 wt % within a bicontinuous LLC template also permits its cross-linking into a high-porosity material. Therefore, judicious selection of the polymerizable TMA-83u/HDDMA/H2O mixture enables synthesis of PHDDMA materials with controlled porosities and mechanical properties. The high level of control over the porous network polymer structure afforded by the TMA-83u LLC template offers enticing opportunities for the development of myriad mesostructured, hydrophobic polymer network materials with far-reaching applications.

from initial GI or GI + DI templates (i.e., PLLC59,13,27 and PLLC50,26,23) displayed enhanced 2-propanol swelling ratios as compared to the isotropic control polymers. Thus, we conclude that the network phase LLC templates exert unusually homogeneous spatial control over cross-link placement in 3D space at both molecular and supramolecular length scales that enable their enhanced swellabilities. Mechanical property testing is another well-established means for assessing differences in the densities and spatial arrangements of cross-links within dry and solvent-swollen polymer networks. While tightly cross-linked networks typically exhibit small mesh sizes and small swelling ratios (W), LLC templating of a cross-linking polymerization offers opportunities for decoupling these structure−property relationships.26 Consequently, we measured the compressive moduli of water-swollen PLLC34,24,41, PLLC50,26,23, PLLC64,25,10, and ISO51,25,0 to evaluate their mechanical properties for potential applications (Figure 8). ISO51,25,0, which swelled significantly

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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.chemmater.7b04183. Azimuthally integrated SAXS patterns for LLC49,25,25, PLLC49,25,25, LLC44,10,45, and LLC14,40,45 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 1+ (319) 335-5015. *E-mail: [email protected]. Phone: +1 (612) 625-4599.

Figure 8. Compressive modulus versus swelling ratio in H2O for polymers prepared from ternary LLC mixtures. Samples were prepared with 25 wt % HDDMA and polymerized in LLC templates with isotropic (△), HI (●), GI or GI + DI (◆), or Lα (■) initial morphologies.

ORCID

C. Allan Guymon: 0000-0002-3351-9621 Mahesh K. Mahanthappa: 0000-0002-9871-804X Present Address

less than LLC-templated analogues, possessed the highest compressive modulus (0.15 MPa) of all samples tested. The LLC-templated polymers displayed nearly linear decreases in compressive modulus with increasing swelling ratio (W). These observations are generally consistent with the fact that solventswollen cross-linked polymers typically display lower moduli than their dry analogues, and the observed moduli depend on the polymer volume fraction.61−63 These data demonstrate the potentially useful ability to specifically tailor the LLC template composition to systematically tune both the swelling ratio and resulting compressive modulus, while maintaining the chemical identity of the underlying polymer network.

∥

J.J.: Department of Chemistry, University of Sheffield, Western Bank, Sheffield S3 7HF, United Kingdom. Author Contributions

The manuscript was written through contributions from all authors, who have given approval to the final version of this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was primarily supported by NSF through the University of WisconsinMadison Materials Research Science and Engineering Center (DMR-1121288), with supplementary support from NSF CBET-1438486, the Wisconsin Alumni Research Foundation, and the University of Minnesota. Partial support for critical core instrumentation facilities derived from NSF Grants CHE-9974839 and DMR-0832760. Synchrotron SAXS analyses were conducted at Sector 12 of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DEAC02-06CH11357.

4. CONCLUSION We have demonstrated the unusually robust ability of the aqueous LLCs derived from the gemini dicarboxylate surfactant TMA-83u to spatially template the cross-linking photopolymerization of the hydrophobic monomer HDDMA into porous polymers with tunable solvent swelling characteristics and compressive moduli. TMA-83u enables the unprecedented formation of well-ordered, normal (Type I) LLC phases with HDDMA loadings as high as 37 wt %, including double gyroid (GI), hexagonally packed cylinders (HI), and lamellar (Lα) phases. Templated photopolymerization of the HDDMA that is spatially localized within the hydrophobic domains of the LLCs affords cross-linked polymeric materials that adopt nonconstant mean-curvature GI nanostructures, irrespective of the original template morphologies. Although removal of the surfactant template collapses the final nanostructure of the LLCtemplated polymer network, these materials exhibit enhanced

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REFERENCES

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