Porating Anion-Responsive Copolymeric Gels - Langmuir (ACS

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Porating Anion-Responsive Copolymeric Gels Dustin England,† Feng Yan,‡ and John Texter* School of Engineering Technology and Coatings Research Institute, Eastern Michigan University, Ypsilanti, Michigan 48197, United States S Supporting Information *

ABSTRACT: A polymerizable ionic liquid surfactant, 1-(11acryloyloxyundecyl)-3-methylimidiazolium bromide (ILBr), was copolymerized with methyl methacrylate (MMA) in aqueous microemulsions at 30% (ILBr w/w) and various water to MMA ratios. The ternary phase diagram of the ILBr/ MMA/water system was constructed at 25 and 60 °C. Homopolymers and copolymers of ILBr and MMA were produced by thermally initiated chain radical microemulsion polymerization at various compositions in bicontinuous and reverse microemulsion subdomains. Microemulsion polymerization reaction products varied from being gel-like to solid, and these materials were analyzed by thermal and scanning electron microscopy methods. Microemulsion polymerized materials were insoluble in all solvents tested, consistent with light cross-linking. Ion exchange between Br− and PF6− in these copolymeric materials resulted in the formation of open-cell porous structures in some of these materials, as was confirmed by scanning electron microscopy (SEM). Several compositions illustrate the capture of prepolymerization nanoscale structure by thermally initiated polymerization, expanding the domain of compositions exhibiting this feat and yet to be demonstrated in any other system. Regular cylindrical pores in interpenetrating ILBr-co-MMA and PMMA networks are produced by anion exchange in the absence of templates. A percolating cluster/bicontinuous transition is “captured” by SEM after using anion exchange to visualize the mixed cluster/pore morphology. Some design principles for achieving this capture and for obtaining stimuli responsive solvogels are articulated, and the importance of producing solvogels in capturing the nanoscale is highlighted.

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INTRODUCTION

An important subclass of ILs is that composed of reactive and functional monomers that can be polymerized into diverse functional materials. The creation of diverse polymers incorporating IL monomers has become a very active area, and a number of reviews are available.15−20 When 1-alkyl-3methylimidazolium ionic liquids contain a long alkyl chain, the IL can function as a surfactant in solution, with a critical micelle concentration (CMC) determined by the chain length of the alkyl substituent. An increase in alkyl chain length decreases the CMC in aqueous solution, due to the increased hydrophobicity imparted by the long hydrocarbon chain. Adding a polymerizable group to the ends of alkyl chains such as these allows the preparation of a wide range of materials that would not be possible with conventional surfactants.21 Conventional ILs have been studied as components in microemulsions.22,23 However, IL surfactants can also be prepared with reactive groups. Yan and Texter used a polymerizable, 1-alkyl-3-methylimidazolium IL surfactant in the microemulsion polymerization of a ternary system including MMA and water.24 The acrylate-functionalized IL, 1-(11acryloyloxyundecyl)-3-methylimidazoleum bromide (ILBr),

Ionic liquids (ILs) have become increasingly popular as alternative fluids and solvents as their application steadily increases.1 Their liquidity beneath 100 °C, very low vapor pressure, and thermal stability have led to their utilization as viable, environmentally friendly alternatives to organic solvents in organic synthesis2 and in inorganic synthesis.3 Other applications utilizing ionic liquids have included dye-sensitized solar cells,4 capacitors,5 and rechargeable Li ion batteries.6 They also have proven to be efficient solvents for polymerization7−9 as well as for inorganic nanoparticle synthesis.10 ILs recently have been shown to be recyclable solvents for sustainable polymerizations of various types and for low-temperature AGET ATRP of methyl methacrylate.11 Increasing interest in ionic liquids and their applications can be attributed to the wide variety of ion pairs available, as each pair possesses different physical properties. Typically, ionic liquids are prepared as quaternary ammonium or cyclic amine salts with cations such as imidazolium, pyridinium, pyrrolidinium, and piperidinium groups. Other cationic groups recently have received attention, such as phosphonium and sulfonium ions.12,13 A wide variety of inorganic and organic anions can be paired with these tetraalkylammonium and cyclic amine cations,14 and physical properties can be dramatically changed by varying ion pairing. © XXXX American Chemical Society

Received: June 24, 2013 Revised: August 19, 2013

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was synthesized and characterized by various methods. The present study follows Yan and Texter24 in further exploring the ternary ILBr, MMA, water system, with respect to the nanostructure of the parent microemulsions and with respect to the structure and morphology obtained after polymerization at 30% ILBr in this ternary system and after targeting the stimuli responsiveness of such materials. Since the report of Yan and Texter,24 many studies examining nanostructured PIL in separation and absorption processes have appeared. These studies have very recently been reviewed by Yuan et al., 15 but particular noteworthy applications deserve further mention in order to illustrate the breadth of this burgeoning application area. Tang et al.25,26 discovered that PIL exhibited important CO2 absorption properties, so important in usefully sequestering this important greenhouse gas. Wilke et al.27 and Zhao et al.28,29 showed that imidazolium-based PIL, similar to those of Yan and Texter,24 could be used to generate high internal surface area absorbent matices with remarkable CO2-absorbing capability. The templating approach of Wilke et al.27 utilized nanoparticle assemblies as templates; this approach was developed over the past dozen years or so as an approach to making new materials based on photonic crystal assembles and disordered nanoparticle assemblies.30,31 PIL based on the pyrrolidinium cation have also been applied to CO2 absorption.32 Over the same time period PIL membranes for gas separation applications have been introduced,33−36 with particular attention paid to factors that influence gas molecular selectivity. Advanced uses of PIL in formulating chromatographic stationary phases recently have been demonstrated by several groups.37−43 Such applications have also been extended to gas chromatography,44 extraction processes,45−47 and capillary electrophoresis.48,49 Microemulsion polymerization,50−52 a specialized form of solution or dispersion polymerization depending on whether the resulting polymer is soluble or insoluble, respectively, has been pursued for several decades. A challenge that has persisted since the earliest of such polymerizations53 has been to learn how to capture nanostructures (morphology, length scale) that exist in a microemulsion prior to polymerization.53,54 Some of these challenges have been reviewed.55,56 It is highly motivating, however, that Gan and coworkers57−59 have repeatedly demonstrated the capture of nanoscopic length scales using photopolymerization methods. In a related communication it was shown that SANS data unequivocally showed that a bicontinuous microemulsion we polymerized succumbed to nanoscopic structure capture even though thermally initiated polymerization was used. This achievement was obtained with only a 20% increase in correlation length.60 Hundreds to thousands percent changes have been the norm in such attempts. We examine here the nanostructure of microemulsions in the ternary water/ILBr/MMA system at 30% (w/w) surfactant (ILBr). We then show that hydrogels obtained by thermally initiated microemulsion polymerization at varying water/MMA ratios appear to substantively capture nanoscale structure or at worst exhibit some nanophase separation. These gels are also stimuli responsive, and we examine how anion exchange of bromide for hexafluorophosphate induces poration and transforms these gels into porous monolithic materials. Thermal analyses show that some of the water is incorporated as nonfreezable water and that other water is incorporated in nanoscopic to macroscopic pores.

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EXPERIMENTAL SECTION

Materials. Acryloyl chloride (96%), 11-bromoundecanol (98%), triethylamine (99.5%), 1-methylimidazole (99%), 2,6-di-tert-butyl-4methylphenol (minimum 99% GC, powder), 2,2′-azobis(isobutyronitrile) (AIBN, 98%), 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50, 97%), ammonium persulfate (APS), methyl methacrylate (MMA, 99%), sodium bicarbonate, magnesium sulfate (anhydrous, reagent grade, ≥97%), potassium hexafluorophosphate (KPF6, 98%), tetrahydrofuran, and THF (anhydrous, ≥99.9%, inhibitor free) were obtained from Aldrich. Inhibitor was removed from MMA by passing the MMA through a column of basic alumina, which also was purchased from Aldrich, as was neutral alumina used in the synthesis of ILBr. Methylene chloride (stabilized, HPLC grade) and diethyl ether were purchased from Fisher Scientific. The principal monomer, ILBr (Scheme 1), was prepared as described below.

Scheme 1. 1-(11-Acryloyloxyundecyl)-3-methylimidazolium Bromide (ILBr)

Methods. The monomer ILBr was prepared by esterifying acryloyl chloride with bromoundecanol. The resulting ester was then quaternized by reacting with 1-methylimidazole. The ester, 11bromoundecyl acrylate, was prepared by dissolving 100 mmol (25.12 g) of 11-bromoundecanol in 100 mL of THF in a three-neck 500 mL round-bottom flask in an ice bath under nitrogen. Triethylamine (120 mmol, 12.14 g, 20% excess) was dissolved in 100 mL of THF and added to the reaction with stirring. Next, 120 mmol of acryloyl chloride (9.7 mL, 20% excess) in 100 mL of THF was added dropwise to the stirred reaction over a period of 30 min by addition funnel. Once addition of acryloyl chloride was completed, the ice bath was removed. Stirring was continued under a nitrogen atmosphere at room temperature for 48 h. The white salt precipitate was then removed by filtration. The light yellow liquid filtrate was washed three times with 2% sodium bicarbonate in deionized water solution in a 500 mL separatory funnel. The washed filtrate was dried overnight over anhydrous magnesium sulfate. The resulting filtrate was diluted with 100 mL of methylene chloride and passed through a gravity column containing approximately 0.75 in. in height of neutral alumina. Solvents were removed by rotary evaporation at 45 °C. The 11bromoundecyl acrylate structure was confirmed by 1H NMR. 1H NMR (400 MHz, CDCl3, δ): 1.30 (m, 14H, −CH2(CH2)7CH2−), 1.65 (m, 2H, −OCHH2CH2(CH2)7−), 1.85 (m, 2H, −(CH2)7CH2CH2Br), 3.40 (t, 2H, −CH2CH2Br), 4.10 (t, 2H, −OCH2CH2(CH2)7−), 5.80 (1H, CH2CH−), 6.10 (1H, CH2CH−), 6.40 (1H, CH2CH−). 11-Bromoundecyl acrylate was then stirred with a 20% molar excess of 1-methylimidazole and 0.01 wt % 2,6-di-tert-butyl-4-methylphenol inhibitor at 40 °C for 48 h under nitrogen. After 48 h the viscous and amber liquid was washed three times with diethyl ether in a separatory funnel. The washed product was diluted with 100 mL of methylene chloride and passed through a gravity column containing approximately 0.75 in. in height of neutral alumina. This filtered solution was placed in a Petri dish to allow evaporation of methylene chloride at room temperature. The resulting waxy ILBr solid was dried under vacuum at room temperature, producing a white, powdery solid. The melting point of ILBr was determined for each batch. The average melting point was 48 ± 4 °C. ILBr structure was confirmed by 1H NMR. 1H NMR (400 MHz, CDCl3, δ): 1.30 (m, 14H, −CH2(CH2)CH2 −), 1.65 (m, 2H, −OCH2CH2 (CH2)7 −), 1.85 (m, 2H, (CH2)7CH2CH2N−), 4.10 (m, 2H, −OCH2CH2(CH2)7−), 4.10 (m, 3H, −N−CH3), 4.30 (t, 2H, −CH2CH2N−), 5.80 (1H, CH2CH−), 6.10 (1H, CH2CH−), 6.40 (1H, CH2CH−), 7.25 (d, 1H, −NCHCHN−), 7.35 (d, 1H, −NCHCHN−), 10.60 (s, 1H, −NCHN−). Ternary phase diagrams of the water/ILBr/MMA system were prepared at 25 and 60 °C. Boundary points between the single-phase B

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microemulsion (solution) and multiphase emulsion domains were visually determined by titration after mixing components in culture tubes. The region above 75% ILBr (w/w) in the phase diagram was not investigated. Further details are given in the Supporting Information. Electrical conductivity measurements of microemulsion compositions was done using a Brinkmann Conductometer E518 and an EA240 probe. The probe was calibrated with 0.1 M potassium chloride solution. Measurements were done by preparing stock solutions of 30% (w/w) ILBr in water and 30% (w/w) ILBr in MMA. A portion of the stock solution of 30% ILBr in MMA was titrated with successive 50 μL aliquots of 30% ILBr in water stock solution in a vial under constant stirring at 25 °C. The conductivity was recorded after addition of each aliquot. A smaller number of conductivity measurements were also taken by adding 100 μL aliquots of the 30% ILBr in MMA stock solution to a portion of the 30% ILBr in water stock solution. ILBr/MMA/water microemulsion compositions were polymerized by thermal initiation at 60 °C in 5 mm o.d. NMR tubes. Stock solutions of ILBr in water, as well as ILBr in MMA, were prepared for each composition series. The ILBr in MMA solutions contained 0.5 wt % AIBN initiator with respect to total monomer. The aqueous ILBr stock solution contained ammonium persulfate (APS) initiator on an equivalent molar basis to the AIBN content of the ILBr in MMA stock solution, resulting in a concentration of 0.7 wt % APS with respect to total monomer. This step was taken due to solubility issues with AIBN in the aqueous ILBr solutions. In each instance the respective stock solutions were combined in a screw-capped culture tube and thoroughly mixed on a vortex shaker before addition to the NMR tubes. All compositions were polymerized at 60 °C for 8 h in a temperature-controlled bath (Haake K20/DC3). The polymerized rods were removed from the NMR tubes by scoring the glass and breaking the tubes. Care was taken to remove any remaining glass from the surface of the rods. 1 H NMR structural analysis of 11-bromoundecyl acrylate and ILBr was performed on a JEOL 400 MHz NMR. Samples were prepared by dissolving 15 mg of analyte in CDCl3 in a 5 mm o.d. NMR tube. Microcalorimetry of microemulsion polymerization of ILBr/MMA/ water was performed on a Setaram C80 microcalorimeter. The microcalorimeter sample cell contained the microemulsion composition, along with 0.5 wt % AIBN initiator with respect to total monomer and a 3 cm long and 1 cm diameter cylindrical PTFE insert. This PTFE insert was necessary to reduce the volume of microemulsion sample in the cell to a volume that kept the heat evolution beneath the saturation limit of the thermal detection during the polymerization. The reference cell contained an equivalent amount of the same microemulsion formulation, without initiator, and a PTFE insert identical to the one placed in the sample cell. The two cells were heated from room temperature to 60 °C at a rate of 3 °C/min and held isothermally at 60 °C for 12 h. The enthalpy of polymerization was calculated by integrating the exothermic curves obtained by using Setaram software. Thermal gravimetric analysis (TGA) was performed on a TA Instruments TGA Q500. Samples were placed in aluminum DSC pans (TA Instruments), which in turn were placed in platinum TGA suspension pans. Samples were heated from room temperature to 525 °C at 10 or 20 °C/min. Nitrogen was used as a purge gas during all runs. DSC (differential scanning calorimetry) analysis was performed on a TA Instruments Q2000 DSC. All samples were analyzed by a heat/ cool/heat cycle in which the sample was initially heated and held isothermally for 5 min at a temperature believed to be above the Tg in order to erase the previous thermal history of the given sample. The samples were then cooled to the desired minimum temperature and held isothermally for 5−10 min before heating again to the desired maximum temperature. All reported transitions were obtained from the second heating portion of the cycle. Scanning electron microscopy (SEM) analysis of ILBr/MMA/water microemulsion polymerized samples was performed on a Hitachi S3400N scanning electron microscope. All materials were sputter-

coated with gold on a Denton Vacuum Desk IV cold sputter/etch unit to reduce surface charging by the electron beam.

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RESULTS AND DISCUSSION Phase Diagram. A partial (but substantial) ternary phase diagram is illustrated in Figure 1. There is not a very significant

Figure 1. Ternary phase diagram of the ILBr/MMA/water system at 25 (■) and 60 °C (○). The shaded area above the curves represents most of the single-phase microemulsion region. The black squares and solid line represent the boundary points and curve at 25 °C, with points at 60 °C represented by open circles and a dotted line. At 60 °C, the single phase microemulsion domain extends to the dotted curve. The dashed line at 75% ILBr concentration (w/w) represents the upper limit verified as being part of the single-phase microemulsion domain. The dashed line at 30% ILBr represents the range of compositions examined and focused upon in this study. The triangle (△) denotes a composition examined in a recent small-angle neutron scattering study60 and in the original paper about ILBr stimuli responsiveness;23 the inverted triangle (▽) denotes a composition examined in the production of stimuli responsive nanolatexes.24,61

difference between the 25 and 60 °C boundaries separating the respective multiphase emulsion domains and large single-phase microemulsion domains. In order to verify that the visually transparent microemulsion domain extended beyond an ILBr concentrations of 30% (w/w), stock solutions at 50, 65, and 75% ILBr were prepared in both water and MMA. At constant 30% (w/w) ILBr concentration, each pair of stock solutions was combined in various ratios to yield compositions that traversed horizontally across the ternary diagram as illustrated. All microemulsion solutions remained transparent and stable. Compositions above 75% ILBr were not investigated; the dashed line in the diagram denotes this limit in our investigation. The crosses (×) illustrated along the 30% ILBr line in Figure 1 at intervals of 10% MMA (H2O) indicate the main compositions investigated in this study by microemulsion polymerization. This aspect is discussed extensively in the sequel. Two other compositions are noteworthy. The weight fraction composition (△) of 0.15/0.10/0.75 in water/ILBr/ MMA has earlier been found to yield a reversibly porating gel and more recently60 was reported in conjunction with SANS (small-angle neutron scattering) measurements before and after polymerization. That study showed that a record retention of C

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microemulsions (solutions) were optically clear and transparent. The polymerized compositions ranged from a sticky gel (70/ 0) to a rigid rod (0/70), and a photograph of the resulting rods, taken before these rods were removed from their respective NMR tubes, is illustrated in Figure 3 (top). The increased

nanoscale structure upon thermal free radical polymerization was obtained with only a 20% increase in correlation length on polymerization.60 We use this composition in the sequel to examine the enthalpy of polymerization. The weight fraction composition (▽) 0.954/0.040/0.027 in water/ILBr/MMA is that reported to yield nanolatexes upon thermally initiated microemulsion polymerization. Thin films of such nanolatexes have been shown to be stimuli responsive and to form pores on various length scales when exposed to stimuli responsive anions.61 Electrical conductivity of microemulsion compositions along the 30% ILBr line of Figure 1 was measured and is illustrated in Figure 2. The asymptotic plateau (right) is due to the existence

Figure 3. (top) Photograph of polymerized compositions as they appeared after polymerization in NMR tubes (left to right, water/ MMA): (70/0); (60/10); (50/20); (40/30); (30/40); (20/50); (10/ 60); (0/70). ILBr is constant at 30% (w/w). (bottom) Same materials as immediately above after removal from NMR tubes and after treatment with (immersion in) aqueous 0.1 M KPF6. Photographed against a black background to highlight the turbidity.

rigidity was a result of decreasing water content and increasing MMA content. The 40/30, 30/40, and 20/50 compositions were slightly turbid after polymerization, suggesting a small amount of nanophase separation during polymerization. The photographs were taken using a black background in order to accentuate any turbidity. The most turbid appearing sample was not opaque. The (60/10) and (50/20) compositions, because of their transparency, appear to have undergone conversion to a polymerized gel without nanophase separation and the (40/30) and (30/40) compositions substantially so. Of course, the (70/ 0) composition has transformed into a gel, but there was no hydrophobic component to drive phase separation. While postpolymerzation does not guarantee preservation of nanoscale structure,63 the earlier documented success with half the present amount of hygroscopic surfactant (ILBr) and proportional amount of MMA preserved transparency and correlation length within 20%.60 Poly(ILBr-co-MMA) solution copolymers are highly water-soluble, and we may therefore infer that the networks formed in our microemulsion polymerizations are highly hydrated, at least for those compositions corresponding to (20/50) and smaller in MMA. Thermal Analyses. In order to evaluate the enthalpy of this microemulsion copolymerization chemistry, we examined the polymerization of an ILBr/MMA/water (0.15/0.10/0.75) microemulsion by microcalorimetry. This composition was that used to produce the stimuli-responsive gel reported by Yan and Texter24 and to examine nanoscale structure retention by SANS.60 The microemulsion was prepared in a screw-capped vial and transferred to a single chamber microcalorimeter cell. V-50 initiator was used because of its superior water solubility (compared to AIBN) and was added to the sample at 0.5% (w/ w) with respect to total monomer. The reference cell contained an equivalent amount of microemulsion without any initiator.

Figure 2. Electrical conductivity at room temperature measured along the 30% (w/w) ILBr compositional line of Figure 1 (from right to left). The compositions are expressed as aqueous ILBr weight (fraction) relative to the total weight when mixed with a 30% ILBr in MMA solution. The arrow at about 0.2 weight fraction denotes a break point in the data corresponding to apparent continuous phase transition in microemulsion nanostructure.

of a continuous aqueous pseudophase. The breakpoint at about 0.2 weight fraction is tentatively assigned to a shift in equilibrium nanostructure from biocontinuous to water in MMA (oil) percolating droplet nanostructure structure.62 A couple of other apparent breakpoints at 0.1 weight fraction and below will be discussed in another report focusing on polymerizations in the predominantly L2 (water in MMA) part of the microemulsion domain (between the ILBr-MMA axis and the emulsion multiphase boundary in Figure 1). These conductivity data, in combination with NMR selfdiffusion data (see Supporting Information), suggest the existence of a bicontinuous ILBr/MMA/water network over the 70/0 to 20/50 range of compositions at 30% ILBr and a series of reverse microemulsion droplets (aqueous ILBr in MMA) and percolating strings of such droplets over the 20/50 to 0/70 range of compositions. Microemulsion Polymerization. Compositions uniformly traversing the 30% ILBr line were prepared and transferred into 5 mm diameter NMR tubes before being thermally polymerized at 60 °C. This series of eight compositions, all 30% in ILBr, had water/MMA weight ratios of 70/0, 60/10, 50/20, 40/30, 30/ 40, 20/50, 10/60, and 0/70, respectively. All of these D

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through 6−25),68 and the reactivity ratios 0.32 and 2.7 from above, we can estimate sequence distributions for these different compositions. For the 0.078/0.1 ILBr to MMA mole ratio we obtain an ILBr sequence distribution {0.80, 0.16, 0.032, 0.0064, ...}, which corresponds to a sequence of probabilities, {p1‑tract, p2‑tract, p3‑tracts, ...}, where pn‑tract is the number frequency probability for a contiguous tract of n sequential ILBr monomers having an MMA on either end of the tract. A quite different distribution for the MMA, {0.22, 0.17, 0.13, 0.10, ...}, is obtained. Here most of the ILBr is in 1tracts and 2-tracts, and the MMA is much more broadly distributed among tracts of varying length. The number-average tract length for ILBr is estimated as 1.25, and for MMA the estimate is 4.5. These distributions and averages apply only for the initial period of polymerization, as the molar excess of MMA is only 28%. Calculated tract distributions for the next higher MMA composition with ILBr/MMA mole ratio of 0.078/0.2 are {0.89, 0.098, 0.011, 0.0012, ...} for ILBr and {0.13, 0.11, 0.098, 0.086, ...} for MMA with number averages of 1.12 and 7.7, respectively. This trend suggests a sharpening of the ILBr distribution and broadening of the MMA distribution. These trends continue, and the 0.078/0.7 mole ratio yields tract distributions of {0.966, 0.033, 0.0011, 3.8 × 10−5, ...} for ILBr and {0.04, 0.0384, 0.0369, 0.0354, ...} for MMA with number averages of 1.04 and 25, respectively. We reiterate that these distribution estimates are only for the initial period of polymerization. The photographs in Figure 3 (top) indicate that essentially complete transparency was maintained in the 70/0, 60/10, 50/ 20, 10/60, and 0/70 compositions. In these cases we may infer that the state of water in these samples is not one that is highly light scattering; the (0/70 sample has no added water). The 0/ 70 sample does lose about 4% over the 0−250 °C interval. This loss, evident in the TGA data of Figure 4, is due to a small amount of water of hydration of the ILBr (∼3 molecules of water per ILBr). The TGA data of Figure 4 illustrate several other phenomena.

MMA was used as received, with MEHQ inhibitor, in the reference microemulsion. The sample was heated isothermally at 60 °C for 8 h. A single sharp and exothermic peak was recorded that peaked at about 40 min incubation, had substantially decayed by about 90 min, and was essentially completely decayed by 180 min (see Supporting Information). The curve was typical for microemulsion polymerization and differs from the classic emulsion three-stage curve that exhibits a monomer reservoir depletion plateau.64 Integration of the exothermic peak yielded the heat of polymerization (ΔHp) for this microemulsion formulation, −54.4 kJ/mol. This enthalpy has contributions from both MMA and ILBr “homopolymerization” sequences as well as ...MMA-ILBr and ...ILBr-MMA cross-“copolymerizations”. ILBr homopolymerization was similarly done. A 15% ILBr in water solution was prepared with 0.5% V-50 initiator with respect to monomer and placed in the sample cell. An equivalent amount of 15% ILBr in water without added initiator was placed in the reference cell. The sample was heated isothermally at 60 °C for 12 h, and a similarly sharp and exothermic peak was recorded. Integration of this curve yielded a ΔHp of −71.7 kJ/mol. This value is within the range of −67.0 to −81.8 kJ/mol reported for various acrylate esters.65 While we do not have actual sequence distribution data, and light cross-linking (see sequel) prevented molecular weight distribution analysis, a simple compositional model indicates these measured heats are self-consistent. Experimental measurements of PMMA polymerization indicate an enthalpy of about −50 kJ/mol.66 The mole fraction of ILBr is 0.28. We assume that all of the ILBr is in isolated 1-tracts, surrounded on each side by MMA, and that the enthalpy of such bonds formation is −61 kJ/mol, an average of the respective homopolymerization values. In this model the mole fraction of ILBr-MMA and MMA-ILBr bonds is 0.56 (x − 61 kJ/mol = −34 kJ/mol), and the remaining bonds are due to MMA-MMA homopolymerization (0.44 × −50 kJ/mol = −22 kJ/mol). This yields −56 kJ/ mol, in reasonable agreement with our measured −54.4 kJ/mol. The thermal decomposition data (illustrated in Supporting Information Figure SI-1) reflect the approximate weight fractions of ILBr and MMA. In these samples the loss of water is first depicted, followed by apparent ILBr decomposition, and then by apparent MMA decomposition. The ILBr decomposition appears to be about 30% in each of the samples, consistent with the composition. MMA propagates 40-fold more slowly than dodecyl acrylate (DA),67 and we might consider DA a suitable model acrylate for ILBr (although a terminal methyl group is quite different than a terminal methylimidazolium bromide group). However, reactivity ratios rMMA (kMMA,MMA/kDA,MMA) and rDA (kDA,DA/kMMA,DA) in MMADA copolymerizations are about 3 and 0.38, respectively, and the impact of the very high acrylate homopolymerization propagation rate is not seen in copolymerization until low mole fractions of MMA in the available monomer pool. Such values argue against de facto block formation and in fact favor MMA contiguous tract formation.68 Reactivity ratios rMMA and rac of 2.7 ± 0.6 and 0.32 ± 0.11, respectively, have been determined by analysis of a mixture of methacrylate/acrylate pairs in copolymerization.69 The molar ratio of ILBr to MMA at the beginning of polymerization is 0.078/0, 0.078/0.1, 0.078/0.2, 0.048/0.3, 0.078/0.5, 0.078/0.6, and 0.078/0.7 respectively for the water/MMA weight ratios of 70/0, 60/10, 50/20, 40/30, 30/40, 20/50, 10/60, and 0/70. Using sequence probability equations from Odian (eqs 6−16

Figure 4. DSC analysis of poly(ILBr-co-MMA) + water sample gels at constant ILBr (30% w/w); (water/MMA) weight ratios: ■ (60/10); ● (50/20); ▲ (40/30); ▼ (30/40); ⧫ (20/50); ▶ (10/60); ◀ (0/ 70). The symbols are not data points but are used to simply help distinguish one curve and composition from another. E

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water, the sharp spikes occur at about −2 to −2.5 °C. These sharp peaks then, using the equation72,73

Each of the decomposition curves (see Figure SI-1) shows that there are essentially two components. The first component, appearing almost constant, is due to the decomposition of the ILBr over 275−325 °C, and the second wave at higher temperature, 350−425 °C, is attributed to decomposition of MMA. This assignment is very well supported by TGA data for a series of poly(ILBr-co-MMA) copolymers polymerized in the absence of solvent (see Supporting Information). An exception is in the case of the 70/0 sample that contains no MMA contribution. In this case the decomposition above 325 °C, and a fraction of the decomposition for the other compositions, is due to a slightly less than 10% component of the ILBr. About half this amount remains as ash at 525 °C. The only decompositions that track fairly closely to their respective compositions are those for the 0/70 and 10/60 compositions. The decomposition for the 0/70 sample has an approximately 30% wave for ILBr and an approximately 70% wave for MMA. The 10/60 sample also exhibits an ILBr loss of about 30% and a loss of about 60% for the MMA. In addition, a 10% loss due to added water is nearly quantitatively realized as a slowly accumulating loss over 25−275 °C. The other decompositions are more quantitatively skewed by loss of about 10−16% water in sample preparation prior to heating. This skewing tends to suggest that the weight fractions of ILBr and MMA are artificially higher than added to the original compositions. It is noteworthy, however, that each of these 20/50 to 70/0 compositions exhibit an increasingly long plateau over the 100−275 °C interval and an increasingly steep approach to this plateau as water content increases. The conductivity data suggested that the 20/50 composition is where bicontinuity begins as water fraction increases in the parent microemulsions, and at and above this composition in the polymerized materials we have a continuous water phase. The polymer network is continuous in all the polymerized compositions. This plateau-like behavior is characteristic of hydrogels, and has also been seen in poly(acrylamide-co-MMA) gels.71 The DSC data of Figure 4 show that the 0/70, 10/60, and 20/50 compositions appear perfectly flat. This behavior is expected for the 0/70 composition that had no added water, except ILBr hydration water. These data, therefore, indicate that up to 20% water is incorporated as tightly bound water that is nanoscopically (most likely molecularly) dispersed and is frustrated from freezing/melting by the detailed nature of this molecular dispersion. The 20/50 sample, however, exhibits the greatest, though moderate, turbidity in Figure 3 (top). We tentatively assign this turbidity to some nanoscopic phase separation of the surfactant ILBr in the polymerized MMA. The DSC for the 30/40 composition shows the existence of some water that has depressed melting, by virtue of a broad tail extending to −15 °C with a spike at about −1 °C, and a more pronounced tail extending up to about 10 °C. The 40/30 sample shows a broad and depressed peak at about −10 °C in addition to a spike at about −1 °C and a more normal water hump at 0 °C and above. With increasing water content, the 50/20 and 60/10 samples have increasing amounts of melting point depressed water and “normal” water. The data illustrate “normal” water, bound water (producing no endothermic behavior), and water with depressed melting behavior. Using the bulk water peaks and shoulders to estimate the equilibrium peak positions relative to the equilibrium freezing point of bulk

r (nm) =

−64.67 + 0.57 ΔTm

where ΔTm is the melting point depression of water, correspond to 50−60 nm pores. The pronounced depressed peak at −10 °C (corrected to −11 to −12 °C) corresponds to 13−15 nm pores. We doubt that the illustrated turbidity is due to any of these water-based pores, but due to some other aspect of nanophase separation such as chain aggregation. The higher melting bulk water corresponds to water in larger micro- and macropores. Spinodal Decomposition. Polymer samples were treated with aqueous 0.1 M KPF6 in order to study the effects of ion exchange between Br− and PF6− in ILBr (Figure 3, bottom). Previous work has shown that treatment of a hydrogel obtained by polymerizing an ILBr/MMA/water microemulsion (0.15/ 0.10/0.75) with 0.1 M KPF6 resulted in the formation of an open-cell porous network.24 This pore formation is governed by a spinodal decomposition process, wherein ion exchanged polymer strands become insoluble in water and condense on themselves to form pore (cell) walls.74 Light cross-linking60 keeps this nonequilibrium phase separation localized in and around pores rather than becoming a macroscopic phase separation process. The result is separation into two phases one continuous in polymer and the other continuous in waterforming an open-cell, interconnecting porous network. The onset of opacity was noticed in the transparent samples, with the exception of the 10/60 and 0/70 compositions, after overnight equilibration in the aqueous KPF6 solution. After 3 days of equilibration, the samples had reached their maximum opacity, as judged by the eye. The 70/0 and 60/10 samples showed the greatest opacity. The 10/60 and 0/70 compositions had only a very slight haze on the outer surface compared to the opaque white appearance of the 70/0 and 60/10 compositions. Selected SEM images of the compositions are shown in Figures 5 (70/0 to 40/30), 6a (30/40), 6b (20/50), and 6c,d (10/60 and 0/70). SEM analysis of fresh freeze fracture surfaces of samples before and after KPF6 treatment confirmed pore formation in the treated compositions. Porosity appears to decrease as the weight ratio of ILBr to MMA decreased (increasing MMA and decreasing water). This is logical, as the ion exchange from poly(ILBr) to poly(ILPF6) is the driving force behind the local phase separation causing pore formation and the aqueous volume distributed within the polymerized microemulsions steadily decreases. In addition, the fraction of water, relative to total water, occluded within the MMA steadily increases. As the weight fraction of MMA increases, the depth of penetration of the pores from the outer surfaces decreases. In the lower MMA weight fraction compositions, pores were densely grouped and in most cases covered the entire fracture surfaces observed. The pores appear random in distribution and in size. This randomness is to be expected because no templating or template is involved, as might be the case if particles of uniform size were used to template pores. In this system the resulting pore size and shape are a consequence of the kinetics of spinodal decomposition, and these kinetics depend in part on diffusion-reaction processes on different time and length scales. The anion exchange is diffusion controlled as the gels are transformed into porous materials. Long-term studies of thin samples show that “unstructured pores” can be F

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such chains eventually reorganize ultimately into porous cell walls. The pores observed in the 70/0 composition ranged from 140 nm to 3 μm in approximate diameter. The 60/10 composition, the first containing MMA (10.5% w/w), showed isolated regions of limited porosity prior to ion exchange, ranging from 820 nm to 1.4 μm. Ion exchange yielded pores on the order of 500 nm to 8.0 μm, giving this sample the largest pores observed in this series of compositions. The next sample, 50/20, also showed a mostly smooth surface with wrinkling due to the vacuum within the sample chamber. Treatment with 0.1 M KPF6 created a network of pores with diameters ranging from 77 nm to 4.4 μm. Sample 40/30 did not initially show porosity. However, its surface roughness was more severe than the earlier discussed samples. Once ion exchange was completed, pores were observed on its outer surface as well as on its fracture cross-section surface. Pore sizes of 117 nm to 3.7 μm were observed, with the pore diameters decreasing as distance from the outer rod surface increased along the fracture surface. The rod with composition 30/40 gave the first indications of microphase separation believed responsible for the turbidity observed in the rods spanning the middle portion of this 30% ILBr compositional line (see Figure 6a). The outer edges of rod 30/40 appeared to be smooth along the fracture surface, with the center of the rod displaying a region of jagged structures (see Figure SI-6a,c). Ion exchange resulted in pore formation located mostly at the rod outer surface, with pore size rapidly decreasing with distance inward from this surface. The observed pore diameters ranged from 30 nm to 5.0 μm. The rod with composition 20/50 showed similar surface inhomogeneity; the central area of the rod fracture cross section contained jagged structures radiating outward toward the edges of the rod (see Figure SI-7). The outermost portions of the rod interior were smooth. Unlike the 30/40 rod, the jagged center of rod 20/50 contained limited pores on the surface of the structures The pores observed were on the order of 45 nm in diameter. Treatment with aqueous KPF6 yielded a very limited number of new pores, with those observed ranging from 30 nm to 2.2 μm (Figure 6b). Rod 10/60 showed surface roughness as well, but it was not as dramatic as the two previous compositions in this series. There appeared to be less phase separation, which is supported by the transparent visual appearance of the rod after polymerization (see Figure SI-8a). Pores were not visible after ion exchange in images taken at 32000× magnification (Figure 6c). The 0/70 composition showed surface roughness similar to the 10/60 sample, but was also visually transparent after polymerization (see Figure SI-8c). Ion exchange resulted in very isolated pockets of pores ranging from 190 nm to 1.7 μm in diameter (Figure 6d). The electrical conductivity titration data of Figure 2 allow us to break the compositional series examined here into two subseries: (1) prebicontinuous and (2) bicontinuous. The second series extending from 70/0 to 20/50 is at first bicontinuous in water and ILBr (70/0) and thereafter bicontinuous in water and ILBr/MMA (10/70 to 20/50). As the MMA weight fraction increases, the porosity evident after poration steadily decreases. The pores in the 30/40 sample appear the most irregular after KPF6 treatment, and those in the 20/50 sample appear very few in number. The opacity and turbidity of these samples still soaking in aqueous 0.1 M KPF6 (lower part of Figure 3) is greatest for the 70/0 and 60/10 compositions, much lower and about the same in the 50/20,

Figure 5. SEM images of freeze fracture surfaces of polymerized samples before (left) and after (right) ion exchange: (a, b) 70/0; (c, d) 60/10; (e, f) 50/20; (g, h) 40/30.

Figure 6. SEM images of freeze fracture surfaces of samples after treatment with 0.1 M KPF6: (a) 30/40; (b) 20/50; (c) 10/60; (d) 0/ 70. Images of freeze fracture surfaces prior to treatment with KPF6 are given in Figures SI-6, SI-7, and SI-8.

dramatically transformed further, when left to equilibrate (Figure SI-10). This second time scale entails diffusion controlled anion exchange in condensed polymer chains, but G

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Figure 7. Cartoon illustrating thermally initiated polymerization of bicontinuous microemulsion while preserving nanoscale structure. Center lower cartoon illustrates an MMA pseudophase (magenta online) stabilized by an ILBr monolayer (lime green online) interdigitated with the aqueous pseudophase (white). Upper left and right cartoons with stick structures of ILBr and MMA illustrate pseudophases before and after thermally initiated polymerization, respectively.

40/30, and 30/40 samples, and slightly greater in the 20/50 sample. After polymerization and prior to illiciting the stimulus response with KPF6 (upper part of Figure 3), the 70/0, 60/10, and 50/20 compositions are transparent, and then the turbidity slowly increases in the order 40/30, 30/40, and 20/50. The greatest turbidity seen after polymerization in the 20/50 sample is consistent with this composition being a “boundary point” in the transition from bicontinuous to reverse microemulsion water-in-oil nanostructures in the precursor microemulsions. While this transition is a continuous (or second order) transition (first order transitions within a single-phase microemulsion domain are thermodynamically excluded),75,76 compositional fluctuations among the mixture of droplet and irregular bicontinuous volume elements would be expected to be maximal at such a transition point. The very recently published SANS study of the composition indicated by the open triangle in Figure 1 was the first demonstration that nanoscale structure in polymerized bicontinuous microemulsion systems could be substantially captured using thermal initiation.60 That long sought after accomplishment was accompanied by two observations: (1) The SANS local scattering maximum related to the pseudophase repeat distance increased only 20% upon polymerization. (2) The resulting gel obtained after polymerization was essentially as transparent as the starting microemulsion. This correlation establishes optical clarity as a marker for nanoscale capture, and in this contribution we see that such nanoscale capture is also obtained in three additional compositions 70/0, 60/10, and 50/20. Substantial capture was also obtained for the 40/30 and 30/40 compositions. These qualitative observations show that the chemical hydrophilic/hydrophobic balancing provided by the ILBr in matching water and MMA in that previous study60 is substantially more widely applicable over additional compositional intervals. The results obtained outside of the apparent bicontinuous compositions, namely for the 20/50, 10/60, and 0/70 compositions, illustrate additionally important results. The SEM of Figure 6b shows that the porosity is substantially reduced further, as expected. This SEM also appears the first to

illustrate the partial capture of percolating droplet and irregular bicontinuous nanostructures. The 10/60 composition yielded essentially no noticeable turbidity after soaking in aqueous KPF6 (Figure 3), and the field of view in Figure 6c reveals only a single pore. The accompanying lack of poration may be due to the ion exchange reaction being highly activated, perhaps as a result of ILBr/MMA copolymerization in a way that precludes extensive poration. The 0/70 composition without added water, on the other hand, appears similarly in Figure 3 as the 10/60 composition, but very regular cylindrical pores are evident in Figure 6d. This geometrically regular pore structure suggests that substantial oligomers of ILBr-co-MMA adopt cylindrical packing motifs, so that upon lengthy exposure to aqueous KPF6 the oligomers are driven to retract to the apparent “cell walls” to produce cylindrical pores. These appear to be the first regularly cylindrical pores induced by an anion exchange process without the benefit of poragen or template of any kind. The 70/0 sample (rod) prior to ion exchange, when placed in deionized water, swelled to a diameter in excess of 1 cm from an initial diameter of about 4 mm. This 6−7-fold increase in volume illustrated that de facto cross-linking existed in this material. Other potentiometric and NMR analyses have suggesteded this cross-linking comes from a 1,3-bis(11acryloyloxyundecyl)imidazolium bromide moiety, obtained as an approximately 4 mol % impurity in the ILBr synthesis (see Supporting Information).60 This cross-linking is significant because it provides a simple explanation for the spinodal decompositions illustrated here. The microemulsion polymerization produces transparent to slightly turbid hydrogel network materials. Transformation from gel to open cell porous material is accomplished by throwing a solubility switch. The hygroscopic imidazolium bromide is transformed into a hydrophobic imidazolium hexafluorophosphate by ion exchange. This cross-linking also contributes to capturing nanoscale structure in these materials prior to treatment with aqueous KPF6. Since the hygroscopic imidazolium bromide is on an undecyl tether, the solubility transition nucleates H

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hydrophilic to hydrophobic by using the anion PF6− to induce this significant stimulus response. This switching has been identified at other compositions in the same ternary system at 15% ILBr as well as in ILBF4 systems. In the cases here where bicontinuous gels were transformed, we may conclude it was by a spinodal decomposition process of local phase separation. Other cases illustrated here at much lower water content exhibit poration also, but these pores are based on ILBr nanostructures within a larger volume fraction PMMA continuous phase. The mostly regular cylindrical pores obtained in the (0/70) composition (Figure 6d) are the first to be obtained by a simple ion exchange process. Percolation and transport in microemulsions have been extensively studied since some of the earliest studies of microemulsion structures, particularly structural organization in reverse microemulsions and the ensuing transformations to bicontinuous nanostructures.90−93 It was shown at the turn of the century that such transformations in nanostructure are thermodynamically defined as complex supramolecular equilibria among reverse micromulsion droplets, strings of such droplets, irregular bicontinuous structures, and other components.76 This was done by producing experimentally derived order parameters and showing their differentiability across compositional transition intervals. Here we see in Figure 6b an SEM that provided the best image to date (based on experimental microscopy) of the gradual transition from percolating droplets to bicontinuous nanostructure. The turbidity illustrated for this (20/50) composition in Figure 3 (top) shows that some features and length scales have been exaggerated relative to those present in the microemulsion prior to polymerization. It is clear that interphase distances, based on apparent droplet or particle dimensions, are of the order of 50−60 nm, 10-fold larger than observed in a transparent polymerized composition.60 We therefore, without equivocation, interpret these observed length scales (Figure 6b) as upper bounds to the lengths scales (interphase distances, correlation lengths) that exist in the precursor microemulsion. A useful design principle for obtaining network gels that may be deduced from this study is to formulate so that the resulting polymer formed is highly solvated in water (or the major nonpolymerizable solvent). Another useful principle is to incorporate small to moderate cross-linking. These factors are key to capturing nanoscale structure during thermally initiated polymerization. If cross-linking is present, any subsequent desolubilization response can only be expressed locally, between cross-links. These principles offer alternative approaches to the design and synthesis of porous membranes and monolithic materials, and we are hopeful of successfully applying these principles in the design of other stimuli responsive networks and materials.

condensation of individual monomer chains in addition to oligomeric segments. The PF6− anion was used in this study because it was the strongest acting stimulus-response inducing anion we were aware of at the time. Extensive studies of model nanolatexes of similar composition have yielded the following Hofmeister series for the strength of interaction with imidazolium cations:61 PF6− > BF4 − > (CN)2 N− > I− > Br − > S2 −

Other popular anions for imidazolium-based IL and PIL such as (CF3SO2)N− have not yet been investigated in this microemulsion-based system. The bis(2-ethylhexyl) sulfosuccinate anion (from the well-known Aerosol-OT sodium salt) has been found to drive poration in gels derived from ILBF4/MMA/ aqueous propanol microemulsions.77,78 It was earlier postulated60 that capturing nanoscale structure in microemulsion and in mesophase polymerization is aided by three empirical principles: (1) using a polymerizable surfactant or surfmer;52,79−89 (2) providing sufficient degrees of freedom so that surfactants can polymerize without destroying interfacial packing;85 (3) increasing the ratio of polymerization rate to the microemulsion/mesophase structural reorganization rate.52,87,88 In this network gel system the inherent solubility (and solvation) of the resulting polymer in water and the number of degrees of freedom in the undecyl tether in addition to those of the growing backbone mitigate against structural reorganization. While solvation of nanogels61,79 also mitigates against flocculation, as has been seen for ILBr and related systems, in bicontinuous systems solvation appears very important in avoiding microphase separation.

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CONCLUSIONS

The polymerizable ionic liquid surfactant, 1-(11-acryloyloxyundecyl)-3-methylimidazolium bromide (ILBr), was used to produce stimuli responsive network gels at various compositions in the 30% ILBr, water, MMA ternary system. Several of these compositions (gels) were completely transparent, suggesting that nanoscale dimensional length scales were retained during polymerization, even though thermal initiation was used (Figure 7). These transparent compositions extend the composition space yielding the capture of nanoscale structure so effectively. The recently documented composition at 0.15/0.10/0.75 that previously was shown to be transparent23 was also documented by SANS to increase in correlation length by only 20% after thermally initiated polymerization.60 These compositions, and that illustrated for the ILBF4/MMA/aqueous propanol system,74 are the only reported bicontinuous microemulsions to be polymerized by thermal initiation and to retain nanoscale structure similar to that of the precursor micromeulsions. All other attempts to date have yielded microscale to macroscale phase separation, as evidenced by reported opacity accompanying the thermally initiated polymerization. These compositions also illustrate important hydrophile/hydrophobe balancing principles and also exemplify the empirical principles 1 and 2 articulated above in the Results and Discussion. Ion exchange between Br− and PF6− in ILBr resulted in the formation of open-cell and closed-cell porous structures in several of these materials, which was confirmed by SEM. Pore structures in other compositions were also observed. Poration was achieved by switching the polymer “solubility” from

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ASSOCIATED CONTENT

S Supporting Information *

Figures SI-1 to SI-10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.T.). Present Addresses

† D.E.: Specialty Coating Systems, 7645 Woodland Drive, Indianapolis, IN 46278.

I

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F.Y.: Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was substantially supported by ONR Grant Award N00014-04-1-0763.

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