Blocky Sulfonation of Syndiotactic Polystyrene: A Facile Route toward

Mar 17, 2017 - The thermal properties and crystallization behavior of a matched set of blocky and random SsPS copolymers containing 3 and 10 mol % sul...
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Blocky Sulfonation of Syndiotactic Polystyrene: A Facile Route toward Tailored Ionomer Architecture via Postpolymerization Functionalization in the Gel State Gregory B. Fahs, Sonya D. Benson, and Robert B. Moore* Department of Chemistry, Macromolecules Innovation Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: Blocky sulfonated syndiotactic polystyrene (SsPS) copolymers were produced using a recently developed postpolymerization functionalization procedure conducted in the gel state. The thermal properties and crystallization behavior of a matched set of blocky and random SsPS copolymers containing 3 and 10 mol % sulfonate groups were compared using differential scanning calorimetry (DSC), which shows that the blocky functionalization architecture displays a much faster rate of crystallization even at low sulfonate contents and a higher crystallizability at high sulfonate contents. The glass transition temperature for gelstate-functionalized copolymers was found to be independent of sulfonic acid content above 4% sulfonation, consistent with behavior observed in previous studies of sodium styrenesulfonate block copolymers. Small-angle X-ray scattering (SAXS) from the blocky copolymer indicates that the sulfonated units are distributed within the amorphous interlamellar domains. Wide-angle X-ray diffraction (WAXD) shows that the blocky architecture facilitates the formation of the β form polymorph of sPS.

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INTRODUCTION Ionomers are polymers that generally contain less than 15 mol % of ionic functionality incorporated into or pendant to a hydrophobic polymer backbone through copolymerization or postpolymerization modification of a homopolymer.1−3 Because of the interesting properties of these materials, governed by electrostatic interactions, great attention has been given to understanding the effect of ionic content and architecture on the thermal, mechanical, solution, and morphological properties of these technologically important materials. Sulfonated atactic polystyrene (SaPS) has been one of the most widely studied ionomers, with numerous reports on the thermal,4−11 mechanical,12−17 solution,18−25 and morphological26−38 properties of SaPS. These ionomers have been prepared via copolymerization of styrene with sodium styrenesulfonate and more commonly by postpolymerization sulfonation of preformed PS homopolymer. Weiss and coworkers conducted studies that compared the properties of SaPS prepared via emulsion copolymerization and postpolymerization modification.39,40 They attributed differences in the thermal and solution properties of SaPS prepared by these two methods to differences in the distribution of sulfonate groups along the hydrocarbon backbone. On the basis of dissimilarities in the glass transition versus ion content behavior of the materials, Weiss and co-workers concluded that their copolymerization of styrene and sodium styrenesulfonate resulted in the incorporation of the sulfonated comonomer as blocks of sulfonate groups along the polymer backbone rather © XXXX American Chemical Society

than a random placement of the sulfonate groups along the polymer backbone for the SaPS ionomer prepared by the conventional postpolymerization modification. Although SaPS, an amorphous ionomer, has been studied in great detail, many of the industrially important ionomers such as Nafion and Surlyn are semicrystalline in nature, and the morphologies of semicrystalline ionomers are complex threephase systems made up of an ion-rich phase, crystalline domains, and an amorphous matrix. Given the complex interactions that can exist in these materials, it has been shown that the process of ionic aggregation significantly influences the crystallization process of semicrystalline ionomers as one polymer chain may traverse all three phases of the semicrystalline ionomer.41 As a model semicrystalline ionomer, Orler and co-workers developed a method to produce sulfonated syndiotactic polystyrene (SsPS)41−44 using a mild postpolymerization sulfonation procedure similar to that used by Makowski to sulfonate aPS.45 Although SsPS may be considered as a model semicrystalline ionomer, it was found that incorporation of as little as 2 mol % of sulfonate groups onto sPS significantly decreased the ability of SsPS to crystallize in the ionized form.42 Such a pronounced decrease in the ability of a polymer to crystallize at low ion contents may be a challenge in utilizing Received: February 24, 2017 Revised: March 7, 2017

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DOI: 10.1021/acs.macromol.7b00408 Macromolecules XXXX, XXX, XXX−XXX

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It is important to note that other groups have reported heterogeneous functionalization reaction schemes for semicrystalline polymers. Polyethylene powders have been suspended in chlorinating reagents, in an unswollen state, to achieve a blocky functionalization.62−64 Borriello and coworkers employed a heterogeneous reaction scheme in the sulfonation of cast or compression-molded sPS films to create materials suitable for absorbing volatile organic compounds from water.65 In this work, the authors proposed that during this solid-state reaction a uniform sulfonation of the amorphous phase is obtained, while the crystalline phase remains unfunctionalized. However, since this solid state or “bulk” functionalization method involves an interplay of diffusion and reaction processes, it was shown for compression-molded films that this procedure created a gradient of sulfonation from the interior to the exterior, similar to that observed earlier for aPS films.66 For cast films, a more uniform sulfonation across the film thickness was observed, along with a noticeable decrease in crystallinity with increasing sulfonation. More recently, Borriello and co-workers created sulfonated aerogels of sPS by exposing bulk gel specimens to chlorosulfonic acid.67 This procedure is similar to that described in this report; however, sulfonation throughout the bulk specimen (6 mm by 35 mm) could be nonuniform compared to small (submillimeter) gel particles. While this study clearly demonstrated preferential sulfonation of the amorphous component in the semicrystalline gels, no evidence was presented to indicate the formation of a blocky architecture that could preserve crystallizability in subsequent processing. Our current study is focused on exploiting the gel-state morphology to template the functionalization chemistry as a means to create a blocky architecture along the polymer backbone that preserves crystallizability.

SsPS in applications where high ion content and a high degree of crystallinity are desirable (e.g., in membrane applications). For example, a proton exchange membrane for fuel cell applications requires high ion content for the transport of water and ions through the membrane.46−49 A proton exchange membrane must also possess mechanical stability to withstand harsh fuel cell operation conditions and low solvent swelling behavior as well.50 The mechanical integrity necessary for operation in harsh environments such as elevated temperature with low swelling may be derived from a membrane containing a reasonable crystalline component.51 Consequently, it would be desirable to produce a semicrystalline ionomer with a degree of sulfonation that is appropriate for proton conduction and a high degree of crystallinity.52 Crystallinity has been linked to improved barrier properties and decreased membrane swelling in the presence of solvents.53 Additionally, crystalline domains have been proposed to act as physical cross-links that enhance mechanical properties.54 In order to produce systems with both a high ion content and high crystallinity, blocky ionomer architectures are being investigated. McGrath and co-workers have shown that block ionomers possess unique and advantageous morphologies that enhance proton conductivity and hydrolytic stability against swelling over random or statistical copolymers.55 For multiblock poly(aryl ether sulfone) (PAES) copolymer systems, the hydrophilic, sulfonated PAES segments provide the ionic character, which is necessary for proton conduction, while the hydrophobic poly(arylene ether) segments lends hydrolytic stability.55 Moreover, the creation of multiblock copolymers with long runs of hydrophilic and hydrophobic segments facilitates the formation of microphase-separated microstructures, which have been shown to enhance proton conduction. The increased proton conductivity of the multiblock system, relative to the random analogue BPSH-35 copolymer membrane, was attributed to enhanced connectivity of the hydrophilic regions within the hydrophobic matrix55 and directly related to the ionic content and the hydrophilic/ hydrophobic block lengths.56 Furthermore, in a similar study, block copolymers were found to exhibit greater proton conductivity than their random copolymer analogues when studied in the partially hydrated state.57,58 In order to take to take advantage of these superior morphology−property relationships, there is great interest in finding experimentally simple and economically desirable methods to produce these block ionomer architectures. Recently, we have developed a postpolymerization method to sulfonate syndiotactic polystyrene to yield a blocky distribution of the sulfonate groups along the polymer backbone.59 This method is based upon conducting the sulfonation process while sPS is in the solvent-swollen, semicrystalline gel state. sPS gels can be formed from a variety of solvents, which forms a physical network consisting of both crystalline and solvent swollen amorphous domains.42,60,61 During the course of the reaction, the sulfonating reagent is sterically excluded from the crystalline domains and thus is only capable of reacting with the solvent-swollen amorphous chain segments in the semicrystalline gel network. Our working hypothesis is that this gel-state reaction creates a blocky architecture, which inherently preserves long sequences of unsulfonated (and thus crystallizable) sPS homopolymer segments separating runs of sulfonated SsPS units along the polymer chains.

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EXPERIMENTAL SECTION

Materials. Syndiotactic polystyrene (Questra 102) having a weightaverage molecular weight (Mw) of 310 000 g/mol was obtained from the Dow Chemical Company. Reagent grade 1,2,4-trichlorobenzene (TCB), methanol, sulfuric acid, and potassium monophthalate were obtained from Fisher Scientific. Hexanoic anhydride, benzyltrimethylammoniuim hydroxide, and benzoic acid were received from SigmaAldrich. Preparation of Sulfonation Reagent. A mild sulfonation reagent (hexanoyl sulfate) was prepared according to previously published procedures42 using TCB as the solvent. TCB was added to a volumetric flask containing 0.03 mol of hexanoic anhydride per milliliter of sulfuric acid. The TCB/hexanoic anhydride solution was cooled in an ice bath for 1 h, and then 1−10 mL (depending on the target degree of sulfonation) of concentrated sulfuric acid was added to the chilled solution and shaken vigorously to allow for complex formation. Additional TCB was then added to the fill the remaining space in the volumetric flask. Random Sulfonation of Syndiotactic Polystyrene. PS was dissolved in TCB under reflux for 1.5 h to yield a 10% w/v solution. After 1.5 h, the solution was cooled to 70 °C, and then additional TCB was added to yield a 1% w/v sPS solution. The sPS solution was allowed to equilibrate at 70 °C under a nitrogen purge for 1.5 h. After equilibration, the appropriate amount of sulfonating reagent was added, and the reaction was allowed to proceed for 1 h at 70 °C under a nitrogen purge. After 1 h, 10 mL of methanol was added to the solution to terminate the reaction. The solution was poured into a large excess of methanol (∼2 L) to precipitate the copolymer, followed by filtration and washing with deionized water. For the random copolymers containing higher than 5 mol % sulfonation, ethyl ether was used to precipitate the polymer instead, and water was used for the entire washing process in order to avoid dissolution of the polymer into methanol. The random SsPS precipitates were filtered and washed B

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Figure 1. Diagram of solution-state and gel-state sulfonation postpolymerization methods. DSC analysis was performed under a continuous nitrogen flow to minimize sample degradation. The glass transition, crystallization temperature, and melting temperatures of the SsPS3.2H+Random and SsPS3.2H+Blocky copolymers were determined from the second DSC scan, after erasing the thermal history, using the TA Instruments Universal Analysis Software. Isothermal crystallization was carried out for the low sulfonic acid content SsPS copolymers. TA Instruments Universal Analysis software was used to analyze the resulting DSC traces. The crystallization half-time, t1/2, the time at which the material reaches 50% of its maximum crystallinity, was obtained from isothermal scans at various temperatures and was used as a measure of the crystallization rates of the random and blocky sulfonated SsPS copolymers. The thermal transitions of the high ion content ionomers were evaluated from the DSC heating scan, post-isothermal crystallization. Small-Angle X-ray Scattering. SAXS experiments were performed using a Rigaku S-Max 3000 3 pinhole SAXS system, equipped with a rotating anode emitting X-rays with a wavelength of 0.154 nm (Cu Kα). The sample-to-detector distance was 1605 mm, and the qrange was calibrated using a silver behenate standard. Twodimensional SAXS patterns were obtained using a fully integrated 2D multiwire, proportional counting, gas-filled detector with an exposure time of 2 h. All SAXS data were analyzed using the SAXSGUI software package to obtain radially integrated SAXS intensity versus

with deionized water repeatedly until all remaining traces of TCB, sulfonating reagent, and methanol were removed. The resulting polymer was dried under vacuum at 100 °C for 12 h. Blocky Sulfonation of Syndiotactic Polystyrene. Syndiotactic polystyrene was dissolved in TCB under reflux for 1.5 h to yield a 10% w/v solution. After 1.5 h, the solution was cooled to room temperature. Upon cooling, the 10% w/v sPS solution was allowed to remain at room temperature for 24 h. During this cooling process, the sPS solution crystallizes and forms a physical gel. The solid gel was manually broken in small, submillimeter-scale particles using a spatula. The sPS gel particles were diluted in additional TCB to yield a 1% w/v sPS suspension. The sPS gel-particle dispersion was allowed to equilibrate at 70 °C under a nitrogen purge for 1.5 h. After equilibration, the appropriate amount of sulfonating reagent was added, and the reaction was allowed to proceed for 1 h at 70 °C under a nitrogen purge. No visible change in the solvent-suspended gel particles was observed during the course of the reaction. After 1 h, 10 mL of methanol was added to the dispersion to terminate the reaction. The dispersion was poured into a large excess of methanol in order to precipitate the polymer. The SsPS particles were filtered and washed with deionized water repeatedly until all remaining traces of TCB, sulfonating reagent, and methanol were removed. The resulting polymer was dried under vacuum at 100 °C for 12 h. Determination of Degree of Sulfonation. The randomly and blocky sulfonated SsPS copolymers were redissolved in 95/5 v/v TCB/methanol for 1.5 h to a yield 0.5% w/v solution. These 0.5% w/v solutions were used to determine the degree of sulfonation via nonaqueous titration with methanolic benzyltrimethylammonium hydroxide. The benzyltrimethylammonium hydroxide was standardized using benzoic acid. SsPS ionomers of low sulfonic acid content and high ion content were prepared. Low sulfonic acid content SsPS containing 3.2−3.5 mol % sulfonate groups are identified as SsPS3.5H+Random and SsPS3.2H+Blocky, where the H+ denotes the copolymers are in the acid form. The Random and Blocky nomenclature denotes homogeneous functionalization of SsPS in solution and functionalization of SsPS in the gel state, respectively. High sulfonic acid content SsPS random and blocky copolymers were also prepared for this study containing 9.2 and 10.5 mol % sulfonate groups, respectively. These copolymers are identified as SsPS9.2H+Random and SsPS10.5H+Blocky. Differential Scanning Calorimetry. Thermal behavior and crystallization kinetics of the SsPS copolymers were studied using differential scanning calorimetry (DSC). A TA Instruments Q2000 DSC was used to probe the thermal behavior of SsPS. Dried powders of the sulfonated copolymers and thin film samples with controlled thermal history were analyzed. Thin film samples were prepared for these experiments by thermally pressing each sample between Kapton sheets at 200 °C and 3000 psi for 3 min using a Carver laboratory press. The SsPS films were allowed to cool to room temperature under ambient conditions after removal from the press. Samples from the thermally pressed films were die cut and placed within aluminum DSC pans. The weight for each sample was maintained between 6 and 8 mg.

scattering vector q, where q =

( 4λπ ) sin(θ),

θ is one-half of the

scattering angle, and λ is the X-ray wavelength. Wide-Angle X-ray Diffraction. WAXD experiments were performed using a Rigaku MiniFlex II X-ray diffractometer emitting X-rays with a wavelength of 0.154 nm (Cu Kα). Samples were scanned from 5° to 35° 2θ at a scan rate of 0.25° 2θ/min and a sampling window of 0.050° 2θ at a potential of 30 kV and current of 15 mA. All WAXD data were analyzed using the PDXL 2 software package to obtain WAXD intensity versus 2θ profiles. WAXD profiles were vertically shifted to facilitate a comparison of the peak positions. Preparation of X-ray Scattering Samples. Films were prepared by thermally pressing each sample between Kapton sheets at 200 °C and 3000 psi for 3 min using a Carver laboratory press. The SsPS films were allowed to cool to RT under ambient conditions after removal from the press. Samples from the thermally pressed films were die cut and placed within aluminum DSC pans. The weight for each film was maintained between 6 and 8 mg. Samples were heated from 50 to 330 °C at 10 °C/min and held at 330 °C for 5 min. Samples were then rapidly quenched to the desired isothermal crystallization temperature, and isothermal crystallizations were carried out at 220, 225, 230, and 235 °C for 60 min for WAXD analysis and 200 °C for 2 h for SAXS analysis. After isothermal crystallization the films were quenched to room temperature at the maximum cooling rate. All samples for SAXS analysis were prepared to have a degree of crystallinity of approximately 20%; DSC scans of heat flow versus temperature are shown in Figure S1 of the Supporting Information. C

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Figure 2. Relative heat flow versus temperature of sPS homopolymer (H), SsPS3.5H+Random (R), and SsPS3.2H+Blocky (B) copolymers. (a) Samples were precipitated from TCB, annealed at 150 °C for 2 h, and then quenched to room temperature prior to scanning at 20 °C/min. (b) Samples were isothermally crystallized from the melt at 200 °C for 2 h prior to scanning at 20 °C/min. All thermograms have been vertically offset to facilitate comparison.

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RESULTS AND DISCUSSION The strategy for producing a blocky SsPS ionomer architecture is depicted in Figure 1. A homogeneous solution of sPS will crystallize upon cooling the solution from elevated temperature to room temperature. Over time, the number and size of the crystalline domains increase until a physical gel is formed from the initial homogeneous solution. The physical network within these gels consists of the crystallites (acting as multifunctional physical cross-links) that are linked together by solvent-swollen amorphous chain segments (acting as ties chains). The presence of crystalline domains that establish the gel structure provides a means by which to preserve sequences of unsulfonated material when the sulfonation process is conducted within the gel state. During this gel-state sulfonation process, the sulfonation reagent (represented by the red circles in Figure 1) is not able to penetrate within the crystalline regions. Thus, the sulfonation reagent is only able to react with the amorphous chain segments spanning the solvent-swollen domains between the physical cross-links. Consequently, we hypothesize that “blocks” of partially sulfonated sPS chains are created, leaving long sequences of unsulfonated sPS homopolymer isolated within the crystallites. In contrast, it can be seen in Figure 1 that conducting the sulfonation process while sPS is a homogeneous solution leads to a “statistical” or random placement of sulfonate groups along the polymer backbone with comparatively shorter sequences of unsulfonated material. The aforementioned method of sulfonating sPS while in the gel state may provide a very simple postpolymerization modification approach to create a blocky placement of sulfonate groups along the polymer backbone. The thermal properties, crystallization behavior, and morphologies of these materials are investigated to elucidate the effect of these distinct sulfonation methods on ionomer architecture and properties. Thermal Analysis. DSC thermograms of the low sulfonic acid content SsPS copolymers SsPS3.5H + Random, SsPS3.2H+Blocky, and the sPS homopolymer are shown in Figure 2. The thermogram for the sPS homopolymer (Figure 2a) contains three thermal transitions: a broad endothermic event near 175 °C, followed by a small exothermic event at 190 °C, and then prominent melting endotherm at 270 °C. In the vicinity of 190 °C, solvent-treated sPS is known to undergo a transformation from a helical γ-form crystal structure to an alltrans α or β crystal form that melts near 270 °C.68,69 Since the

sPS homopolymer sample in this analysis was precipitated from TCB (consistent with the process used to isolate the SsPS copolymers), the observed thermal events below 200 °C are thus attributed to the helical to all-trans crystal transformation. Similar to the behavior of solvent treated sPS, the lightly sulfonated copolymers also show thermal transitions near 190 °C (Figure 2a), attributed to the helical to all-trans crystal transformation, and the melting of the α and/or β crystal polymorphs near 270 °C. It is interesting to note, however, that the exothermic event near 190 °C for the SsPS3.5H+Random sample is much more distinct than that observed for both the SsPS3.2H+Blocky and sPS homopolymer samples. During the time frame of this heating scan, we suspect that the reorganization event that establishes the all-trans crystal forms is delayed in the random copolymer69 (yielding a more pronounced separation of the overlapping endothermic and exothermic events) due to the close proximity of the interacting defects (i.e., the sulfonated styrene units) to the crystalline stems. Given the consequence of the sulfonate groups acting as structural defects along the sPS chains, it is not surprising that the melting point for both of the sulfonated copolymers during the heating scans (as shown in Figure 2a) is observed to be depressed from 270 °C (for the sPS homopolymer) to 260 and 266 °C for the SsPS3.5H+Random and SsPS3.2H+Blocky copolymers, respectively. Following isothermal crystallization at 200 °C for 2 h (Figure 2b), the melting point of the SsPS3.5H+Random copolymer is further depressed to 257 °C, while the melting point for the SsPS3.2H+Blocky copolymer remains at 266 °C (nearer to that of the sPS homopolymer at 272 °C). The higher melting point for the blocky copolymer suggests the presence of thicker crystalline lamellae relative to the random analogue. This behavior may be attributed to the gel state functionalization, which is expected to inherently preserve longer runs of crystallizable sPS units between the functional defects, and thus allow for the formation of thicker crystals during this isothermal crystallization process. The area under the melting endotherms of the data in Figure 2b was used to determine the overall degree of crystallinity, Xc, for these samples using the following relationship:

Xc = D

ΔHf ΔHf°

(1) DOI: 10.1021/acs.macromol.7b00408 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The value of ΔH°f for sPS is 82.6 J/g.70 The degree of crystallinity for the SsPS3.2H+Blocky copolymer (Xc = 35%) is nearly identical to that of the sPS homopolymer (Xc = 36%). In contrast, the degree of crystallinity of the SsPS3.5H+Random copolymer is significantly reduced (Xc = 26%). Although the thermal properties of the random and blocky copolymers are only moderately different at low degrees of sulfonation, our previous work has demonstrated that the crystallizability of random SsPS at ion contents above 3 mol % is severely limited.42 However, based on our working hypothesis that the gel-state functionalization can preserve crystallizability and effectively decouple the functional group aggregation phenomenon from the crystallization process, it is of interest to explore the impact of this prospective route to create blocky architectures at significantly higher degrees of sulfonation. Figure 3 compares the DSC thermograms of the

strong evidence for the blocky architecture stemming from the gel-state functionalization process, which preserves a significant quantity of long runs of crystallizable sPS units along the functionalized chains. Crystallization Kinetics. The effect of ionomer architecture (random versus blocky) on the crystallization behavior of the SsPS materials was evaluated using DSC isothermal crystallization experiments. The development of crystallinity within a matched set of SsPS3.2H + Random and SsPS3.2H+Blocky copolymers was monitored by applying the following relationship: Fc(t ) =

t dH dT ∞ dH 0 dT

∫0

dt

∫

dt

(2)

The expression given in eq 2 represents the bulk fractional crystallinity of the functionalized copolymer systems, Fc(t), and is equal to the heat evolved during isothermal crystallization at a specific time t divided by the total exothermic heat generated during the entire crystallization process. Figure 4 displays plots of Fc(t) versus ln time for isothermal crystallization temperatures of 220, 225, 230, and 235 °C for the SsPS3.2H+Random and SsPS3.2H+Blocky copolymers. Comparing the isotherms of the SsPS3.2H+Random and SsPS3.2H+Blocky copolymers, it is evident that both copolymers are crystallizing on a comparable time scale for the lower crystallization temperatures. For higher crystallization temperatures, however, the two systems diverge where the SsPS3.2H+Blocky copolymer is observed to crystallize significantly faster than the random analogue. Polarized light microscopy of spherulites growing at an isothermal crystallization temperature of 235 °C show (in Figure 4) that the SPS3.2H+Blocky copolymer yields a lower nucleation density relative to the SsPS3.2H+Random copolymer. This behavior may also be attributed to the blocky vs random architecture. The long runs of crystallizable sPS segments in the SPS3.2H+Blocky copolymer, that are well removed from the sulfonated units, are expected to have a relatively high mobility. In this nucleation-controlled regime, high segment mobility reduces the rate of primary nucleation. For the SsPS3.2H+Random copolymer, however, the shorter runs of crystallizable sPS segments, that are in close proximity to the interacting sulfonated units, are relatively restricted in their mobility. Consequently, the rate of primary nucleation is enhanced in the SsPS3.2H+Random copolymer. To quantify the differences in the rates of crystallization between the SsPS3.2H+Random and SPS3.2H+Blocky copolymers, crystallization half-times, t1/2, were extracted from each of the crystallization isotherms. In addition, a complete Avrami analysis of the crystallization isotherms is provided in the Supporting Information, included in Figures S2R and S2B as well as Table S1. It is clear from the data in Figure 5 that the crystallization half-times for the SPS3.2H+Blocky copolymer are significantly shorter (i.e., a faster rate of overall crystallization) than that for the SsPS3.2H+Random copolymer, especially at higher crystallization temperatures. The faster rates of bulk crystallization for the SPS3.2H+Blocky copolymer may be attributed to a higher population of long sequences of unsulfonated sPS runs between sulfonated units that more readily associate into crystallites compared to the random analogue containing a statistically lower population of long sPS sequences. Upon cooling from the melt, as the polymer chains attempt to pack into crystalline structures, sulfonic acid groups

Figure 3. Relative heat flow versus temperature of sPS homopolymer (H), SsPS10.5H+Blocky (B), and SsPSsdsd9.2H+Random (R) copolymers. All samples were isothermally crystallized at 200 °C for 2 h prior to scanning at 10 °C/min. All traces have been vertically offset to facilitate comparison.

sPS homopolymer to that of the highly sulfonated SsPS9.2H+Random and SsPS10.5H+Blocky copolymers, following isothermal crystallization. It is clear from these data that even after thermal annealing, the SsPS9.2H+Random copolymer is not able to crystallize and remains completely amorphous. In contrast, a large melting endotherm is observed at 244 °C for the SsPS10.5H+Blocky copolymer, indicative of the development of a high degree of crystallinity during the annealing period. This behavior demonstrates that the gel-state sulfonation process indeed preserves the crystallizability of the SsPS copolymer at a high degree of sulfonation and lends strong evidence (albeit indirect) supporting our hypothesis that the heterogeneous gel-state functionalization reaction yields a blocky architecture that conserves long runs of pure sPS units between functional units. In contrast, the homogeneous solution state functionalization yields a random architecture, with statistically shorter runs of pure sPS units between sulfonated units, which completely inhibits crystallization. Moreover, after isothermal crystallization at 200 °C for 2 h, the degree of crystallinity for the SsPS10.5H+Blocky copolymer is measured to be Xc = 23%. Surprisingly, this high degree of crystallinity for the blocky SsPS copolymer containing 10.5 mol % sulfonate groups is 66% of that for the pure sPS sample (Xc = 35%) crystallized under the same conditions. Again, this remarkably enhanced crystallizability of the SsPS10.5H+Blocky E

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Figure 4. Fc(t) versus ln time at 220, 225, 230, and 235 °C for (R) SsPS3.2H+R and (B) SsPS3.2H+B copolymers. Polarized light micrographs for (R) SsPS3.2H+R and (B) SsPS3.2H+B copolymers captured at 30 min during isothermal crystallization at 235 °C.

copolymer occurs despite a greater population of the slower crystallizing β-form71 relative to the kinetically favored α-form (see Figures S3 and S4). Glass Transition Temperature Analysis. As mentioned in the Introduction, Weiss and co-workers found that changes in the glass transition temperature with increasing sulfonate content could be used to distinguish between random and blocky architectures of sulfonated atactic polystyrene (SaPS) produced by the conventional postpolymerization route and an emulsion polymerization route, respectively. In their study, the random SaPS system showed a linear increase in glass transition temperature (Tg) with sulfonate content over all compositions, while the Tg of the blocky SaPS system was found to become independent of sulfonate content at high degrees of functionalization.40 Assuming that the intermolecular interactions that affect the glass transition (i.e., the aggregation of polar sulfonate groups yielding physical cross-links that restrict chain mobility)28 are operative in both atactic SaPS and the syndiotactic SsPS systems, it is of interest to see if a similar analysis would provide evidence of blockiness with the gel-state sulfonated SsPS. Figure 6 shows a plot of the glass transition temperature versus mol % sulfonation for both the random and blocky SsPS systems. Up to about 4 mol % sulfonation, both systems show a linear increase in the glass transition temperature with increasing degree of sulfonation. Beyond 4 mol % sulfonation, the random system continues to display the expected linear trend, while the blocky SsPS deviates significantly. In contrast to the random system, the glass transition temperature for blocky SsPS reaches at maximum at 104 °C at 4 mol %

Figure 5. Crystallization half-time versus crystallization temperature for SsPS3.2H+Random and SsPS3.2H+Blocky copolymers.

act as defects in the polymer chain and are rejected from the growing crystalline interface. With a homogeneous distribution of defects (as expected for the SsPS3.2H+Random copolymer), it is reasonable to suspect that the probability of encountering a “defective stem” diffusing to the crystal growth front is higher when compared to a blocky distribution of defects. Rejection of these encountered defects increases the time required for crystallization to occur and leads to longer crystallization halftimes, as observed for the SsPS3.2H+Random copolymer. Given the polymorphic nature of sPS, it is also of interest to note that the faster crystallization kinetics for the blocky F

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homopolymer and the SsPS10.5H+Blocky copolymer. Note that the SsPS9.2H+Random copolymer remained completely amorphous as shown in Figure 3. The SAXS profile for the SsPS10.5H+Blocky copolymer shows a prominent scattering maximum at q = 0.39 nm−1, which is attributed to an interlamellar long period of ca. 16 nm. As expected, the SAXS profile for the completely amorphous SsPS9.2H+Random copolymer is featureless within this q-range, consistent with the absence of stacked lamella. While the sPS homopolymer certainly contains crystalline lamella of a composition comparable to that of the SsPS10.5H+Blocky copolymer, the sPS SAXS profile also shows a featureless, monotonic decrease in intensity over this q-range. As demonstrated by Barnes and McKenna, this behavior is attributed to a lack of scattering contrast between the crystalline and amorphous components of sPS.72 The crystalline densities of the α and β forms are 1.033 and 1.067 g/cm3, respectively, while the density of amorphous sPS is 1.04 g/cm3.73 With an expected mixed composition of both α and β polymorphs in the isothermally crystallized sPS sample, it follows that the electron density contrast in sPS is very weak regardless of polymorphic composition. At elevated temperatures, however, a differential coefficient of thermal expansion between the amorphous and crystalline components yields sufficient contrast to observe a scattering maximum.72 On the basis of this fundamental principle of scattering contrast, we propose that the prominent scattering maximum observed at room temperature for the SsPS10.5H+Blocky copolymer is attributed to the presence of relatively electron dense sulfonate groups dispersed within the amorphous layers between the lamella. Upon crystallization, the sulfonate groups are concentrated within the amorphous domains, intercalated between the lamella, thus yielding a relatively high electron density contrast between the correlated amorphous and crystalline components of the SsPS10.5H+Blocky copolymer. If the sulfonate groups were somehow isolated to regions well removed from the crystalline domains, then the electron density contrast between the amorphous and crystalline layers would be expected to be identical to that of the homopolymer. A future publication concerning USAXS investigations will focus on the hierarchical ordering of crystallites and polar domains within the complex morphology of these semicrystalline ionomers.

Figure 6. Glass transition temperature versus mol % sulfonation of SsPS Random and SsPS Blocky polymers. Linear fit lines have been added as a visual guide.

sulfonation and then becomes independent of the degree of sulfonation at higher contents. Since quench rescans were used to obtain the Tg values in Figure 6, the observed behavior is a purely amorphous response, without influence from a crystalline contribution. As such, the long runs of unsulfonated sPS units along the chains of the blocky SsPS are expected to possess dynamics more comparable to that of amorphous chains in the parent sPS homopolymer (having a Tg near 100 °C). Therefore, in perfect agreement with the earlier results for SaPS, this comparison of the influence of sulfonate content on Tg between the random and blocky SsPS systems lends strong support for the blocky architecture of SsPS produced via the gel-state sulfonation process. Morphological Analysis. Small-angle X-ray scattering (SAXS) is used here to probe the ordering and scattering contrast of crystallites within the SsPS copolymers. Figure 7

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CONCLUSION We have demonstrated a very simple postpolymerization method to produce a blocky sulfonated sPS that preserves crystallizability in highly functionalized copolymers. By performing the sulfonation reaction in the gel-state, long sequences of pure sPS units within the crystallites are effectively shielded from the reactive reagents. The longer sequences of unsulfonated material of the blocky SsPS facilitates the ability of these crystallizable stems to pack efficiently into crystalline structures relative to that of the randomly sulfonated sPS analogue. At relatively low degrees of sulfonation (3 mol %), the blocky copolymer crystallizes significantly faster than that of the random copolymer. At high ion contents of 9.2 mol %, the randomly sulfonated ionomer is rendered totally amorphous due to the presence of homogeneously distributed defects along the polymer backbone. In contrast, however, the highly sulfonated SsPS10.5H+B copolymer is able to achieve a high degree of crystallinity during isothermal annealing. Additionally, further evidence has been presented showing that for gel-state functionalized copolymers the glass transition temperature

Figure 7. One-dimensional SAXS profiles for sPS homopolymer (H), SsPS10.5H+B (B), and SsPS9.2H+R (R) copolymers. All samples were isothermally crystallized at 200 °C prior to analysis. SAXS profiles have been vertically offset to facilitate comparison between curves.

contains SAXS profiles of the SsPS9.2H+Random and SsPS10.5H+Blocky copolymers in comparison to that of the sPS homopolymer. In order to provide a comparable volume of phase separated structures in this analysis, all samples were isothermally crystallized at 200 °C, prior to analysis. The crystallization time was controlled to yield an equivalent degree of crystallinity of approximately 20% for both the sPS G

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Macromolecules Notes

becomes independent of the degree of sulfonation above 4% sulfonation, which is consistent with previous studies conducted by Weiss and co-workers on sodium styrenesulfonate block copolymer systems. Enhanced lamellar SAXS scattering contrast of the crystallized blocky copolymer suggests that the polar sulfonate groups are distributed within the amorphous, interlamellar domains. It is important to note that the gel-state functionalization approach described here is not expected to be limited to sulfonation. Other chemistries, such as halogenation, are quite possible, provided the reaction is not inhibited by the solvent chosen to sustain the gel state. Halogenated copolymers produced from this facile approach will greatly broaden the scope of this methodology in creating precursory blocky architectures that could be further modified with a wide range of useful functionality. With respect to the morphological consequence of this gel-state blocky functionalization approach, it is also of interest to consider the technological benefit of developing a highly sulfonated polymer with high crystallizability. In proton exchange membrane fuel cells (PEMFC) applications, the crystalline component within the membrane can enhance mechanical durability and limit water solubility. However, conventional postpolymerization sulfonation used to create efficient membranes, such as sulfonated poly(ether− ether−ketone) (SPEEK), yields a random copolymer that profoundly prohibits crystallizability. To effectively decouple crystallization from functional group aggregation, blocky architectures created through controlled copolymerization methods are becoming state-of-the-art. With the facile approach described in this report, the process of crystallization within the blocky architecture also inherently increases the functional group concentration in the amorphous phase. This crystallization-induced concentration of the functional groups may then lead to enhanced aggregation and thus improved transport properties in applications such as PEMFC. Recently, we have discovered that this gel-state functionalization approach is applicable to new thermoreversible gels of poly(ether ether ketone),74 which extends this platform of facile blocky copolymer synthesis to materials (i.e., blocky SPEEK) ideally suited to the harsh chemical environments encountered in PEMFC operations.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants DMR-1507245 and DMR0923107. Sonya Benson gratefully acknowledges the financial support of Procter & Gamble through the Procter & Gamble Industrial Polymer Science Graduate Fellowship Award.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00408. Table S1: Avrami analysis parameters; Figure S1: DSC thermograms after isothermal crystallization; Figure S2: Avrami crystallization kinetics plots; Figure S3: X-ray diffraction data after isothermal crystallization to determine polymorphic composition; Figure S4: polymorphic composition following isothermal crystallization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(R.B.M.) Phone (540) 231-6015; Fax (540) 231-8517; e-mail [email protected]. ORCID

Gregory B. Fahs: 0000-0002-4400-6995 Robert B. Moore: 0000-0001-9057-7695 H

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