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Synthesis and mechanical properties of poly(styrene-b-isobutylene-b-styrene) block copolymer ionomers.

INTRODUCTION

Ionomers are polymers that possess a primarily hydrocarbon backbone and about 12.0 mol% or less of structurally attached ionic groups. Traditionally, ionomers possessing a random ionic microstructure have been the most thoroughly investigated. These ionomers are typically produced by copolymerization of a functionalized monomer with an unfunctionalized monomer or by direct functionalization of a preformed polymer.

In addition to random ionomers, ionic polymers possessing a non-random ionic microstructure have also been reported. End-functionalized or telechelic ionomers composed of a polybutadiene and/or poly-isoprene backbone and carboxylate end-groups have been synthesized by anionic polymerization (1, 2) or free radical polymerization (3). Polyisobutylene (PIB)based, sulfonate telechelic ionomers possessing either a broad (4-6) or narrow molecular weight (7) distribution between ionic groups have been produced by carbocationic polymerization in conjunction with derivatization (8) of [Alpha],[Omega]-difunctional polyisobutylenes.

Another type of ionomer microstructure is based upon block copolymers. Block copolymer ionomers typically possess a linear ABA or star-branched block architecture where the outer blocks (A blocks) contain ionic moieties, and the inner (B) block is a nonionic hydrocarbon segment. For example, using anionic polymerization, Storey et al. (9) have synthesized and characterized star-branched block copolymer ionomers consisting of a mid-block of poly(ethylene-co-butylene) and short end-blocks (about 4 to 12 units) of sulfonated polystyrene (PS).

A closely related system, also based upon the rubbery poly(ethylene-co-butylene) mid-block and polystyrene end-blocks, is sulfonated poly(styrene-b-ethylene-co-butylene-b-styrene) (S-EB-S) synthesized and extensively characterized by Weiss et al. (10-14). These polymers were prepared by lightly sulfonating commercially available S-EB-S using acetyl sulfate as the sulfonating reagent. They differ from the ionomers synthesized by Storey et al. in that they possess long, lightly sulfonated as opposed to short highly sulfonated polystyrene blocks. These block copolymer ionomers were reported to possess a unique three-phase morphology that consists of ionic domains within glassy polystyrene domains that are dispersed in a rubbery matrix.

In both the Storey and Weiss systems, the saturated mid-block was produced by hydrogenating polybutadiene of appropriate microstructure. Sulfonation of these block copolymers occurs predominantly at the phenyl rings of the polystyrene blocks, but may also occur at any residual double bonds present in the rubbery block. In fact, the latter sites are more reactive towards the sulfonating reagent and thus, represent defects in the pure block copolymer structure.

Being interested in the synthesis of well-defined polymers based on polyisobutylene, as well as the physical properties of ionomers, we have synthesized, characterized, and examined the physical properties of poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymers and their corresponding sulfonate ionomers. Unlike S-EB-S block copolymers, sulfonation of PS-PIBoPS block copolymers can take place only at the phenyl rings of the polystyrene blocks; thus, these block copolymer ionomers may potentially possess a more well-defined ionomeric structure than S-EB-S block copolymer ionomers.

EXPERIMENTAL

Synthesis of PS-PIB-PS Block Copolymers

The synthesis of linear and three-arm star PS-PIB-PS block copolymers possessing equivalent compositions on a per arm basis have been described elsewhere (15-17).

Synthesis of PS-PIB-PS Block Copolymer Ionomers

Sulfonate ionomers of PS-PIB-PS block copolymers were prepared using the procedure of Makowski et al. (18). A representative sulfonation was as follows: Into a 500 ml round-bottomed flask equipped with a magnetic stir-bar were charged 15 g of a three-arm star radial PS-PIB-PS block copolymer (37,400 g/mol PIB, 12,600 g/mol PS) and 200 ml methylene chloride. Acetic anhydride, 1.04 g (0.0102 mol), was added to the stirred solution and stirring was continued for 10 min, after which time 1.00 g sulfuric acid (0.0102 mol) was added. The reaction was allowed to proceed for 24 h at 42 [degrees] C with stirring. The lower acetyl sulfate layer was discarded, and the organic layer was steam stripped by pouring the solution into vigorously stirring, boiling deionized (DI) water. The white, crumb-like material was collected by filtration, washed with DI water several times, kneaded in acetone to remove the water and residual acetic acid, and vacuum dried at 60 [degrees] C for 3 h.

The concentration of sulfonic acid groups in sulfonated PS-PIB-PS was determined as follows: To a tared 250 ml Erlenmeyer flask equipped with a magnetic stir-bar were charged 2 g of sulfonated block copolymer in 100 ml THF and a few drops of phloxine B solution in ethanol. The solution was titrated with 0.05 N tetramethylammonium hydroxide (TMAH) solution in ethanol. Standardization of the TMAH was accomplished using a 0.1 N aqueous solution of HCl. The precise mass of polymer titrated was determined by evaporating off most of the THF/ethanol in an oven at 50 [degrees] C and vacuum-drying the flask at 60 [degrees] C until a constant weight was reached. The mol% of styrene units that were sulfonated was deduced from the titration value and the mass fraction of styrene in the copolymer, determined by 1H NMR.

Large-scale preparative titrations were performed in essentially the same manner as described above with the following exceptions: 1) the concentration of sulfonated PS-PIB-PS was approximately 0.05 g per ml of THF; 2) the titration temperature was increased to 50 [degrees] C to maintain solubility; 3) the ethanolic titrant differed according to the counterion desired; and 4) the neutralized block copolymer ionomer was isolated by pouring the solution into a 1000 ml PTFE resin kettle and evaporating the solvents in an oven at 50 [degrees] C for two days followed by drying in vacuo for another 3 days at 60 [degrees] C.

Film Preparation

Block copolymer ionomer films were prepared by compression molding between polytetrafluoroethylene (PTFE)-coated metal plates in a temperature controlled hydraulic press for 1 or 24 h at 180 to 200 [degrees] C and 15,000 psi.

Instrumentation

Tensile properties were measured using an MTS Model 810 Universal Test Machine. Microdumbbell samples possessing a gauge length of 11.00 mm and a width of 1.54 mm were stamped from films ranging in thickness from 0.25 to 1.0 mm. A 100 lb load cell with a 100 lb load range cartridge and a 10 in displacement cartridge were used in conjunction with a 1 mm/sec continuous strain rate to obtain tensile data.

Dynamic mechanical (DMA) spectra were obtained using a Seiko model SSC/5200H dynamic mechanical spectrometer equipped with a DMS 210 tension module. Rectangular samples possessing gauge lengths of 20 mm and widths in the range of 0.5 mm to 0.8 mm were used for testing. Spectra were obtained by holding the frequency constant at 1 HZ and sweeping temperature.

Thermogravimetric analysis (TGA) was preformed on a DuPont 9900 thermal analyzer equipped with a 951 Thermogravimetric Analyzer module. Temperature scans on 5 to 20 mg samples were performed at a heating rate of 10 [degrees] C/min under a nitrogen atmosphere.

RESULTS AND DISCUSSION

Synthesis of PS-PIB-PS Block Copolymer Ionomers

The polymers chosen for this study were a carefully matched pair of block copolymers, one linear and one three-arm star. As shown in Fig. 1, the molecular weights of the polystyrene (PS) blocks were the same for both polymers (4000 g/mol), as was the molecular weight of the polyisobutylene (PIB) "arm," i.e., the PIB segment extending from the center of the molecule to a PS block (12,300 g/mol). This provided each sample with the same composition, i.e., the same volume fraction of PS, and the same molecular weight PIB segment (24,600 g/mol) connecting two PS blocks. The synthesis of these two polymers by living carbocationic polymerization has been described elsewhere (16). The formation of ionomers from these block copolymers involved first the sulfonation of a fraction of the phenyl rings present in the polystyrene blocks using acetyl sulfate, followed by an analytical titration of a portion of the sulfonated sample to determine the extent of sulfonation, and then neutralization of the balance of the sample with a stoichiometrically equivalent amount of the appropriate ethanolic metal hydroxide or metal acetate solution.

Tensile Properties of PS-PIB-PS Block Copolymer Ionomers

Figure 2 shows representative stress-strain curves for the two block copolymer precursors. It may be seen that the three-arm star block copolymer showed much higher engineering stress than the linear block copolymer, at strains of 3 and higher; yet the ultimate strains at break for the two polymers did not differ greatly. This result is reasonable since each molecule of the three-arm star would be expected to possess one additional point of attachment to the network relative to the linear polymer; however, the span length of the elastomeric segment, which determines ultimate elongation, is approximately the same in both samples. It was, however, unexpected that the three-arm star block copolymer would exhibit such high strength considering its relatively short polystyrene blocks.

When ionic groups were incorporated into the polystyrene blocks of either block copolymer, tensile strength increased while elongation at break decreased. The average values of tensile results are listed in Table 1 for all the block copolymer ionomers; each value represents the average of approximately six sample pulls. The composition of each ionomer is identified in the column "Sample ID" using a sample identification system consisting of a number at the far left indicating the number of arms in the polymer (2 = linear, 3 = three-arm star), followed by the chemical symbol of the counterion, and a number at the right indicating the mol% of styrene repeat units sulfonated. The designations 2-PS-PIB-PS and 3-PS-PIB-PS correspond to the unsulfonated linear and three-arm star block copolymer precursors, respectively.
Table 1. Tensile Data for PS-PIB-PS Block Copolymer Ionomers.

 Film
 Formation
 Time (h)
 180 [degrees] C, 15,000 Stress at Strain at
Sample ID psi Break (MPa) Break (%)

2-PS-PIB-PS 1 8.5 572
2-Mg-8 1 9.0 411
2-Mg-8 24 8.5 363
2-Zn-8 1 13.0 524
2-Zn-8 24 13.0 498
2-K-23 1 13.0 449
2-Zn-23 1 10.5 293
3-PS-PIB-PS 1 17.0 696
3-K-15 1 20.0 531
3-K-15 24 19.0 354
3-Mg-15 1 19.0 603
3-Zn-15 1 25.0 635
3-Zn-15 24 25.0 572


The tensile properties show that the zinc ionomers were the strongest materials studied. The higher strength imparted by zinc counterions has been observed by others and has been attributed to the presence of outer-shell d-electrons of zinc that form stronger, more covalent interactions with sulfonate anions (11, 19).

A high degree of variability in tensile measurements was observed for the block copolymer ionomers as compared with their unsulfonated precursors. To illustrate this point, four separate microdumbbell samples of the three-arm star precursor, 3-PS-PIB-PS, yielded tensile strength and elongation values within the narrow ranges of 16 to 18 MPa and 6.7 to 7.4, respectively. In contrast, six separate tensile specimens of 3-K-15 yielded values in the much broader ranges 15-31 MPa and 4.4-7-4, respectively. Thus, for the ionomer, the range of tensile strengths broadened and the average tensile strength increased by inclusion of samples with much higher strengths; while at the same time, the range of elongations broadened and the average elongation decreased by inclusion of samples with much lower elongations. The Young's modulus remained constant for both the ionomer and the precursor. The high degree of variance in tensile properties of the block copolymer ionomer can be attributed to a lower degree of processability, and thus a higher incidence of defects in these materials as compared with unsulfonated block copolymers. When viewed through a light microscope, compression-molded block copolymer ionomer films were found to be heterogenous. A number of imperfections in the film such as air bubbles were apparent. This result indicates that the tensile measurements obtained were influenced not only by the physical properties of the material but also by their processability. Attempts to solution-cast block copolymer ionomer films largely failed because of poor surface film formation and the low solubility of some ionomers.

Dynamic Mechanical Properties

The viscoelastic properties of PS-PIB-PS ionomers were found to be different from those of their unsulfonated precursors. Figure 3 illustrates the storage modulus (E[prime]), Fig. 4 the loss modulus (E[double prime]), and Fig. 5 tan /5 as a function of temperature for linear, zinc-based PS-PIB-PS ionomers with two different levels of sulfonation (8 and 23 mol%), compared with their unsulfonated block copolymer precursor. The dual phase morphology of unsulfonated PS-PIB-PS is revealed by the peak in E[double prime] at approximately -65 [degrees] C, which corresponds to the [T.sub.g] of the PIB matrix as well as a peak at about 80 [degrees] C corresponding to the [T.sub.g] of the PS domains.

The loss modulus and tan [Delta] data in Figs. 4 and 5, respectively, show that the [T.sub.g] of the PIB matrix remained the same, while the [T.sub.g] for the PS domains decreased from approximately 80 [degrees] C to 55 [degrees] C for the block copolymer ionomer possessing an 8.0% sulfonate content and to 45 [degrees] C for the ionomer possessing a 23.0% sulfonate content. The PS [T.sub.g]s for the ionomers were weak and difficult to resolve; however, the [T.sub.g] at 55 [degrees] C for the Zn-8 ionomer can be best seen in the tan [Delta] curve [ILLUSTRATION FOR FIGURE 5 OMITTED], and the [T.sub.g] at 45 [degrees] C for the Zn-23 ionomer is most visible in the E[double prime] curve [ILLUSTRATION FOR FIGURE 4 OMITTED]. The reduction in PS domain [T.sub.g] was attributed to the formation of an ill-defined, kinetically controlled domain structure, with considerable phase-mixing of PIB into the sulfonated PS, resulting from transient crosslinking and hindered mobility of the PS segments caused by the presence of ionic groups. In addition, a peak at approximately 180 [degrees] C in the E[double prime] spectrum of the 23.0% sulfonate containing polymer was attributed to the [T.sub.g] of ionic domains since these are the only domain structures that could be present at this high temperature. Weiss et al. (10) have also observed an ionic domain [T.sub.g] at this temperature for lightly sulfonated atactic PS ionomers.

As illustrated in Fig. 3, ion incorporation was found to cause an increase in E[prime] in the region below the [T.sub.g] of PS with its magnitude in this region increasing as a function of increasing ion content. In addition, ion incorporation resulted in a complex rubbery plateau above the PS [T.sub.g] that was not observed in the unsulfonated precursor. The presence of an upward inflection in E[prime] in the temperature region just above the polystyrene domain [T.sub.g] indicated that a domain reorganization occurred as a result of the thermal energy provided by the experiment. The occurrence of this reorganization process showed that the morphology of the unheated sample (original, compression molded sample) was a nonequilibrium, kinetically controlled morphology. In the temperature region between the [T.sub.g]s of PIB and PS, i.e., [approximately]60-100 [degrees] C, the observation of a higher elastic modulus in the block copolymer ionomer samples as compared with that of the parent block copolymer also indicated that the ionomer samples were in a nonequilibrium morphology, since, as will be seen shortly, an equilibrium microphase-separated morphology (attained by annealing) consisting of partially sulfonated PS domains in the PIB matrix produced the same elastic modulus as that of the unsulfonated precursor.

Further support for the formation of a nonequilibrium, kinetically controlled morphology was obtained by comparison of the viscoelastic properties of block copolymer ionomers prepared using a 24 vs. 1 h compression molding time at 180 [degrees] C and 15,000 psi. As illustrated in Fig. 6, which is representative of the general behavior, the use of longer molding times re-suited in a reduction of the rubbery plateau modulus to the point where it was essentially an extension, to higher temperatures, of the plateau modulus of the corresponding unsulfonated block copolymer precursor. Elimination or suppression of the upward inflection in E[prime] that was observed for the samples that were compression molded for only 1 h, indicates that the samples were in a state much nearer to the thermodynamically favored, equilibrium morphology.

The effect of annealing was also demonstrated by conducting a DMA experiment on 1 h compression molded 2-Zn-8, in which temperature was cycled several times between -120 and 250 [degrees] C. Figure 7 illustrates the DMA spectra obtained for the first and second heating of this sample. It can be seen that the inflection in the modulus observed during the first heating through the [T.sub.g] of PS was absent in the second heating, indicating that a more thermodynamically stable equilibrium morphology was obtained as a result of thermal and mechanical energy provided during the experiment.

The fact that annealing the ionomers produced an E[prime] response identical to that of the block copolymer precursor below the PS [T.sub.g], and that further heating of the ionomer yielded the same modulus to significantly higher temperatures, indicates that the microphase-separated morphology produced by annealing consists of discrete, partially sulfonated PS domains of the same density and size as the PS domains of its unsulfonated precursor, with the ionic domains fully contained within these PS domains.

Block copolymer ionomers of higher ion content (23 mol%) were observed to reorganize to a morphology different from that of block copolymer ionomers of lower (8.0%) ionic content. Figure 8 displays the results of a DMA experiment (E[prime]) in which the sample 2-Zn-23, formed using a 1 h compression molding time, was analyzed by first scanning upward in temperature to 250 [degrees] C (first pass), scanning back down to -120 [degrees] C (second pass), and then scanning back up to 250 [degrees] C (third pass).

On the first pass the ionomer displayed a relaxation process, attributed to the presence of ill-defined, phase-mixed PS domains, that started at approximately 40 [degrees] C and caused E[prime] to descend to the level of the rubbery plateau modulus of the unsulfonated block copolymer precursor. This transition can be more readily observed in the tan [Delta] curve shown in Fig. 9. Once the PS domains passed through this transition, allowing for motion, a large upward inflection in E[prime] was observed, indicating a reorganization of these domains. Upon further heating, a transition associated with ionic domains was observed at approximately 160 [degrees] C as shown in the E[double prime] spectrum [ILLUSTRATION FOR FIGURE 10 OMITTED]. As indicated in the E[prime] spectra of the second and third passes, the thermal energy that provided for domain reorganization resulted in a morphology characterized by a plateau modulus more than an order of magnitude higher than the rubbery plateau modulus of the unsulfonated precursor, indicating a higher crosslink density for the annealed block copolymer ionomer. As illustrated in Figs. 9 and 10, a relaxation process at approximately 65 [degrees] C was still observed in the second and third temperature sweeps; however, it is much less prominent than that in the first sweep.

These results indicated that block copolymer ionomers of higher ion content that possess lower chain mobility caused by increased intermolecular ionic interactions, cannot assume an equilibrium morphology characterized by microphase-separated sulfonated PS domains of similar size and density as those of their unsulfonated precursor. Rather, these less mobile polymers assume a morphology upon annealing characterized by a high plateau modulus and a depressed P$ domain [T.sub.g] indicative of a more phase-mixed, and less discrete domain morphology than that of their unsulfonated precursors or analogs of lower sulfonate content.

It may be seen from Fig. 6 that the block copolymer ionomers did not show a terminal zone in modulus until approximately 370 [degrees] C. The sharpness of the decrease in modulus in this temperature region coupled with the physical appearance of the samples after the test, indicated that most of the samples were essentially degrading instead of entering a regime of viscous flow. However, it should be noted that these samples did form films when compression molded and thus did exhibit viscous flow at 180 [degrees] C, under conditions of high pressure (15,000 psi). The need for high pressure and long times to enable flow is in accord with a flow mechanism involving repetitious breaking and reforming of ionic associations, which has been termed "ion-hopping" (20).

As mentioned previously, solution casting of block copolymer ionomers largely failed; however, solution-cast films were obtained from the zinc ionomers with lower ion contents, i.e., 2-Zn-8 and 3-Zn-15, by slow evaporation of 5 to 10% solutions of the ionomer in THF/methanol (95/5, v/v) contained in PTFE-lined aluminum pans. Figure 11 shows E[prime] as a function of temperature for solution-cast 3-Zn-15 using several passes in which the temperature was cycled between -120 and 250 [degrees] C. Solution-cast 2-Zn-8 showed similar behavior as an as-cast film; it was not, however, subjected to multiple passes. In Fig. 11, it may be observed that E[prime] of the solution-cast film, within the rubbery regime below the [T.sub.g] of the PS domains, was similar in magnitude to the moduli of compression molded films of high ion content [ILLUSTRATION FOR FIGURE 8 OMITTED], and considerably higher than those of compression molded films of low ion content [ILLUSTRATION FOR FIGURE 7 OMITTED]. However, the second and third heating passes in Fig. 11 show the establishment of a reversible PS domain [T.sub.g] that was not observed in the compression molded films; clearly such a [T.sub.g] is not visible in the second heating pass for sample 3-Zn-23 in Fig. 8 and for 2-Zn-8 in Fig. 7. This suggests that solution casting and compression molding produce completely different morphologies, which cannot be readily interconverted by thermal annealing.

Thermal Stability

The thermal stability of the linear PS-PIB-PS block copolymer and its tetramethylammonium ionomer were measured under a [N.sub.2] atmosphere by thermo-gravimetric analysis, as reported in Fig. 12. The mid-point of the major mass-loss process of the parent block copolymer was observed at 417 [degrees] C, while that of its tetramethylammonium ionomer at 431 [degrees] C. Weiss et al. (11) found that sulfonation of a similar styrenic block copolymer, S-EB-S, resulted in approximately a 100 [degrees] increase in thermal stability from 300 [degrees] C for the unsulfonated block copolymer to 400 [degrees] C for the Na ionomer. The large increase in thermal stability upon sulfonation of S-EB-S can be attributed to the presence of residual double bonds in the mid-block resulting from incomplete hydrogenation of the butadiene-based precursor. These double bonds are a source of thermal instability, but show high reactivity toward sulfonation, even higher than that of aromatic rings. Thus, the exceptionally high increase in thermal stability upon sulfonation can be attributed to a scavenging of double bonds in the polymer mid-block. The much more modest increase in thermal stability upon sulfonation of PS-PIB-PS reflects the already higher stability of the fully saturated polyisobutylene mid-block.

CONCLUSIONS

The incorporation of sulfonate groups into the polystyrene blocks of PS-PIB-PS block copolymers was found to greatly affect the mechanical properties of these materials. Ion incorporation significantly increased tensile strength while decreasing elongation at break for compression molded films. Zinc counter-ions were found to produce the highest strength materials. In addition, ion incorporation resulted in the persistence of the rubbery plateau in the dynamic tensile modulus well above the [T.sub.g] of polystyrene, attributable to the presence of ionic interactions.

The strong ionic interactions present in these materials caused significant processing problems. Solvent casting largely failed, while compression molding produced films containing defects. Ionic interactions hindered the formation of an equilibrium morphology, which is thought to be characterized by discrete, partially sulfonated polystyrene domains of roughly the same density and size as the polystyrene domains of the unsulfonated precursor. It was found that ionomers possessing the monovalent potassium counter-ion were less resistant to flow than ionomers containing the divalent magnesium or zinc counterion.

Nonequilibrium morphologies were evidenced by a decrease in polystyrene domain [T.sub.g] and an upward inflection in the dynamic tensile modulus above the [T.sub.g] associated with partially sulfonated polystyrene domains. The reduction in polystyrene [T.sub.g] upon ion incorporation was attributed to considerable phase mixing resulting in incorporation of the rubbery matrix into the partially sulfonated polystyrene domains, while the upward inflection in dynamic tensile modulus above this [T.sub.g] was attributed to domain segregation resulting from the thermal energy provided by the experiment. Further evidence for the formation of a nonequilibrium morphology was provided by either annealing samples using longer compression molding times or by conducting DMA experiments using multiple heating and cooling cycles. Annealing of samples containing lower levels of sulfonation was found to result in equilibrium morphologies as evidenced by a dynamic tensile modulus response identical to that of the block copolymer precursor below the polystyrene [T.sub.g] and the fact that further heating yielded the same modulus to significantly higher temperatures.

Block copolymer ionomers with higher ion contents were observed to possess a higher rubbery modulus than those with lower ion contents, even after annealing. The modulus values of the materials of low ion content were similar to those of the unsulfonated precursor. Solution-cast ionomers of low and intermediate ion contents (zinc counterions) exhibited unique viscoelastic properties after annealing, characterized by a higher rubbery plateau in the temperature range between the [T.sub.g]s of PIB and PS, similar in magnitude to that observed for compression molded samples of high ion content, and a lower modulus above the [T.sub.g] of PS, the value of which was intermediate between those of compression-molded ionomers with high and low ion contents.

ACKNOWLEDGMENTS

The research upon which this material is based was supported by the National Science Foundation through Grant No. EHR-9108767, the State of Mississippi, and the University of Southern Mississippi.

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Author:Storey, Robson F.; Chisholm, Bret J.; Lee, Youngkwan
Publication:Polymer Engineering and Science
Date:Jan 1, 1997
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