a environ 25 C. Cette valeur est tres semblable de celle observce anttrieurement par Otocka et Eirich dans leur ttude d'un

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1 Vinylpyridinium ionomers Influence of the matrix glass transition temperature on the dynamic mechanical properties of random acrylate-based systems1 DENIS DUCHESNE~ AND ADI EISENBERG~ Department of Chemistry, McGill Universify, 801 Sherbrooke St. W., Montreal, Que., Canada H3A 2K6 Received November 27, 1989 This paper is dedicated to Professor Arthur S. Perlin on the occasion of his 67th birthday DENIS DUCHESNE and ADI EISENBERG. Can. J. Chem. 68, 1228 (1990). The thermal and dynamic mechanical properties of random butyl acrylate- and plasticized ethyl acrylate-based vinylpyridinium ionomers have been investigated. The properties of the ionomers were found to be dependent on the glass transition temperature of the matrix material. Ionomers having a glass transition temperature lower than ca. 25 C exhibited all the features associated with the presence of phase-separated microdomains or clusters while the materials with higher glass transition temperatures were not. It was also observed that the dispersion associated with the vinylpyridinium clusters for a butyl acrylate-based ionomer with 12 mol% of ionic units occurs at ca. 25'C. This value is very close to that observed previously by Otocka and Eirich in their study of a butadiene-based vinylpyridinium ionomer with the same ion content. Key words: ionomers, plasticization, clustering, glass transition, dynamic mechanical properties. DENIS DUCHESNE et ADI EISENBERG. Can. J. Chem. 68, 1228 (1990). On a examint les proprictts dynamiques et mtcaniques d'ionomeres du vinylpyridinium bases sur de l'acrylate de butyle et de I'acrylate d'cthyle plastifit. On a trouvc que les proprittts des ionomeres dtpendent de la temperature de transition vitreuse du mattriel de la matrice. Les ionomeres posstdant une temperature de transition vitreuse inferieure a environ 25 C prtsentent toutes les caracttristiques assocites a la prtsence des microdomaines ou des agrtgats stparts par des phases; ce n'est pas le cas avec les mattriaux dont les temptratures de transition vitreuse sont plus tlevtes. On a aussi observt que la dispersion associte aux agrtgats de vinylpyridinium d'un ionomere bast sur I'acrylate de butyle et contenant 12 mol% d'unitts ioniques se produit a environ 25 C. Cette valeur est tres semblable de celle observce anttrieurement par Otocka et Eirich dans leur ttude d'un ionomhe de vinylpyridinium bast sur le butadiene et contenant la m&me quantitt d'ion. Mots elks : ionomeres, plastification, agrtgation, transition vitreuse, proprittts dynamiques et mtcaniques. [Traduit par la revue] Introduction In other publications of this series (1, 2), rheological evidence was presented to support the hypothesis that the formation of phase-separated microdomains or clusters does not occur in ethyl acrylate- and styrene-based vinylpyridinium ionomers. Indeed, the investigation of their dynamic mechanical properties has shown that the properties of the materials, aside from an increase in the glass transition temperature with increasing ion content, were unaltered by the incorporation of up to 24.2 mol% of vinylpyridinium units. This is in contrast with the results obtained by Otocka and Eirich (3, 4), for butadiene-based vinylpyridinium ionomers. Not only did they observe a large increase in the glass transition temperature of the materials with increasing ion concentrations, they also observed the appearance of a second inflexion point in the Young's modulus versus temperature curves and a decrease in the slope of the same curves in the transition regions. They concluded that all these modifications in the rheological properties of the butadiene-vinylpyridinium ionomers were most probably due to the formation of ionic aggregates and that these, according to their stress relaxation data, had a decomposition temperature of ca. 25 C. A comparison of all these data suggests that while the butadiene-vinylpyridinium ionomers appear to be clustered, cluster formation in the ethyl acrylate- and styrene-based vinylpyridinium ionomers does not occur, possibly because h his paper is based, in part, on Duchesne's Ph.D. dissertation "Structure-property relations of vinylpyridinium ionomers". McGill University, Present address: 3M Canada Inc., P.O. Box 5757, London, Ont., Canada N5A 4Tl. 3 ~ whom o all correspondence should be addressed. their glass transition temperatures are higher than the vinylpyridinium cluster decomposition temperature previously reported by Otocka and Eirich, or because no clusters exist in these styrene or ethyl acrylate ionomers. Another factor that could be invoked to explain the absence of clusters in the ethyl acrylate-based ionomers is the dielectric constant of the matrix, which is higher than that for the butadiene-based system. Eisenberg and co-workers (5) have compared the stress-relaxation behaviour of ethyl acrylate-sodium acrylate copolymers with that of styrene-sodium methacrylate copolymers and concluded that clusters become rheologically significant at higher ion contents in the former system. The threshold concentration values above which the clusters start to dominate the rheological behaviour were found to be 6 mol% and 12 mol% for the styrenic system and the acrylic system, respectively. Neppel et al. (6, 7), using Raman spectroscopy techniques, showed that the degree of solubility of multiplets is higher in the acrylates than in styrene. Hence, both the glass transition temperature and the dielectric constant of the matrix material seem to be parameters of utmost importance for cluster formation. Many studies, reviewed elsewhere (8-lo), have dealt with the influence of the ion structure as well as the dielectric constant of the matrix material on the formation of clusters. None, as yet, has specifically tried to establish the effect of the matrix glass transition temperature alone on cluster formation. In order to do this, the viscoelastic properties of the low glass transition temperature butyl acrylate-based vinylpyridinium ionomers as well as those of plasticized ethyl acrylate-based ionomers were investigated. The plasticizer, dimethyl malonate, was chosen to match the dielectric constant of the ethyl acrylate matrix so as to allow for the examination of the effect of one parameter alone, i.e., the glass transition. The results of

DUCHESNE AND EISENBERG this investigation will be compared with the results that already exist on butadiene- (3,4), styrene- (I), and ethyl acrylate-based (2) vinylpyridinium ionomers to obtain a

2 DUCHESNE AND EISENBERG this investigation will be compared with the results that already exist on butadiene- (3,4), styrene- (I), and ethyl acrylate-based (2) vinylpyridinium ionomers to obtain a better understanding of the effect of the matrix glass transition on cluster formation in vinylpyridinium ionomers. Experimental Materials Ethyl acrylate (Aldrich, 98%, inhibited with 200 ppmof monoethylether hydroquinone) and butyl acrylate (Aldrich, 98%, inhibited with 10-50ppm of monoethylether hydroquinone) were fractionally distilled under reduced pressure, discarding the first and last 10% of the distillate. The monomers were then stored under a nitrogen atmosphere at -20 C prior to use. 4-Vinylpyridine (Aldrich, 98%) was treated the same way. Benzoyl peroxide (Baker, 99%) benzene (Fisher, reagent grade), hexanes (A&C, technical grade), and methanol (A&C, technical grade) were used without further purification. Polymerization The copolymers were prepared by the free radical copolymerization in solution. The amount of initiator necessary to get the required polymerization rate and molecular weight was assumed to be the same as that for the monomer constituting the major part of the copolymer. For each mixture, the monomers were transferred through a double-tipped needle under flowing nitrogen into an ampoule and kept under nitrogen. Next, the solvent was weighed and the calculated amount of benzoyl peroxide (to get the desired molecular weight) was added and dissolved in the solvent. Once the catalyst was dissolved completely, the solvent-catalyst mixture was transferred through a double-tipped needle under flowing nitrogen into the ampoule already containing the monomers, and again kept under a nitrogen atmosphere. Each ampoule was immediately degassed by several freeze-thaw cycles under vacuum (lo-'rnrn Hg), sealed off under vacuum, and immersed in a thermostatted bath preset to 56OC or 60 C. The polymerization time varied with the nature of the comonomers and the composition of the mixtures. The conversions were kept low to give less than 30% compositional heterogeneity in the copolymers. Subsequently, the ampoules were opened and the copolymers isolated by precipitation in a 10 volume excess of hexanes. The gel-forming copolymers were soaked overnight in fresh nonsolvent. Following this step, they were removed from the non-solvent. To insure total dryness, all the copolymers were placed in a drying oven at 60 C under reduced pressure (1 mm Hg) until constant weight was reached. The details of the copolymerization runs are given in Tables 1, 2, and 3. Analysis The vinylpyridine content was determined by potentiometric titration using a solution of perchloric acid in glacial acetic acid according to the procedures described by Burleigh et al. (1 1). These results were further verified for the series of ethyl acrylate-4-vinylpyridine copolymers by elemental analysis. Variations between the different techniques were less than 0.2 mol%. Molecular weight determination An approximate molecular weight for at least one copolymer of each series was obtained by measuring their intrinsic viscosities, [q], in acetone solutions. The determinations were made at 25 C (BuA) and 30 C (EA) using a Ubbelhode viscometer. The values of K and a were taken as K = 6.8 x dl/g and a = 0.75 for the BuA (12) copolymers and 20 x lop5 dl/g and a = 0.66 for the EA copolymers (13). The intrinsic viscosities ranged from 0.7 to 1.5 dl/g corresponding to molecular weights of 2-6 x lo5 g-mol-'. Copolymer modificarion Materials Methyl iodide (A&C, reagent grade) was fractionally distilled under nitrogen and iil the absence of light a few hours before use. Tetrahydrofuran (THF) was dried by refluxing over potassium metal TABLE 1. Copolymerization conditions Bz202 T Copolymer X102M Solvent Ea-4-VP 0.78 benzene* 56 BuA-4-VP 2.8 benzene? 60 TABLE 2. Monomer feed, copolymer composition, polymerization time, yield, and intrinsic viscosity for ethyl acrylate-4-vinyl-pyridine (EA-4-VP) copolymers Vinylpyridine content in Monomer Copolymer Polymerization feed (mol%) (mol%) time, (min) Yield (%) [q] TABLE 3. Monomer feed, copolymer composition, polymerization time, yield, and intrinsic viscosity for butyl acrylate-4-vinly-pyridine (BuA-4-VP) copolymers Vinylpyridine content in Monomer Copolymer Polymerization feed (mol%) (mol%) time (min) Yield (%) [q] and benzophenone until the deep purple colour characteristic of the potassium-benzophenone complex persisted. It was then distilled directly into the flask containing the polymer. Absolute ethanol (loo%, Consolidated Alcohols Ltd.) and hexanes (A&C, technical grade) were used without further purification. Quarernizarion The vinylpyridinium copolymers were obtained by the reaction of the vinylpyridine copolymers with methyl iodide. The procedure given below is a combination of those described by Otocka and Eirich (3,4), Selb and Gallot (14), and Gauthier and Eisenberg (15). The 4-vinylpyridine copolymers were quaternized by adding a five-fold excess of methyl iodide to a boiling 3% copolymer solution. The mixture was then refluxed under nitrogen for 2 h. The final products had a yellow colouration most due to the presence of triiodide ions. The solvents in which the reactions were conducted. ethanol for the EA copolymers and THF for the BuA copolymers were chosen to avoid the precipitation of quaternized material during the reaction. The ionomers were recovered by precipitation into 10 volumes of stirred hexanes. The gel-forming materials were soaked overnight in the non-solvent and then dried in a vacuum over at 60 C for at least 24 h. Analysis The extent of quaternization of the ethyl acrylate- and butyl acrylate-based vinylpyridinium ionomers was determined by NMR

3 1230 CAN. 1. CHEM. VOL. 68, 1990 spectroscopy (Varian XL-200, Varian T-60). The ionomers were found to be at least 80% quaternized. This value was obtained by taking the ratio of the peak intensity for the PY+CH31- protons (N-methyl- E 4-vinylpyridinium iodide (4-VPQ): 4.65 ppm) to the peak intensity for 2 some of the protons on the pendant ethyl or butyl acrylate groups. w Plasticization The plasticized ethyl acrylate-4-vinylpyridium iodide (EA-4-VPQ) cn mol% samples were prepared by mixing pieces of the previously --I 3 dried rubbery material with dimethyl malonate (Aldrich, 99%) in a 6 compression mold. 'The mixtures were then molded at pressures no 0 higher than 2000 psi at 70 C for 1 h and were used without further treatment. The plasticizer content was determined by measuring the c3 difference in weight for the samples before performing the dynamic mechanical testing and after that, once the samples had been heated at 80 C under reduced pressures until a constant weight was reached. It is assumed that no loss of plasticizer occurred during the testing since dimethyl malonate has a boiling point of 182 C (19). Calorimetry The glass transition temperatures of the non-quaternized copolymers and the ionomers were determined using a Perkin and Elmer Differential Scanning Calorimeter (D.S.C.-11). The determinations were done on samples weighing about 10 mg. Indium and n-octane were used to calibrate the instrument. The calibration and experimental runs were carried out at a heating rate of 20 C min-'. Prior to the determination of 2 their glass transition temperatures, all the samples were dried for at -I least 48 h in a vacuum oven at 60 C. W 0 The non-ionic copolymers were annealed at Tg+20 C for 5 min and quenched before each scan. Three scans were usually made to ensure 5 reproducibility, the last one being retained. t- The alkyl acrylate ionomers were annealed at Tg+ 10 C for 5 min, c3 quenched, and then scanned. Three runs were also made, the last one 0 being retained. --I In all cases the values obtained for the glass transition temperature were reproducible within 2 C. Dynamic mechanical measurements The dynamic mechanical properties were investigated with a torsional pendulum which is described elsewhere (17). The frequency varied from ca. 3 Hz in the glassy region to ca Hz in the low modulus region. Rectangular samples obtained by compression molding of 0.7 to 0.9 g of polymer at a temperature of Tg+20 C and at pressures no higher than 2000 psi were used in this study. All samples were subjected to the same drying procedures as those adopted for the calorimetry study. The experiments were performed under a nitrogen atmosphere. Heating rates of 1-2 C rnin-' were employed and the temperature was controlled to 1 C. The values of the shear storage modulus (G'), the shear loss modulus (G"), and the loss tangent (tan delta) acquired and calculated by a computerized system, were obtained directly. Results Butyl acrylate-n-methyl-4-vinylpyridinium iodide ionomers The variation of the shear storage modulus (G') with the temperature for a series of butyl acrylate ionomers with 4.6 to 18.3 mol% of 4-VPQ ions (BuA-4-VPQ) is plotted in Fig. 1. It can be seen that as the ion content increases, the curves shift upward in temperature, and the shapes of the curves are modified extensively in the glass to rubber transition region with noticeable broadening. The loss tangent curves, shown in Fig. 2, display two distinct relaxations. The high temperature relaxation is seen to gain in intensity as more ions are incorporated and, at high levels, to become the dominant one. The position of both relaxations depends to a different degree on the ion content, as can be seen in Fig. 3. The low temperature relaxation moves progessively upward in temperature at a rate of about 1 /mol% while the high temperature relaxation is shifted at a rate of about 4.3"/mol% I FIG. 1. Shear storage modulus as a function of temperature for butyl ac~1ate-n-meth~1-4-vin~1~~ridinium ionomers. q -- FIG. 2. LOSS tangent as a function of temperature for butyl acrylate- N-methyl-4-vinylpyridinium iodide ionomers.: Tg, Q. (0. S. C. ) A: Tg, N. Q. (0. S. C. ) : Tg, Q. (T. P. ) *: Tg, c (T. P. ) ION CONTENT (Mol%) FIG. 3. Glass transition temperature (Tg) as a function of ion content as determined by D.S.C. and dynamic mechanical measurements (T.P.) for the non-quaternized (N.Q.) and quaternized (Q.) poly(buty1 acrylate-co-4-vinylpyridine) copolymers, and high temperature peak position (Tg, c) as a function of ion content for the quaternized copolymers

DUCHESNE AND 1 EISENBERG 123 1-90 -70-50 -30-10 10 30 50 70 FIG. 4.

4 DUCHESNE AND 1 EISENBERG FIG. 4. Shear storage modulus as a functionof temperature for ethyl acrylate-n-methyl-4-vinylpyridinium iodide ionomers plasticized with dimethyl malonate (DMM). -2 I FIG. 5. LOSS tangent as a function of temperature for ethyl acrylate- N-methyl4vinylpyridinium iodide ionomers plasticized with dimethyl malonate (DMM). Only one transition is detected by calorimetry. Its position, also plotted in Fig. 3, is influenced by the ion concentration in the same way as the low temperature relaxation detected by dynamic mechanical measurements. For the sake of comparison, results obtained for the non-quaternized copolymers (N.Q.) are also presented. Plasticized ethyl acrylate-n-methyl-4-vinylpyridinium iodide ionomers The dynamic mechanical properties of an ethyl acrylatebased vinylpyridinium ionomer (EA-4-VPQ) 10.5 mol% plasticized with dimethyl malonate (DMM) were investigated. Figure 4 shows the variation of the shear storage modulus with temperature for samples of poly(ethy1 acrylate) with 0, 10, 20 wt% of dimethyl malonate which was used as a plasticizer. A marked shift downward in temperature is observed as the amount of plasticizer increases. Along with this shift, the 20 wt% sample also exhibits some broadening in the glass to rubber transition region. Similarly, the loss tangent curves, Fig. 5, move progressively downward in temperature indicating, as expected, a severe depression of the glass transition temperature with increasing plasticizer content. More importantly, the 10 and 20 wt% sample display two relaxations. The significance of this second relaxation will be discussed in detail in the next section. Discussion The damping behaviour of BuA-4-VPQ ionomers indicates that these materials are phase-separated. Indeed, the appearance of a high temperature relaxation can easily be related to the formation of clusters. As can be seen in Fig. 2, the high temperature relaxation gains substantially in intensity relative to the low temperature relaxation as the ion concentration increases. In the light of these results, it is believed that the low temperature relaxation is due to microbrownian segmental motions associated with the glass transition of the ion-poor region, while the high temperature relaxation is associated with the same phenomenon in the ion-rich phase. Thus, the BuA- 4-VPQ ionomers fall in the same category of ionomeric material as, among others, styrene - sodium methacrylate (1 8), styrene - sodium-p-styrene sulfonate (19), ethyl acrylate - sodium acrylate copolymers, sulfonated ethylene - propylene rubbers (EPDM) (20), and carboxylated polypentenamers (9). The plot for the unplasticized EA ionomer in Fig. 5, along with results for a more extensive ion concentration range reported in ref. 2, tend to indicate that the EA ionomers are not clustered. By contrast, it is clear that ionomers based on BuA are clustered. The question of whether clustering takes place because the butyl acrylate matrix has a lower glass transition temperature or a lower dielectric constant than the ethyl acrylate matrix will now be addressed. The plasticization experiments provide the answer. The results in Figs. 4 and 5 suggest that phase separation is induced and progressively enhanced as the glass transition is depressed by the addition of the plasticizer. As the plasticizer content increases, clusters are formed which have observable mechanical properties of their own. It should be noted that the plasticizer mimics the backbone: therefore, the clustering cannot be ascribed to changes in the dielectric constant, but only to changes in Tg. Thus, vinylpyridinium ionomers with a low glass transition temperature should be clustered while those with high glass transition temperature should not. Similar observations have been made in a parallel study for the poly(styrene-co-4-vinyl pyridine) system (2 1). Furthermore, while the dielectric constant of the matrix increases the solubility limit of the multiplets, it will not prevent cluster formation by itself unless it has a very high value, as in the case of the polyphosphates (22). In the light of these results, it now seems surprising that the low concentration (< 10 mol%) EA-4-VPQ ionomers did not show any sign of phase separation. It appears that these materials are a borderline case, and would probably be clustered if their glass transition temperatures were a few degrees lower. Conclusions The BuA-4-VPQ ionomers were shown to behave like clustered materials. In addition, the investigation of the viscoelastic properties of EA-4-VPQ 10.5 mol% samples plasticized with DMM suggest that while the EA ionomers are not clustered, in the absence of plasticizer, the plasticized samples are clustered, and that the essential condition for vinyl pyridinium ions to form clusters in this material is a lowering of the matrix glass transition temperature. 1. S. GAUTHIER, D. DUCHESNE, and A. EISENBERG. Macromolecules, 20, 760 (1987). 2. D. DUCHESNE, and A. EISENBERG. J. Polym. Sci. To be published. 3. E. P. OTOCKA, and F. R. EIRICH. J. Polym. Sci. Part A-2,6,921 (1968).

5 1232 CAN. J. CHEM. VOL. 68, E. P. OTOCKA, and F. R. EIRICH. J. Polym. Sci. Part A-2,6,933 (1968). 5. A. EISENBERG, H. MATSUURA, and T. TSUTSUI. J. Polym. Sci. Polym. Phys. 18, 479 (1980). 6. A. NEPPEL, I. S. BULTER and A. EISENBERG. Macromolecules, 12, 948 (1979). 7. A. NEPPEL, I. S. BUTLER, and A. EISENBERG. J. Polym. Sci. Polym. Phys. 17, 2145 (1979). 8. A. EISENBERG and M. KING. Ion-containing polymers. Academic Press, New York W. J. MACKNIGHT and T. R. EARNEST. J. Polym. Sci., Macromol. Rev. 16, 41 (1981). 10. R. LONGWORTH. In Developments in ionic polymers-i. Edited by A. Wilson and H. Prosser. Applied Science Publishers, Essex. 1983, Chap J. E. BURLEIGH, 0. F. MCKINNEY, and M. G. BARKER. Anal. Chem. 31, 10 (1959), 31, 1684 (1959). 12. E. SAINI and L. TROSSARELLI. Atti. Accad. Sci. Torino, Classe Sci. Fis. Mat. Nat., 90, 410 ( ). 13. H. SUMITOMO and Y. HACHIHAMA. Kobunshi Kagaku. 10, 544 (1953). 14. J. SELB and Y. GALLOT. J. Polym. Sci. Polym. Lett. 13, 615 (1975) GAUTHIER and A. EISENBERG. Polym. Prepr. Am. Chem. Sot. Div. Polym. Chem. 22, 2 (1981); 22, 293 (1981). 16. R. C. WEAST. (Editor). Handbook of chemistry andphysics. 56th ed. CRC Press, Cleveland B. CAYROL. Ph.D. Thesis, McGill University, Montreal A. EISENBERG and M. NAVRATIL. Macromolecules. 6, 604 (1973). 19. M. RICDHAL and A. EISENBERG. J. Polym. Sci. Polym. Phys. 19, P. AGARWAL, H. S. MAKOWSKI, and R. D. LUNDBERG. Macromolecules. 13, 1679 (1980). 21. D. WOLLMANN and A. EISENBERG. TO be published. 22. A. EISENBERG, H. FARB, and L. G. COOL. J. Polym. Sci. Part A-2. 4, 855 (1966).

Microphase Separation in Sulfonated Polystyrene Ionomers

Microphase Separation in Sulfonated Polystyrene Ionomers D. G. PEIFFER, * R. A. WEISS, t and R. D. LUNDBERG, Corporate Research Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45,1600