No abstract

No abstract

No abstract

Three different sets of styrene-based ionomer homoblends containing ca. 5 mol% of ionic repeat units, i.e. poly(styrene-co-sodium itaconate) [P(S-co-ITANa)]/ poly(styrene-co-sodium methacrylate) [P(S-co-MANa)] ionomer blends, sodium sulfonated polystyrene [P(S-co-SSNa)]/P(S-co-MANa) ionomer blends, and P(S-co-ITANa)/P(S-co-SSNa) ionomer blends, were prepared and their dynamic mechanical properties were investigated. It was observed that with increasing itaconate content in the blend of P(S-co-ITANa)/P(S-co-MANa) ionomers the multiplet of the P(S-co-MANa) ionomer disrupted initially very rapidly, and ionic modulus of the blend increased drastically. In the case of P(S-co-ITANa)/P(S-co-SSNa) ionomer blends, again, the ionic modulus increased significantly with increasing itaconate content. In these two ionomer blend systems, the itaconate, having two ionic groups per ionic repeat unit, influenced the properties of the blends more noticeably than the other two ionic units, containing only one ionic group per ionic repeat unit. In the blend system of P(S-co-SSNa)/P(S-co-MANa), as expected, the cluster loss tangent peak shifted to higher temperature, ionic modulus decreased, but the ionic plateau extended more with increasing the ratio of the P(S-co-SSNa) content. However, it should be mentioned that when the ion contents of the methacrylate and sulfonate ionomers increased to over 6 mol%, at which the cluster phase of the P(S-co-MANa) ionomers is known to become dominant, compared to the matrix phase, the three loss tangent peaks were observed, which implies that methacrylate-rich and sulfonaterich phases exist together. This might be due to the fact that a copolymerization effect becomes stronger; that is, with increasing ion content the role of ionic units in random copolymer ionomers becomes more important, compared to the role of host non-ionic units. As a concluding remark, the properties of these three ionomer blend systems depend on the degree of clustering, type of ionic groups, and the number of ionic groups per repeat unit.

The molecular weight dependence of the viscoelastic properties of polystyrene-based ionomers was examined. To measure the physical properties, a Rheometrics dynamic analyzer (RDA) was utilized for the poly(styrene-co-sodium methacrylate) (4 mol %) system, while for the poly(styrene-co-sodium styrenesulfonate) samples (5 mol %) the studies were performed on a dynamic mechanical thermal analyzer (DMTA). In the methacrylate samples of high molecular weight (>100 000), the plot of tan vs temperature showed two peaks, as in other ionomers. However, for samples of the lowest molecular weight (ca. 21 000), the tan plot showed only one peak and a shoulder. Peak deconvolution, however, showed that, even in that sample, there are two peaks and that the deconvoluted peak areas do not change significantly with molecular weight. The flow behavior of the matrix in the low molecular weight samples partially masks the relaxation process of the clusters. In the terminal region, the viscosities of the ionomers are molecular weight dependent in a way similar to that in nonionic polystyrene; above the critical molecular weight for entanglements, compliance values of the ionomers are independent of the molecular weight. In the case of the sulfonates, many trends are similar to those seen in the methacrylates. As the molecular weight increases, the cluster tan peak becomes more visible, and slopes of storage modulus curves in the terminal region become less steep. However, there are some differences, which are due to the difference in the strength of the interaction between ionic groups in the multiplets. In the methacrylates, because of weaker interactions, the ionomers start to exhibit appreciable ion-hopping and chain mobility at a relatively low temperature (ca. 170 °C), which is the onset of the glass transition (Tg) of the cluster phase and is only ca. 50 °C higher than the matrix Tt. Above that temperature, the multiplets can no longer act as long-term cross-links, and the ionomers show a strong molecular weight dependent behavior. On the other hand, in the sulfonates, due to the stronger interaction, ion-hopping becomes important only at much higher temperature (ca. 220 °C), which is, again, the beginning of the cluster T¡; above that point these ionomers also start to show appreciable molecular weight dependent behavior. In addition, in the sulfonates, the activation energies for the matrix and cluster transitions and for ion-hopping are independent of the molecular weight. All these results suggest that the stability of multiplets is independent of the molecular weight. The thermal stabilities of the present ionomers are similar to that of polystyrene.

Electron spin resonance (ESR) spectroscopy has been used to obtain information on chain mobility in spin-doped poly[styrene-co-(sodium methacrylate)] random ionomers. The results show that spin probe molecules randomly dispersed in ionomers respond to the matrix glass transition. However, when the probe molecules are anchored to the ionic multiplets, their mobility is reduced, indicating that the polymer segments adjacent to the ionic multiplets are constrained, and their mobility is restricted. Significant motions of the probe molecules start only when the kinetics of the ion-hopping process becomes relevant. Plasticization of the ionomers with polystyrene oligomers allowed for the simultaneous observation of two different mobility regimes, assigned to the matrix and cluster phases. These findings support the suggestion, which is a major component of the EHM model for the morphology of random ionomers, that regions of restricted mobility exist around the ionic multiplets.

No abstract

The morphologies of Cs-neutralized poly(styrene-ran-methacrylic acid) (Cs-SMAA) random ionomers have been studied using scanning transmission electron microscopy (STEM). The Cs-SMAA ionomers exhibit Cs-rich aggregates in the shape of spherical shells that are randomly distributed in a polystyrene-rich matrix. For simplicity, we refer to these as vesicular aggregates. The vesicular aggregates are typically 5-20 nm in diameter and have shell thicknesses of ∼3 nm. The vesicular aggregates persist after annealing with their dimensions and number density unchanged. The aggregate diameters and shell thicknesses are independent of the copolymer concentration during neutralization and the rate of neutralizing agent addition. We also demonstrate that our specimen preparation for STEM (microtomy parameters and storage conditions) avoids substantial changes to the bulk ionomer morphology. Overall, the microstructures of the current materials are similar to those recently reported in sulfonated polystyrene ionomers solution-neutralized with Zn.