Molecular Weight Dependence of the Viscoelastic Properties of Polystyrene-Based Ionomers

Abstract

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.

Microphase-separated structure of telechelic urethane acrylate anionomers and their network in various solvents

“…Ionic aggregation in ionomers has been confirmed by small-angle X rays and inferred from mechanical measurements with dynamic mechanical analyzers. [37][38][39][40][41] Both the elastic modulus and the glass-transition temperature of ionomers were shown to increase with an increase in the ionic content of a sample in agreement with the expectation of an increase in ion clusters.…”

Correlation between Morphology and Mechanical Properties in Zn‐Neutralized Sulfonated Polystyrene Ionomer/Poly(ethylene glycol) Blends: Contribution of EFTEM

“…When only 20% of stickers remain effectively closed, as observed on Figure 10, the stickers quickly disassemble and the material becomes viscous. This terminal region corresponds to the chain disentanglement strongly delayed by the ionic interactions [41] in comparison with the terminal region of the pristine PS.…”

Clustering in carboxylated polysulfone ionomers: A characterization by dynamic mechanical and small-angle X-ray scattering methods

“…The best fits were achieved by using an exponential function as a background and fitting the matrix and cluster peaks with Gaussian functions. This combination of curves has been found to work well for other ionomer systems; 19 this suggests that it may work in this case. Curve deconvolutions were carried out over the temperature range from 140°C to approximately 20°C above the minimum following the cluster peak.…”

“…This behavior is strongly reminiscent of that of an ionomer of low molecular weight in which the cluster peak is "buried" under the part of the tan ␦ curve which rises rapidly because of polymer flow. 19 In samples of higher molecular weight, where the flow region is moved to higher temperature, the cluster peak should become more visible. To see the cluster peak clearly, the peak deconvolution was performed.…”