Effects Of Counterion On Morphology And Thermal Transitions Of Ionomers

In this work, 18 types of ionomers have been prepared through neutralization of a carboxylated poly(methyl methacrylate), i.e., poly(methyl methacrylate-ran-methacrylic acid)(MMA-MAA), and a sulfonated polystyrene(SPS) with alkali metals(Li+, Na+, K+, Rb+ and Cs+) and alkaline-earth metals(Mg2+, Ca2+, Sr2+ and Ba2+) counterions, to systematically explore their ionic-aggregate morphology as well as thermal-transition behavior.It was observed from transmission electron microscopy(TEM) that the ionic aggregates of the SPS alkali-metal-salts ionomers all showed a morphology of monodisperse nanospheres of ca. 2-3 nm in mean diameter, which were relatively homogeneously distributed in the polystyrene matrix. Characteristic multiple Bragg-peaks were detected via wide-angle X-ray scattering(WAXS) from the ionic aggregates of the SPS Na-salt ionomer, which, in combination with an observed electron-diffraction schlieren texture via TEM from the ionic aggregates of the SPS alkali-metal-salts ionomers, directly evidenced the establishment of order inside the(anhydrous) ionic aggregates of the ionomers upon room-temperature physical aging. The ionic-aggregate size of the SPS Na-salt ionomer estimated from WAXS was reconciled quantitatively to that observed from TEM, revealing that both the WAXS and TEM results were reliable.Differential scanning calorimetry(DSC) studies showed that there existed a low-temperature, wide endothermic-peak at 60-145 oC for each of the 18 ionomers only upon room-temperature physical aging, before which(at 25 oC) a primary WAXS Bragg-peak characteristic of the(ordered) ionic aggregates of the representative SPS-Na-salt-ionomer appeared while after which(at 190 oC) it disappeared during variable-temperature WAXS experiments; this contrast strongly indicates that the peak originated from an order-to-disorder transition within the ionic aggregates of the physically aged ionomers. In addition, with an extension of the physical-aging time at room temperature, both the peak temperature and the enthalpy of the DSC low-temperature endothermic peak were increased for all of the ionomers, suggesting the relaxation behavior of a gradual improvement in the degree of order inside the ionic aggregates with time.Further systematic investigations via a combination of TEM and DSC disclosed that, for the 9 alkali-metals- and alkaline-earth-metals ionomers of the same matrix(i.e., MMA-MAA or SPS), with an increase in the ratio of the(valid) electric charge(q) of the counterion to the counterion-ionic-group interionic distance(a), or the q/a ratio of the counterion(i.e., the strength of ionic interactions), the mean size(and thus the aggregation number) of the ionic aggregates was reduced, as revealed from TEM, due to a decrease in the ionic-aggregates-matrix interfacial tension, which in turn led to an improvement in the number density of the ionic aggregates(i.e., the ionic cross-linking density); therein, in terms of the chain dynamics, the effect of an increase in the q/a ratio on the enhancement of chain-end immobilization by the ionic aggregates had a tendency of offsetting the effect of a decrease in the aggregation number on the reduction in the degree of chain close-packing in the coronas of the ionic aggregates, and hence the matrix and cluster glass-transition temperatures(Tg’s) of the ionomers primarily were determined by the ionic cross-linking density—as observed from DSC, both the Tg’s were raised with increasing the ionic cross-linking density(originally with increasing the q/a ratio of the counterion). Therefore, in terms of the interpretation of the structure-property relationships of ionomers, it is imperative that the traditional ion-hopping theory developed by Eisenberg et al. be extended and modified: based on the retention of the structural factor, the strength of ionic interactions, solely involved in the source theory, one should extend to accommodate two morphological factors, the aggregation number of ionic aggregates and the ionic cross-linking density; a net effect of the joint interactions of the three factors is that the properties of ionomers predominantly are dictated by their ionic cross-linking density.