Synthesis and rheology of tailored poly(dimethylsiloxane) zinc and sodium ionomers

We describe a synthesis scheme for the preparation of "model" poly(dimethylsiloxane) ionomers with tailored number of monomers between ions and number of ions per chain. Melts of low ion concentration (0.3-1.3 mol %) model zinc and cobalt ionomers, and their unneutralized COOH precursors are found to precipitate as polymers that flow and exhibit a zero-shear viscosity but equilibrate to physical gels. The gel time follows an Arrhenius relationship and is used to predict and verify physical gel formation at room temperature over a time scale from several months to years. The gel time depends chiefly on the ion concentration (calculated here as the;average number of ions per monomer units (x 100)) and functionality (defined as the average number of ions per chain); for ionomers with comparable overall molar mass, the ion concentration and functionality act counter to each other. Unlike traditional hydrocarbon ionomers, gel formation in these polysiloxane ionomers is favored by high temperature, and the gel moduli are comparable to moduli of end-linked PDMS networks. We propose this is due to transformation of intramolecular interactions to intermolecular interactions at high temperatures. Sodium ionomers form critical gels immediately after precipitation which we attribute to the more ionic nature of sodium relative to the transition-metal ionomers.