Neutron Spin Echo Insight into Dynamic Networks Formed by Ionomers*

Association of ionic groups strongly affects the rheology of ionomers in melts and in solutions. The constraint arises from formation of ionic clusters affects the macroscopic dynamics. While the network is constrained, the chains remain dynamic on the length scale of the network mesh. Using NSE we probe chain dynamics in networks formed by polystyrene sulfonate (PSS) as the physical crosslinks are perturbed by addition of ethanol. Solutions of 20Wt% PSS, 11,000 g/mol (PD=1.02) with 3 and 9% mole sulfonation were studied. S(q,t) measured across a broad temperature range, was analyzed in terms KWW and characteristic times extracted. At length scales that correspond to the ionic domains, the motion is constrained in both solvents; however, ethanol enhances the dynamics. While the ionic domains expand, the dynamics remain restricted, pointing to formation of “ionic clouds” where the ionic clusters are swollen with alcohol, but remain segregated, retaining the physical crosslinks of the network. At smaller dimensions, the dynamics becomes significantly faster and no differences are observed as ethanol was added. Increasing the temperature results in equal acceleration of the motion across all q values, indicative of retaining the dynamic, swollen, physical crosslinks.