Molecular Design of Highly-Stretchable Ionomers

S. Wu1, X. Cao1, Z. Zhang1, Q. Chen1, Y. Matsumiya2, and H. Watanabe2

1 State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences; University of Chinese Academy of Sciences, Changchun 130022, China
2Yumi Matsumiya and Hiroshi Watanabe
Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan

Application of ionomers is often disturbed by their brittleness originating from limited stretchability of the network strands physically crosslinked by the ionic sites therein. Thus, an effective method of improving the ductility is to increase the length of network strands (and/or entanglements). Considering this point, this study examined linear viscoelasticity (LVE) and nonlinear elongational rheology of unentangled copolymers of hexyl methacrylate (HMA) and the ionic monomer, Sodium 4-vinylbenzenesulfonate hydrate (SSNa). The ionized SSNa monomer, being randomly distributed along the chain backbone at a concentration ranging from less than one to ~four monomers per chain, served as the physical crosslink (or physical branching point). The LVE data showed a sol-to-gel transition, and the ductility of the sample turns out to be strongly related to the degree of gelation.[1] Analysis of those data gave an average length of the network strands, and the ductility of the ionomer samples detected in the nonlinear elongational test was well correlated with this strand length in most cases. An exception was found for the sample slightly above the gel point: the ductility of this sample was much larger than expected from the strand length, possibly due to the “pseudo-yielding” behavior that reflected exchange of the ionic, physical crosslinks and the resulting motion/displacement of the ionomer chains.

[1] Q. Chen, C. Huang, R. Weiss, and R. Colby, Macromolecules 48, 1221 (2015).