Rheological Properties of ABA-Type Copolymers Physically End-Cross-Linked by Polyoxometalate

In this study, we designed an ABA-type copolymer physically end-cross-linked by polyoxometalate and developed a versatile and robust pathway to tune its structural and rheological properties. The copolymer precursors are composed of a central poly­(n-butyl acrylate) (nBA) block with DP ≅ 100 and two short vinyl imidazole (VIm) moieties as chain ends. Phase segregation occurs gradually for the precursors with an increase of the number of VIm per end, N VIm, from 1 to 7.6, and clear segregation is observed for precursor with the highest N VIm = 7.6, using small-angle X-ray scattering. For those precursors having N VIm ≤ 4.6, introduction of a small amount of silicotungstic acid, H4[SiW12O40], induces phase segregation, owing to a strong electrostatic attraction between [SiW12O40]4– ion and protonated VIm monomer. As a result, the copolymer chains are physically cross-linked, forming thermoplastic copolymers, for which the size of core, plateau modulus, and solid-to-liquid transition temperature can be tuned in wide ranges by the content of silicotungstic acid. The solid-to-liquid transition temperature is controlled by an energy barrier for dissociation of the VIm end moieties. Analysis of the solid-to-liquid transition temperature enables us to determine a scaling relationship between the energy barrier and the size and degree of protonation of the end moiety. The near-proportionality between the activation energy E a and the length of the end moiety suggests a strong motional coupling of the ionic associations belonging to the same end moiety.