Analysis of Crosslinking Structure of Vulcanized Rubber and Pressure-Sensitive Adhesive using Equilibrium Swelling Method, Mechanical Properties and Pulse NMR

Abstract

The 100% modulus measured by a tensile test for crosslinked poly (butyl acrylate-acrylic acid) random copolymer (P (BA-AA)) as pressure-sensitive adhesive showed the clear tensile rate dependence, whereas that for vulcanized isoprene rubber (IR) never showed the dependence. To clarify this reason, the crosslinking structure of both systems was compared in terms of the molecular weight between crosslinking points (M_c) values determined via equilibrium swelling, dynamic mechanical analysis and tensile tests. There are two kinds of crosslinking points: the chemical crosslinking points and the entanglement points of polymer chains. It was found that the proportion of entanglement points was far greater in the crosslinked P (BA-AA). The entanglement points can readily disentangle with a stress relaxation in response to slow deformation. This is the reason why only the crosslinked P (BA-AA) showed the tensile rate dependence. The glass transition temperature (T_g) was higher for the crosslinked P (BA-AA) than for the vulcanized IR. From the ¹H pulse nuclear magnetic resonance analysis, the restriction of molecular mobility by the intermolecular interaction was found to be larger in the crosslinked P (BA-AA) than in the vulcanized IR. Therefore, the crosslinked P (BA-AA) shows higher T_g.