Stress relaxation of reptating chains of acrylonitrile–butadiene copolymer in end‐linked acrylonitrile–butadiene copolymer networks

Abstract

AbstractStress relaxation of unattached chains of acrylonitrile–butadiene copolymer in TDI end‐linked acrylonitrile–butadiene copolymer has been studied. The advantage of this system is that the polar nature of the TDI crosslinking agent in end‐linked acrylonitrile–butadiene copolymer can be compensated if an unattached acrylonitrile–butadiene copolymer with moderate acrylonitrile content is selected. It was found that with increasing unattached chain content, the position of the relaxation spectra transitions shifted to shorter times. The frictional resistance for the relaxation of networks containing unattached chains was proportional to the amount of crosslinked components. This means that crosslinked components not only bear load but also resist the relaxation of unattached chains. The relaxation moduli contribution from unattached chains first increased and then decreased with increasing amount of crosslinked components. This can be explained by the presence of two kinds of entanglements, i.e., those between unattached chains themselves and those between unattached chains and crosslinked components, which in turn depend on the amount of unattached chains. At lower concentrations of unattached chains, relaxation of unattached chains requires that molecules pass only through obstacles formed by one kind of entanglement; this results in lower relaxation modulus. With increasing unattached chain content, the relaxation of those chains require molecules reptation through both kinds of entanglements, which results in higher relaxation modulus. Therefore, the enhancement of mechanical properties for networks containing unattached chains originates not only from load‐bearing of crosslinked components but also from the relaxation of unattached chains.