Strain-induced crystallization in an unfilled polychloroprene rubber: Kinetics and mechanical cycling

Polychloroprene rubber is often quoted as an example of strain-crystallizing elastomer; however, this fundamental property, known to impart good mechanical properties and fatigue resistance, has received much less attention in this class of compounds than natural rubber. The present paper relates systematic investigations combining mechanical and X-ray diffraction measurements. Crystallization kinetics are first investigated based on tensile impact tests. An approximately logarithmic time-dependence is found for the crystalline content and the time-constants are estimated at various temperatures. The impact of strain-induced crystallization on stress-strain curves under mechanical cycling conditions is further assessed. It is demonstrated that one main effect of strain-induced crystallization is a partial relaxation of the strain experienced by the amorphous fraction, although stress-hardening is simultaneously observed. Based on the stress-optical law, the contribution of the amorphous phase to the stress is shown to level off after crystallization onset, and stress-hardening is attributed to crystallite networking. The crystallite network rapidly disintegrates at the beginning of recovery, and the stress-optical law shows that the amorphous fraction becomes the only contributor to the stress at still non-negligible crystalline contents. (C) 2018 Elsevier Ltd. All rights reserved.