Mechanisms of abrupt fracture transitions in crystallizable elastomers

Abstract

We carried out theoretical analysis and experimental investigations to update our understanding concerning rupture and fracture transitions in continuous stretching of vulcanized natural rubber (NR). Besides a previously reported rupture transition in notch-free NR as a function of test temperature, we predict and confirm at elevated temperatures the existence of a rupture transition with respect to the applied stretch rate. In addition to the previously observed fracture transition in prenotched NR with respect to notch size, we also predict and observe two other transitions when either stretch rate or temperature is varied. For the first time we explain, based on our recently proposed kinetic theory of bond dissociation (KTBD) for elastomeric rupture and fracture, why these five transitions are abrupt with respect to the three variables of notch size, stretch rate and temperature for both unnotched and prenotched NR. Since the transition is determined by whether strain induced crystallization (SIC) in NR occurs – SIC reinforces NR by prolonging its lifetime, the essential physics behind these transitions can be understood by answering whether the polymer network could undergo sufficient stretching to produce SIC before the network breakdown through chain scission. The lifetime of the network competes with the timescale required for strain-induced crystallization, and the experimental timescale prescribed by the stretch rate is the third timescale. The competition among these timescales exhibits itself in the form of these abrupt transitions.