Stretching behavior of the butene‐1/ethylene random copolymer: A direct correspondence between triggering of II‐I phase transition and mechanical yielding

Abstract

Quiescent phase transition from the tetragonal form II into hexagonal form I was utilized to prepare II/I mixture systems in the butene-1/ethylene random copolymer with 1.5 mol% ethylene units, where aging duration was varied to tune the fraction of form I in the initial crystallites. The mechanical test and in-situ wide-angle X-ray diffraction were combined to quantitatively explore stretching-induced phase transition of the II/I mixture systems. Interestingly, the triggering of stretching-induced further phase transition from forms II to I always corresponds to its mechanical yielding. This may be associated to formation of new tie chains by the lamellar collapsing with yielding. Moreover, for all mixture systems, the threshold stresses for triggering phase transition, that is, the yield stress, follow the master form I fraction-stress correlation that is obtained from the pure initial form II system. Such form I fraction-stress correspondence confirms the stress-determined kinetics for phase transition and also provides an approach to predict the yielding stress for the II/I mixture system.