Investigation on the phase transition from Form II to Form I in iPB-1 after pre-stretching

Abstract

Mechanical stretching can accelerate Form II to Form I phase transitions in isotatic polybutene-1 (iPB-1). This study in-situ investigated the effect of pre-stretching on further phase transitions. Three tensile modes were considered. In mode A, the samples were stretched to their breaking point, and the phase transition was wholly completed. In mode B, the samples were pre-stretched to specific strains and annealed under quiescent conditions at room temperature. Mode C was studied under the same conditions as Mode B, but the samples were allowed free shrinkage during the phase transition. The phase transition process were investigated by mechanical testing and in situ wide-angle X-ray diffraction. The phase transition kinetics were obviously faster in the pre-stretched samples (even when stretched in the elastic zone) than in the un-stretched samples. The pre-stretching may affect the conformation or the lattice arrangement of the segments in Form II, thereby accelerating the subsequent phase transition. In the samples with pre-strain beyond the yield strain, the accelerated transition rate increased with the increased pre-strain, meanwhile, the growth dimension of Form I reduced, owing to the confinement effect of stretch-induced Form I. The residual stress in the mode-B stretched samples did not significantly affect the phase transition, because the residual stress was lower than the critical stress that triggers Form I nucleation. In summary, a small pre-strain accelerated the phase transition and affected the growth dimension of Form I.