Strain-Rate-Dependent Phase Transition Mechanism in Polybutene-1 during Uniaxial Stretching: From Quasi-Static to Dynamic Loading Conditions

With the assistance of an ultrafast time-resolved synchrotron radiation wide-angle X-ray diffraction technique and a homemade hyphenated high-speed tensile apparatus, structural evolutions in the crystalline domain of polybutene-1 (PB-1) are elucidated even within several milliseconds. The stretching-induced phase transition mechanism of PB-1 consisting of form II crystals is investigated in three strain-rate regions (A, B, and C) spanning six orders of magnitude (from 0.005 to 100 s–1). During the quasi-static loading process in region A (0.005 s–1 < ε̇ < 0.5 s–1), metastable form II crystals progressively transform into the stable form I ones. This classical transition is ascribed to the stress-induced change of the long-range chain position and conformation. Under dynamic loading conditions, in region B (1 s–1 < ε̇ < 10 s–1), besides the form II to I transition, several form II crystals are directly melted with increasing stress. In region C (ε̇ > 50 s–1), not only the form II crystals but also a large amount of the form I crystals are melted when the imposed stress reaches the threshold value. This abnormal stretching-induced phase transition of PB-1 is related to both the high strain rate and accompanied heating effect. When the lattice is subjected to an ultrahigh rate of energy input, the appearance and growth of conformational defects, which are related to the change of short-range contour shape of chains, can be involved. These defects lower the energy barrier of phase transition between the crystalline and amorphous structures significantly. For the crystals containing a large number of defects, they tend to be melted with increasing stress rather than undergoing a phase transition in the crystalline domain as those in the quasi-static loading region.