Stretching-Induced Melting and Recrystallization Polymorphism Revealed in Polybutene-1

Abstract

The crystalline structures of polymorphic polymer not only are determined by the processing parameters but also could further evolve within the service period. In this work combining in-situ wide-angle X-ray diffraction and ex-situ differential scanning calorimetry, the stretching-induced crystallite evolution was studied for polymorphic polybutene-1 (PB) within a broad stretching temperature (Tstretching) range of 30–115 °C. First of all, it was found that although the initial crystallites were in the most thermodynamically stable modification of the trigonal phase, the metastable but kinetically favored tetragonal phase was generated during stretching at the elevated temperatures because of crystallite melting and subsequent recrystallization. What is more interesting is that those stretching-generated amorphous fractions also have the possibility of recrystallizing into the trigonal form I′ when cooled to room temperature, depending on Tstretching. These results demonstrate that the polymorphism selection occurs in the stretching-induced recrystallization, where form II might be generated during stretching at the elevated temperature and form I′ is obtained by cooling. The unusual re-appearance of the trigonal form I′ may be associated with the local confinement crystallization caused by the residual crystallites. The combination of structural evolution with mechanical response disclosed that the stretching-induced recrystallization requires macroscopic yielding to destroy the crystallite skeleton for melting. Furthermore, the increase of lamellar thickness decreases the amount of stretching-induced amorphous phase and increases the critical Tstretching of recrystallization into forms II and I′. Based on the quantitative results, a threshold criterion of stretching-melted PB, which is crystallinity of around 12.6%, was identified to be required for recrystallization of the trigonal phase form I′ by cooling the stretched form I crystallites.