Analysis of formation mechanism of crystal form I in isotactic polybutene-1 based on a melting memory method

In this paper, the melting memory effect generated by a novel cyclic thermal treatment protocol with form II as “seeds” on the polymorphic transformation in isotactic polybutylene-1(iPB-1) was investigated. Polar Optical Microscope (POM), wide angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR) were used to study the changes of morphology and crystal form II → I transition during aging. Selected crystallization temperature (T_sc) and melting temperature (T_sm) were applied to generate residual nuclei of form II. Both cycled and uncycled samples were analyzed to demonstrate the formation of crystal I and polymorphic transformation. The results indicate that the melting memory has a significant effect on tetragonal modification, leading to an increased nucleation density and an accelerated nucleation speed. The ordered chain sequences in crystal II initiated the formation of crystal nuclei of form I, resulting in a rapid conversion and increased transition degree during the hexagonal crystal growth. Form I was found to have two formation pathways, including the transformation of the unstable crystal form II and the direct formation from residual crystal and melt, which have been rarely discussed before and may shed light on understanding of crystal formation mechanism in polymorphism polybutene-1.