Influence of Initial Crystallization Temperature of Form II on the Nucleation and Growth of Form I IPBu Crystals during II–I Phase Transition

The effects of crystallization temperature of isotactic poly­(1-butene) (iPBu) form II flat-on crystals on the nucleation and growth of its form I during II–I solid phase transition has been in situ studied by atomic force microscopy. It was found that the initial crystallization temperature of used form II samples affects both the nucleation and growth of iPBu form I crystals. The growth rate decreases simply with increasing the crystallization temperature, originating from a better stability of form II crystals grown at high temperature. On other hand, the effect of crystallization temperature on the nucleation of form I crystals is related to aging time. At an early stage, the nucleation of form I crystals in samples fabricated at high temperature is more efficient, associating to a higher residual thermal stress caused by a larger temperature difference and contraction of thicker lamellar crystals after cooling. In a later stage, the effect of residual thermal stress may vanish, or at least be less pronounced, which results in a more efficient form I nucleation for form II crystals grown at low temperature owing to poor stability. Based on a statistical location analysis of 200 form I crystals and morphological features of the nucleation site, it was found that more than 50% form I crystals nucleate at gaps between adjacent flat-on crystals with width smaller than 2R_g of the used iPBu material. This implies the important role of tie molecules and entangled chain loops on the II–I phase transition of iPBu. These findings are helpful for guiding the processing and of great significance for broadening the application of iPBu materials.