The II to I Phase Transition of Isotactic Poly(1-butene) Single Crystals at an Early Stage

Regular and dendritic (i.e., composed of multiple flat-on dendritic crystals with a fixed 90 degrees branching angle) single crystals of isotactic poly(1-butene) (iPBu) in its form II were obtained by ultrathin film melt crystallization. Their II-I phase transition behavior was studied by atomic force microscopy (AFM) and electron diffraction techniques. Ex-situ electron diffraction studies on both the regular and dendritic single crystals reveal that the II-I phase transition leads generally to the formation of untwinned hexagonal form I iPBu crystals. In-situ AFM studies on the regular single crystal show that the II-I phase transition occurs easier at its edge facets than inside the crystal, analogous to the preferential form I nucleation at the edges of individual crystal sheet within the dendritic single crystals. It is also found that inside the regular single crystal rowlike nuclei form preferentially along [100]/[010] directions of the parent form II single crystal, which may be related to a larger shrinkage in these directions caused by densification during the II-I phase transition. These obtained results may shed more light on the II-I phase transition mechanism and kinetics of the iPBu.