The Origin of Solvent-Accelerated II–I Phase Transition of Isotactic Poly(1-butene)

Abstract

The II–I phase transition process of isotactic poly(1-butene) (iPBu) in xylene and the natural environment has been systemically investigated by FTIR, WAXD, and AFM techniques. It is found that while the nucleation of form I crystals during aging in a natural environment takes a long time, normally over hours, the nucleation of form I crystals in xylene occurs immediately, generally in minutes or even in seconds. In addition, AFM in situ study on the phase transition process of iPBu form II single crystals demonstrates that the growth rate of form I iPBu crystals during phase transition in xylene is about 61 ± 1.7 nm/min, which is 218 times faster than that during aging in a natural environment (0.28 ± 0.05 nm/min). It is thus clear that the quick nucleation and fast growth of form I crystals in xylene result in an accelerated II–I phase transition of iPBu. Moreover, it is found that the thickness of form I iPBu single crystals obtained through the II–I phase in xylene is similar to that of its form II precursor, whereas that of the form I crystal attained in a natural environment decreases by around 15% compared to the original form II crystal. This suggests that xylene vertically expands the chain fold loops at the single-crystal surface and results in the packing of some chain segments in connect with the chains in crystals into regular crystals, which compensates for the contraction of crystals in chain direction and leads to a slight (approximately 5%) increase of crystallinity after transformation in xylene. These results clearly shed more light on the mechanism of the solvent-accelerated II–I phase transition of iPBu.