Structure development and phase transformation after processing polybutene-1

An important phase transformation encountered after the melt processing of isotactic polybutene-1 is the lattice rearrangement from tetragonal to hexagonal in the solid state. This phase trans-formation of PB-1 was characterized quantitatively by the x-ray diffraction technique. The application of melt orientation during tubular film blowing and a small level of strain during cold rolling were found to accelerate the phase transformation of PB-1 significantly.

A "crystallinity index" of each of the two crystal forms, XII and X₁, was calculated by a new method which combined the two techniques of wide angle x-ray diffraction (WAXD) and density measurement. During the phase transformation of PB-1 crystallinity and density all increase to an asymptote value. The substantial increase of crystallinity (ca. 30% to ca. 60%) was the result of secondary crystallization during the crystal transformation. An attempt was made to analyze rigorously the development of crystallinity by combining the Avrami analysis of the crystal transformation and the secondary crystallization equation. This led to the conclusion that crystal Form I would nucleate (heterogeneously) directly from the melt at very large supercooling.

Based on the Avrami exponent, the mechanism of the crystal transformation was interpreted as involving the spontaneous nucleation of crystal Form I followed by the growth of these nuclei along the polymer chain-axis. With this interpretation, an accelerating mechanism of the crystal transformation was proposed in connection with the consequent morphologies of crystal superstructure depending on the draw-down ratio ( DDR = 1 DDR3, DDR = 6).