Effect of pressure on the polymorphism of melt crystallized polybutene-1

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Abstract
The melting, crystallization, and Form II to Form I transformation of isotactic polybutene-1 were studied under hydrostatic pressures up to 5000 atm. The melting behavior of Form I was similar to that of other polymers, showing an increase in melting temperature and decrease in the volume of melting with pressure. Crystallization of the melt under pressure resulted in the formation of negatively birefringent spherulites. At pressures below 1000 atm, the melt crystallized initially in Form II, which rapidly transformed to Form I. Above 1000 atm, the melt crystallized directly in Form I′, which shows the same X-ray diffraction pattern as Form I but has a lower melting temperature (96 vs. 130†C), the resulting melt recrystallizing in Form II. Upon annealing, Form I′ showed an increase in the long period measured by small-angle X-rays and an increase in melting point to 102† C but did not convert to any other structure. This behavior suggests the possible existence of differences in chain conformation between Forms I and I′, within the limits of an identical or very similar crystalline unit cell. A study of the II to I transformation under pressure indicated that it can take place in the temperature range between the glass transition and melting temperature of Form II, and its rate is drastically accelerated by pressure and unidirectional stretching and affected by internal strains. These effects suggest a strain-controlled transformation mechanism.

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