Some observations on the melting behavior of isotactic polypropylene crystals

Abstract

AbstractThe results of DSC investigations carried out at different heating rates for isotactic polypropylene (IPP) samples with various degrees of morphology and crystallinity have been correlated with the results of SAXS experiments and optical microscopy observations. The SAXS experiments have been performed mainly to calculate the long‐period, thickness of crystalline layers and the degree of crystallinity. The multiple endotherms have been observed for samples crystallized at temperatures higher than 100°C. These endotherms are connected with different crystal types formed during crystallization of IPP. These crystals show superheating phenomena when the heating rate in DSC measurements increases. These effects lead to the conclusion that crystals of larger size and perfection grow in these samples rather than in those crystallized at lower temperatures. The latter ones show reorganization phenomena in DSC measurements. The DSC and SAXS investigations performed for the samples after their thermal oxidation in air at 130°C and in fuming nitric acid have shown the long‐period, thickness of crystalline layers and the degree of crystallinity changes correlated with the melting peak temperature variations. The polymer oxidation in air occurs mainly in the amorphous phase and causes the chain scissions. This effect results in the chain mobility increase and their recrystallization on the lamellae surfaces. At the same time (at sufficiently high temperature) the reorganization of the polymer crystals and the crystal form transformation occur and allow for elucidation of the observed changes in melting temperatures and DSC curves.