Pre-Treatment Effects on Isothermal Crystallization Behavior of Isotactic Polypropylene

Abstract

The effects of crystal nuclei or crystallization on the isothermal crystallization behavior of isotactic polypropylene homopolymer are examined using fast scanning calorimetry (FSC) and wide-angle X-ray scattering (WAXD). During crystalline nucleation below the glass transition temperature as described by Tammann's two-stage nucleation approach, an increased crystallization rate is observed, regardless of whether the mesophase structures formed in the low-temperature region or α-crystals formed in the high-temperature region. No increase in isothermal crystallization rate is detected following self-nucleation just above the melting point. Additionally, the influence of the pre-existing crystals on the subsequent crystallization behavior is investigated by combining crystallization time and temperature profiles, whereby crystallization at high temperatures is followed by slight crystallization at low temperatures, and in reverse order. Surprisingly, it is confirmed that the presence of pre-existing crystals do not affect the subsequent crystallization rate. The crystallization rate of iPP is influenced only by low-temperature pre-annealing that enhances the pre-nucleation.