The effect of pressure pulses on isotactic polypropylene crystallization

High hydrostatic pressure pulses, up to 80 MPa, have been imposed for different time and at various temperatures on molten iPP during cooling. The absence of any effect on the crystallization behavior after pressure release and further cooling demonstrates either that no nucleation precursor is produced by this thermo-mechanical history or that the lifetime of pressure-induced precursors is extremely short, as if no crystallization occurs during pressurization. Even when crystals develop during the pressurized stage, when high pressure is released, subsequent crystallization takes place in the same temperature range in which the polymer crystallizes at atmospheric pressure. Structure and morphology of samples partially crystallized under pressure were investigated. WAXD show that, as expected, the fraction of γ-crystallinity firstly increases with increasing the time under pressure up to impinging of γ-spherulites and then levels off at a constant value. Morphological investigation performed on microtomed samples reveals that the γ-spherulites formed under high pressure exhibit positive birefringence and that their skeleton continues to grow during cooling when pressure is released. On heating, these spherulites melted first, but incompletely: the radial lamellar texture is still present up to relatively high temperature. These features seem to be consistent with the spherulitic architecture described by Lotz: infilling γ-lamellae grow by epitaxy on a skeleton of α-lamellae. Under the adopted experimental conditions, pre-imposed pressure pulses do not promote formation of extra nuclei, while they largely affect the crystal structure and the morphology of melt crystallized isotactic polypropylene.