Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of shear and pressure

Key issue in studying the crystallization process of semi-crystalline polymers, is the need for controlled (or known) boundary and initial conditions. Here dilatometry (PVT) is used to reveal the crystallization kinetics and the resulting morphology of isotactic polypropylene homopolymer as a consequence of the combination of non-isothermal cooling at elevated (isobaric) pressure and application of shear flow. Based on the flow strength the crystallization kinetics can be classified in three regimes; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. Using ex-situ wide angle X-ray diffraction and small angle X-ray scattering we revealed the details of the (oriented) crystalline structures for the different regimes including the different phases (α-, β-, and γ-phase) that can occur in iPP. Some unique oriented structures of combined α- and γ-phase were found. The distribution of the crystal phase content is explained by the interplay of nucleation density and crystal phase dependent growth rate, which are functions of both temperature and pressure.