Atmospheric pressure and elevated pressure crystallization of isotactic polypropylene

The crystallization of isotactic polypropylene was studied at atmospheric pressure and elevated pressures up to 200 MPa (2 kbar). At atmospheric pressure, two homopolymers and an 0.5% ethylene copolymer were studied. The investigation included studies of crystallization rates under quiescent isothermal conditions. The morphology and thermal behavior were also studied. The Avrami model of crystallization behavior was applied to the bulk crystallization data. Secondary nucleation theory was also applied to the crystallization data.

At atmospheric pressure the bulk crystallization kinetics exhibited similar Avrami exponents for the three polypropylene samples. However, the results of the secondary nucleation analysis using reciprocal crystallization halftimes showed a Regime II Regime III transition only for the low molecular weight homopolymer. The elevated pressure kinetics studies showed differences in bulk crystallization kinetics and secondary nucleation kinetics analysis as a function of pressure.

The morphology at atmospheric pressure for the homopolymers exhibited similar behavior as a function of crystallization temperature, but the copolymer exhibited distinct differences from the homopolymers. The elevated iv pressure crystallization samples exhibited spherulitic morphology at all observed pressures and crystallization temperatures.

The thermal behavior of the atmospheric pressure crystallized samples exhibited the complex behavior common to isotactic polypropylene. The low molecular weight homopolymer showed somewhat different behavior from the high molecular weight homopolymer and the copolymer.