Analysis of the pressure effect on the crystallization kinetics of polypropylene: Dilatometric measurements and thermal gradient modeling

Dilatometric measurements in isobaric cooling mode were performed to study the pressure effects on the crystallization kinetics of polypropylene (PP) up to 100 MPa. The experimental specific volume curves were analyzed by tak ing into account the thermal gradient that appears in the sample even for r elatively low cooling rates. The Tait equation was used to describe the spe cific volume of the purely amorphous phase, and linear variations of the pu rely crystalline phase specific volume are considered. The relative crystal linity was modeled using the Nakamura equation, which is relevant for nonco nstant cooling rates. Considering an Avrami exponent of 3, the Nakamura rat e constant was obtained first at atmospheric pressure and then generalized for higher pressures considering the equilibrium melting temperature variat ion. The obtained intrinsic specific volume was validated by computing the thermal gradient in the sample and comparing the calculated average specifi c volume to the experimental one.