Crystallization of isotactic polypropylene from mesomorphic phase: a constant heating rate study

We have studied crystallization behaviour of isotactic polypropylene (iPP) from mesomorphic phase in structural point of view. Time-resolved wide-angle X-ray diffraction (WAXD) measurements during a heating process have been performed using a synchrotron radiation (SR) X-ray beam line at SPring-8, Japan. The heating process was so programmed to reproduce a thermal trace of differential scanning calorimetry (DSC) with a constant heating rate (10 degrees C/min) in order to compare the structural change with thermal behaviour. SR-WAXD sensitively detected the crystallization behaviour and we have obtained fractions of alpha-crystal, mesomorphic phase and amorphous phase as a function of temperature by analysing the data. The results showed that the crystallization from mesomorphic phase proceeds in between 60 and 120 degrees C (meso-alpha transition). During this process, the crystallization from amorphous hardly takes place. The crystalline fraction shows almost constant in between 120 and 140 degrees C; meanwhile, the mesomorphic fraction still decreases above 120 degrees C. The crystalline fraction starts to decrease above 140 degrees C and the most extensively decreases at around 165 degrees C (melting point). We have also determined the energy level of the mesomorphic phase (meta-stable state) relative to that of alpha-crystal (stable state), considering the balance among the fractions of alpha-crystal, mesomorphic phase and amorphous.