Influence of the stereoregularity on the crystallization of the trans planar mesomorphic form of syndiotactic polypropylene

The crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene by quenching the melt at 0°C is investigated as a function of the stereoregularity of the samples. The formation of the trans-planar mesomorphic form at 0°C is followed as a function of the permanence times at 0°C by X-ray diffraction and FTIR spectroscopy. Samples kept at 0°C for short time rapidly crystallize into the helical form I at room temperature, whereas longer permanence times at 0°C increase the stability of the trans-planar mesomorphic phase which remains stable and inhibits the normal crystallization of the sample into the helical form at room temperature. The stereoregularity of the polymer sample strongly influences the rate of formation of the mesomorphic form at 0°C. Higher the syndiotacticity, easier the formation of trans-planar mesomorphic form. Very short permanence times (few hours) at 0°C are enough to form and stabilize the mesomorphic form for highly syndiotactic samples, whereas for stereo-irregular samples the amount of mesomorphic form observed when the sample is removed from the 0°C bath and heated to room temperature, remains always very low, even for very long permanence times (months) at 0°C and the crystallization into the normal helical form is not inhibited.