Formation and disappearance of the rigid amorphous fraction in semicrystalline polymers revealed from frequency dependent heat capacity

For semicrystalline polymers the observed relaxation strength at glass transition is often significantly smaller than expected from the non-crystalline fraction. This observation leads to the introduction of a rigid amorphous fraction (RAF) which does not contribute to the heat of fusion or X-ray crystallinity nor to the relaxation strength at glass transition. The RAF is non-crystalline and in a glassy state at temperatures above the common glass transition. Complex heat capacity in the high frequency limit allows for the measurement of base-line heat capacity also at temperatures above the glass transition. From that the temperature and time dependence of the RAF can be obtained. For PC, PHB and syndiotactic polypropylene (sPP) it is possible to study the creation and disappearance of the RAF in situ during isothermal crystallization and on stepwise melting. If crystallization is not limited by the stability (melting point) of the crystals to be formed the total RAF is created during the isothermal crystallization. Simultaneously with the melting of the smallest crystals the RAF disappears. For these polymers vitrification and devitrification of the non-crystalline material detected as the RAF at glass transition is structural (conformational) and not temperature induced. The formation of the last growing crystals, which melt first, are responsible for the vitrification of the amorphous material around them and, consequently, by that they limit their own growth.