Can spherulitic growth rate accelerate before impingement for a semicrystalline polymer during the isothermal crystallization process?

Isothermal crystallization processes in the temperature range from 110 to 140 degrees C were studied by using polarizing optical microscopy (POM) for poly(L-lactide) (PLLA) film samples with and without confinement conditions. The PLLA film samples were prepared by melt pressing between two cover glasses to provide the surface confinements and by removing the top cover glasses or by solution cast to keep the top surface free of confinements, respectively. Through carefully following the changes of spherulite radius with crystallization time, it is surprisingly found that there exists an acceleration of spherulitic growth rates at the late stage following the normal spherulitic growth rates at the early stage of isothermal crystallization for PLLA film samples with both surface confinement (by cover glasses) and planar confinement conditions (by aggregation of adjacent spherulites to form lake-like regions) at certain temperatures, while the spherulitic growth rates remain constant at each isothermal crystallization temperature for PLLA film samples without the confinement conditions. On the basis of data analyses on the spherulitic growth rates under various conditions from several different aspects, it is finally concluded that the decreases of glass transition temperature induced by crystallization under confinement conditions during isothermal crystallization of PLLA film samples cause the peculiar spherulitic growth rate acceleration at the late stage.