Modeling of polymorphic composition development during isothermal crystallization of poly(L-lactide acid)

Development of polymorphic composition during isothermal crystallization of unstressed amorphous PLLA is considered in the introduced model. The model analysis of the overall crystallization half-time indicates involvement of both alpha' and alpha polymorphs in the crystallization kinetics at substantial impact of the alpha' form strongly dependent on temperature. Above 120 degrees C effective creation of alpha form takes place at much slower amorphous-to-alpha transformation and proceeds via alpha'-to-alpha transformation which occurs through orders of magnitude faster direct solid-to-solid transition mechanism. The model allows to predict development of individual alpha' and alpha phases during isothermal crystallization of PLLA basing only on the overall temperature dependent crystallization half-time, t(1/2). The predictions are in good agreement with the experimental data reported by different authors in the literature. The model indicates that the transformation degrees to the individual alpha' and alpha polymorphs vs. crystallization temperature, as well as the overall transformation degree are independent of the nuclei concentration in terms of the multiples of the crystallization half-time, because the concentration of nuclei is directly involved in t(1/2).