Non Isothermal Crystallization of Polymers

Traditional studies of crystallization kinetics are often limited to idealized conditions, in which the parameters of state (temperature, pressure, etc.) are constant. In real situations, however, the external conditions change continuously, which makes the kinetics of crystallization dependent on instantaneous conditions, as well as on rate of change. This article provides an overview of the current state-of-the-art of non-isothermal crystallization of polymers during the cooling from the melt. The majority of the proposed theoretical formulations that predict non-isothermal crystallization kinetics, concern bulk crystallization and are based on modifications of the Avrami equation. The Ziabicki, Nakamura and Ozawa models are examined here in some detail together with treatments from other authors. The basic hypotheses of the various models, as well as their relative drawbacks, are underlined. Alternative empirical approaches to calculate the main parameters of non- isothermal crystallization and to compare the crystallization rate of different polymeric systems are also discussed. This article reviews the data concerning non-isothermal crystallization processes for different classes of polymers. Major attention is directed towards the dynamic crystallization of polyolefins, a class of materials of large industrial interest. Other results for polyoxyolefins, polyesters, polyamides and polyketones are also examined.