Crystallization kinetics analysis of poly(trimethylene terephthalate) including the secondary crystallization process

Since its development for fibers and engineering thermoplastics in the 1990s [1–3], poly(trimethylene terephthalate) (PTT) has attracted much attention due to its outstanding characteristics, such as high elastic recovery, chemical resistance, and resilience [4, 5]. Like poly (ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), PTT is a typical semicrystalline polymer and the investigation of its crystallization kinetics has significant importance both from scientific and technological points of view. PTT crystallizes faster than PET but slower than PBT [6]. Its maximum crystallization rate occurs at 415 K. It has clearly been observed that a primary crystallization process and a secondary crystallization process consecutively occur during isothermal annealing of PTT [7], nevertheless no detailed kinetics analyses of the latter have been reported so far. Although the nature of the secondary crystallization of polymers is still unclear, several models have been proposed to modify the original Avrami theory to include this process [8–11]. Hillier [8] has proposed a model, which considered that the secondary crystallization of polymers is a first-order process with an Avrami form (i.e., the Avrami exponent is a constant and always equals to 1). Despite this model being a better fit than the Avrami equation to some isotherms analyzed, the imposed invariable Avrami exponent seems quite arbitrary. Velisaris and Seferis [10] have found another way to consider secondary crystallization by describing the time dependence of crystallization degree as a linear combination of two Avrami expressions: