On the crystal stabilization during two-step isothermal crystallization of poly(butylene terephthalate) examined by fast scanning calorimetry

Abstract

Crystallization behaviors on two-step isothermal crystallization at Tc1 and Tc2(<Tc1) were examined by fast scanning calorimetry for poly(butylene terephthalate) in order to identify the dominant process during the secondary stage of crystallization at Tc1 after the completion of spherulitic growth under isothermal condition. From the difference between the melting peaks obtained before and after the low-temperature crystallization step at Tc2, the distinction of the following two processes became realizable: the growth of crystals formed at Tc2 and the stabilization of crystals formed at Tc1. The dependences on Tc2 and the duration of times Δtc1 and Δtc2 revealed the following: The formation and growth of new crystals are controlled by thermodynamic driving force determined by supercooling, and hence are suppressed at Tc1 higher than Tc2. The crystal stabilization is controlled by the mobility of polymer chains, and became more pronounced at Tc1. Therefore, the dominant process of the secondary crystallization under isothermal condition at Tc1 should be the stabilization of crystals formed in the primary stage and the formation of new crystals will only have a secondary effect.