Non-isothermal crystallization of fluoropolymers

Abstract

Crystallization kinetics of polymers has received increasing attention since the pioneering work on isothermal crystallization by Avrami in 1939 [M. Avrami, J. Chem. Phys. 7 (1939) 1103.]. More recently the need to understand crystallization phenomena under non-isothermal conditions has forced many researchers to propose modifications of the original Avrami theory. Under both isothermal and non-isothermal crystallization conditions two different kinetic regimes, indicated as primary and secondary crystallization, can occur. In fact, for many polymers the primary stage is followed by a secondary process which involves a continuous increase in crystallinity over a period of time which often exceeds the experimental observation. Most of the theoretical models ignore the presence of this secondary crystallization. The aim of this paper is the study of the non-isothermal crystallization of fluoropolymers with different chemical structures (PVDF, PTFE, ECTFE and PFA). The work has been performed by Differential Scanning Calorimetry (DSC) at constant cooling rates (2–40 °C/min). The non-isothermal crystallization model proposed by Ozawa [T. Ozawa, Bull. Chem. Soc. Jpn. 57 (1984) 952] has been modified to account properly for the two kinetic regimes. This methodology allows better definition of the Avrami index in the primary stage and a more precise identification of the critical temperature, T cr, for the onset of secondary crystallization. Master plots for non-isothermal crystallization have been constructed as a function of temperature for the polymers under study. While for PVDF primary crystallization is the dominant kinetic regime under all cooling rates, secondary crystallization is not negligible for the other fluoropolymers considered here. Deviations from each master curve correspond to the critical temperature T cr.