Influence of structural and topological constraints on the crystallizationand melting behavior of polymers. 2. Poly(arylene ether ether ketone)

The secondary crystallization and its influence on the glass transition are studied as a function of crystallization temperature and time by different ial scanning calorimetry for PEEK. The multiple melting behavior resulting from isothermal annealing from the glass or crystallization from the melt i s discussed in the context of models considering either a melting-recrystal lization-remelting process or a bimodal population of primary and secondary crystals. The heating rate dependence of the multiple melting behavior ind icates that reorganization of primary crystals occurs during heating for sa mples annealed from the glassy state but is insignificant for those crystal lized from the melt. For either mode of crystallization, the high- and low- temperature endothermic regions are associated with the melting of primary and secondary crystals, respectively. Investigations of the low endotherm t ransition temperature and heat of fusion as a function of crystallization t ime and temperature lead to the following conclusions: the melting temperat ure of secondary crystals increases linearly with the logarithm of secondar y crystallization time at a rate, B(T), increasing linearly with decreasing temperature; the Avrami exponent, which characterizes the initial stage of secondary crystallization, is constant below ca. 310 degrees C (n = 1/2) b ut increases gradually with temperature above 310 degrees C; the late stage of secondary crystallization is characterized by a linear increase in crys tallinity with logarithm of time. Studies of the evolution of the glass tra nsition after secondary crystallization indicate that the calorimetric T-g increases linearly with the logarithm of time at a rate, b(T), increasing w ith decreasing temperature. Finally, a qualitative model of polymer crystal lization of semiflexible polymer chains is proposed. This model considers t he effect of structural constraints (chain stiffness) on the nature of the amorphous phase after primary crystallization and the effect of topological constraints (pinning of amorphous chains) on the secondary crystallization behavior.