EFFECT OF MOLECULAR-WEIGHT ON THE CRYSTALLIZATION BEHAVIOR OF POLY(ARYL ETHER ETHER KETONE) - A DIFFERENTIAL SCANNING CALORIMETRY STUDY

The effect of molecular weight on the kinetics of the crystallization of poly(aryl ether ether ketone) has been investigated by means of a series of fractions of relatively narrow molecular-weight distribution. In this study the crystallization has been monitored by differential scanning calorimetry (d.s.c.). Analysis of the sigmoidal crystallization curves by means of the Avrami equation leads to n almost-equal-to 2. In order to obtain a uniform set of rate constants (k), small adjustments in the zero time were made. Measurements have been made of the crystallization of amorphous samples just above the glass transition temperature. The temperature dependence of the observed rate constants follows a Vogel equation, log k = U*/2.3R(T(c) - T-infinity), where U* almost-equal-to 4500 cal mol-1 and T-infinity = T(g) - 55. These parameters are consistent with the universal WLF parameters. The experimental glass transition temperature of each sample has been used. It is found that the rate of crystallization considered under iso-free-volume conditions depends on M-2 as expected for a diffusion-controlled process that involves entangled linear polymers. This conclusion is supported by an analysis of the onset temperature of crystallization in dynamic d.s.c. scans of the different molecular-weight samples. In another series of experiments the crystallization rates have been measured below the melting point of the samples. Analysis of the temperature dependence of the rate constants reveals that high-molecular-weight polymers crystallize at lower temperature (T(c) = 270-290-degrees-C) according to regime III. Low-molecular-weight samples require higher temperatures in order to measure the rates of crystallization (T(c) > 300-degrees-C). The temperature dependence of the rate constants suggests that in that case crystallization occurs according to regime II. One sample (M(w) = 32000) showed a regime III-regime II transition at about 298-degrees-C. These conclusions are not drastically affected by reasonable variations in U*, T-infinity and the equilibrium melting temperature T(m)-degrees.