NON-ISOTHERMAL CRYSTALLIZATION KINETICS OF A TYPICAL SEMICRYSTALLINE POLYIMIDE

A semicrystalline polyimide was synthesized from 3, 3', 4, 4'-biphenyltetracarboxylic dianhydride (s-BPDA)/1, 3-bis-(4-aminophenoxy) benzene (TPER)/phthalic anhydride (PA). According to the crystallization exothermic peaks obtained from differential scanning calorimetry (DSC) when cooling from melt at 2, 5, 10, 20 K/min, respectively, the method of Jeziorny, Ozawa and an approach proposed by MO Zhishen were applied to study the non-isothermal crystallization behaviour of this polyimide. The results suggested that s-BPDA/TPER/PA was a very special system, the onset crystallization temperatures were almost the same at different cooling rates. The apparent Avrami exponent n obtained by the Jeziorny method was 2. 4 and then changed to 3. 5 when cooling rate reached 20 K/min, indicating the mechanism of nucleation or the way of growth have changed. Besides, the kinetic parameter Z(c) increased with the increase of the cooling rate. Whereas deviations and fluctuations occurred in the later stage due to the secondary crystallization. The Ozawa method was also limited in obtaining the reliable kinetic parameters because of the substantial curvatures in the Ozawa plots. However, the approach proposed by Mo Zhishen appeared applicable due to the better linear relation, according to the new parameter a, the relationship between Ozawa exponent m and apparent Avrami exponent n was confirmed (n/m = 1. 3) and exact values of m were obtained. Finally, Kissinger method was used to get the value of the crystallization activation energy Delta E, about -936 kJ/mol, which meant a fast nucleation rate of this system.