Structure development in a thermoplastic polyimide. Cold crystallization as revealed by microhardness

The isothermal "cold crystallization" of the thermoplastic polyimide (New T PI) has been studied using the microhardness technique to examine the prope rty-microstructure correlation. By using wide- and small-angle X-ray diffra ction and differential scanning calorimetry, the influence of the experimen tal parameters, i.e., treatment temperature T-c and time t(c) on the microm echanical properties of the "cold crystallized" samples has been examined. It is shown that both macroscopic hardness H and volume crystallinity x(c) increase with T-c and t(c). For the samples prepared in the secondary cryst allization range, it is demonstrated that N values strongly depend on the h ardness of the crystalline units H-c. In this range, long spacing L, crysta l thickness l(c) and crystallinity (both linear x(CL), and derived from den sity x(c)) slightly increase with T-c. From the DSC study, it is demonstrat ed that the proportion of "liquid-like" and "rigid" amorphous fraction pres ent in each sample is directly related with the crystallization conditions. Finally, from the SAXS and DSC combined study, information concerning the secondary crystallization mechanism has been obtained.