The isothermal `cold crystallization' of the thermoplastic polyimide (New TPI) has been studied using the microhardness technique to examine the property-microstructure correlation. By using wide- and small-angle X-ray diffraction and differential scanning calorimetry, the influence of the experimental parameters, i.e., treatment temperature Tc and time tc on the micromechanical properties of the `cold crystallized' samples has been examined. It is shown that both macroscopic hardness H and volume crystallinity xc increase with Tc and tc. For the samples prepared in the secondary crystallization range, it is demonstrated that H values strongly depend on the hardness of the crystalline units Hc. In this range, long spacing L, crystal thickness lc and crystallinity (both linear xCL, and derived from density xc) slightly increase with Tc. From the DSC study, it is demonstrated that the proportion of `liquid-like' and `rigid' amorphous fraction present 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.

The authors are indebted to DGICYT (Grant PB94-0049), Madrid (Spain). We wish also to thank NEDO's International Joint Research Programme, Japan, for the generous support of this investigation. Partial financial support was provided by the Polymeric Composites Laboratory Consortium at the University of Washington.

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