Elastic modulus of the crystalline regions of polyimide derived from poly(amic acid)-biphtalic dianhydride and p-phenylene diamine

The elastic modulus E-l of the crystalline regions in the direction paralle l to the chain axis was measured by X-ray diffraction for polyimide derived from polyamic acid (biphthalic dianhydride and p-phenylene diamine). These specimens were cured by two different routes: curing at 200 degrees C, and at 400 degrees C for 1 h, respectively (2STEPS), and curing from 80 degree s C to 400 degrees C stepwise (nine steps) for 1 h at each step (STEPWISE). The E-l values of 54-169 GPa were obtained for the STEPWISE specimen and 8 0-178 GPa for the 2STEPS specimen, depending on the meridional reflection e mployed for measurement of the E-l value. A linear relationship between the E-l value and the fiber identity period was obtained from each meridional diffraction, such that the E-l value increased with an increase in the fibe r identity period. The E-l value of the fully extended structure was estima ted to be 210 GPa. These are considered to be due to the coexistence of pol ymorphs with different skeletal structures. The crystalline regions of the 2STEPS specimen seems to consist of a more extended skeleton than those of the STEPWISE specimen. (C) 1999 John Wiley & Sons, Inc.