Abstract
The polypyromellitimide exhibits an excellent thermal stability due to its rigid structure of the chain molecules. In the present paper, the mechanical and dielectric secondary dispersion of such a polymer are discussed, and are correlated to their structure. The complex dielectric properties of polypyromellitimide and its relating polymers were measured at frequencies from 30 to 3MHz in the temperature range of -90°∼120°C. The viscoelastic properties of Kapton H polypyromellitimide film were measured at a higher temperature range of 20°∼500°C by means of a torsional pendulum. The stress-strain curves of H film were measured with a tensile testing apparatus at 20°C, and the thermal contraction of a stretched H film sample was also measured with the thermo-mechanical analysis (TMA) apparatus in the temperature range of 20°∼600°C. Three dispersions have been found in these measurements. The lowest one found at -60°C has been ascribed to the absorbed water. The middle one found at 70°C has also been related to the absorbed water. The highest one, which was revealed in this work for the first time, has been assigned to rearrangement of oriented metastable molecules at higher temperatures. The experimental results of TMA and the X-ray diffraction patterns of unstretched and stretched H films support this conclusion.
