Thermal expansion behavior of various aromatic polyimides

The thermal expansion behavior for various aromatic polymides was investigated. Usally polymers, including polyimides, have high thermal expansion coefficients (3–6 × 10−5 K−1), compared with metals and ceramics. However, there are some polyimides which have very low thermal expansion coefficients below 1 × 10−5 K−1. This property was observed for the polymides obtained from pyromellitic dianhydride or 3,3′ 4,4′‐biphenyltetracarboxylic dianhydride and aromatic diamines which were constituted of only benzene rings fused at para positions. It was proposed that their low thermal expansion coefficient related to the linearity in their polymer molecular skeltons.