Approaches to improve the film ductility of colorless cycloaliphatic polyimides

This study described approaches for improving the film ductility of colorless cycloaliphatic polyimides (PIs). An unexpected toughening effect was observed when a PI derived from pyromellitic dianhydride (PMDA) and 4,4′‐methylenebis(cyclohexylamine) was modified by copolymerization with a low isophoronediamine (IPDA) content of 5 to 30 mol%, despite there being no film‐forming ability in the homo PMDA/IPDA system. For example, at an IPDA content of 20 mol%, the copolymer showed significantly improved film toughness (maximum elongation at break, ε b max = 57%), excellent optical transparency (light transmittance at 400 nm, T 400 = 83.7%), and a high glass transition temperature ( T g = 317°C). This toughening effect can be interpreted on the basis of the concept of chain slippage. In this study, the PIs derived from bicyclo[2.2.2]octane‐2,3,5,6‐tetracarboxylic dianhydride (H‐BTA) with various diamines were also systematically investigated to evaluate the potential of H‐BTA‐derived systems. The combinations of H‐BTA with ether‐containing diamines led to highly tough PI films ( ε b max > 100%) with very high T g s, strongly contrasting with the results of an earlier study. The observed excellent properties are related to the steric structure of H‐BTA. Our interest also extended to the solution processability. A copolyimide derived from H‐BTA with a sulfone‐containing diamine and an ether‐containing diamine achieved a very high optical transparency ( T 400 = 86.8%), a very high T g (313°C), and good ductility ( ε b max = 51%) while maintaining solution processability. Thus, these approaches enabled us to dramatically improve the ductility of cycloaliphatic PI films that have, to date, been considered brittle.