Solution-Processable Colorless Polyimides Derived from Hydrogenated Pyromellitic Dianhydride: Strategies to Reduce the Coefficients of Thermal Expansion by Maximizing the Spontaneous Chain Orientation Behavior during Solution Casting

In this study, practically useful colorless polyimides (PIs) with low coefficients of thermal expansion (CTEs) and other desirable properties were prepared from hydrogenated pyromellitic dianhydride (1-exo,2-exo,4-exo,5-exo-cyclohexanetetracarboxylic dianhydride, H-PMDA). A modified one-pot polymerization method afforded a high-molecular-weight PI with sufficient film-forming ability from 2,2′-bis(trifluoromethyl)benzidine (TFMB) with a rod-like structure and H-PMDA. However, the PI film cast from its homogeneous solution did not have low CTEs, similar to the analogous system using meta-tolidine. To solve this problem, a series of amide- and amide-imide-containing diamines were designed and synthesized. The modified one-pot polymerization of H-PMDA and the diamines in γ-butyrolactone produced homogeneous, viscous, and stable solutions of high-molecular-weight PIs with high solid contents. The cast films of certain systems examined in this study simultaneously achieved low CTEs, high optical transparency, considerably high glass transition temperatures (T(g)s), and sufficient ductility. A possible mechanism for the generation of low CTEs, which is closely related to the spontaneous in-plane orientation behavior during solution casting, was proposed. Certain H-PMDA-based PIs developed in this study are promising colorless heat-resistant plastic substrates for use in image display devices and other optical applications.