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.

Masatoshi Hasegawa,Junichi Ishii,Shuichi Takahashi,Katsuki Ichikawa

doi:10.3390/polym14061131

Abstract

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 (Tgs), 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.

Highlights

Aromatic polyimides (PIs) have been used as the most reliable electrically insulating materials in various electronic devices because of their excellent combined properties, including their considerably high physical and chemical heat resistances, good mechanical properties, high flame retardancy, extremely high purity, resistance to various chemicals, and high dimensional stability [1–11]
Our first attempt to obtain a free-standing flexible hydrogenated pyromellitic dianhydride (H-PMDA)/TFMB PI film was unsuccessful because the film obtained via the conventional two-step process included many cracks
We aimed to develop practically useful colorless PIs with low coefficients of thermal expansion (CTEs) and other desired properties by overcoming the great difficulties involved in reducing

Summary

Introduction

Aromatic polyimides (PIs) have been used as the most reliable electrically insulating materials in various electronic devices because of their excellent combined properties, including their considerably high physical and chemical heat resistances (e.g., resistance to the soldering process at 260 ◦ C), good mechanical properties, high flame retardancy, extremely high purity (absence of the residues of monomers and solvents, as well as ionic, metallic, and halogen impurities), resistance to various chemicals, and high dimensional stability [1–11]. The intense coloration of wholly aromatic PI films, which arises from intra- and intermolecular charge–transfer (CT) interactions [12], often disturbs their optical and optoelectronic applications (e.g., their use as plastic substrates and as alternatives to current non-alkali glass substrates in image display devices). The most effective way to completely remove the coloration of PI films is to use aliphatic (usually, cycloaliphatic) monomers either in diamines, tetracarboxylic dianhydrides, or both [12–38], thereby preventing CT interactions. When aliphatic diamines are used, poorly soluble salts are formed in anhydrous amide solvents in the early stage of polyaddition [30], which hampers the formation of the PI precursors, viz

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