Methyl Side-Group-Regulated Terphenyl Polyimides with High Heat Resistance and Ultralow Thermal Expansion for OLED Applications

Polyimides have attracted increasing attention due to being promising candidate materials for flexible display substrates. However, to meet the rigorous demands of thin-film transistor processing, the heat resistance and dimensional stability of polyimides need to be further improved. In this work, we designed a series of terphenyl diamines with small side methyl groups. This rodlike linear molecular terphenyl structure provides the self-orientation ability to achieve the purpose of reducing the coefficient of thermal expansion (CTE), while the methyl group restricts the molecular chain rotation, thus further increasing the glass-transition temperature (Tg) of the polyimides. The terphenyl diamines with different quantities of methyl groups were polymerized with commercial 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA), and the obtained polyimides showed outstanding performance, with high Tgs ranging from 399 to 439 °C, remarkable thermal decomposition temperature at a 5% weight loss (Td5) exceeding 500 °C, and extremely low CTEs ranging from −0.76 to 11.92 ppm·K–1. The regulation of the methyl groups on the performance of polyimides was investigated and explained by the analysis of their condensed structures and theoretical simulation. This strategy provides an approach for preparing polyimides with excellent comprehensive performance.