Near-Zero Thermal Expansion and High Heat-Resistance Polyimide Films Based on a Symmetric and Rigid Pyrazine Structure

Regarding the applications of polyimide (PI) films in emerging flexible devices and precision apparatus, their low coefficient of thermal expansion (CTE), especially near-zero expansion, is an urgent requirement for weakening the internal stress between the organic/inorganic layers. Other properties of PI films including the high heat-resistance and superior toughness are also demanded in actual scenarios. Herein, a set of PI films was developed by copolymerization of 2,5-bis(4-aminophenyl)pyrazine (PRZ) into a pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) system. The monocrystal analysis of the PRZ monomer reveals its rigid, symmetric, and nearly coplanar steric structure. Thanks to the synergy and balance of flexible ODA and rigid PRZ units in PI chains, the near-zero expansion, high heat-resistance, and optimal mechanical properties of the PI films are simultaneously achieved. Particularly, the PI film with ODA/PRZ = 4.5/5.5 presents a near-zero CTE of −0.41 ppm/K, high glass transition temperature (Tg) of around 417 °C, superior fracture strength of 191 MPa, and initial modulus of 3.83 GPa. Interestingly, via adjusting the ODA/PRZ ratios, the precise control of CTE values can be realized, enabling the thermal expansion matching of PI films on broad types of device substrates, e.g., Cu (15–17 ppm/K) and Si (3–5 ppm/K).