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).