Preimidized auto-photosensitive polyimides containing methyl-substituted benzanilide units with increased high-temperature dimensional stability for advanced optical applications: preparation and properties

A series of solution-processable and photosensitive polyimide (PSPI) coatings with enhanced high-temperature dimensional stability were designed, synthesized and characterized. For this purpose, the rigid-rod benzanilide (Ph − CONH − Ph) units with methyl substituents were incorporated into the molecular structures of an intrinsically auto-photosensitive PSPI system via the two-step chemical imidization copolymerization of a specially designed aromatic diamine, 2,3'-dimethyl-4,4'- diaminobenzanilide (MMDABA) and an aromatic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), together with an aromatic diamine, 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane (TMMDA). The ortho -substituted methyl group in MMDABA, on one hand, was designed to guarantee the solubility of the derived PSPI resins in the organic solvents; and on the other hand, maintain the intrinsic photosensitivity of the pristine PSPI (BTDA-TMMDA, PSPI-1). Meanwhile, the rigid benzanilide unit was applied to reduce the coefficients of thermal expansions (CTE) of the derived PSPI coatings. Four PSPI copolymers with the TMMDA/MMDABA (molar ratio) of 95/5 (PSPI-2), 90/10 (PSPI-3), 85/15 (PSPI-4), and 80/20 (PSPI-5) were prepared. The derived PSPI resins were all soluble in polar aprotic solvents of N -methyl-2-pyrrolidone (NMP) and N,N -dimethylacetamide (DMAc). All the PSPI coatings maintained good photosensitivity to the ultraviolet (UV) light (UV-A, 320 ~ 460 nm) except PSPI-5, indicating the deterioration of the photosensitivity of the polymers by incorporation of MMDABA units. Incorporation of MMDABA units into the PSPI systems basically maintained the thermal stability of the polymers, while apparently increased the dimensional stability of the coatings at elevated temperatures. PSPI-5 possessed the CTE value of 42 × 10 –6 /K in the temperature range of 50 to 300 °C, which was obviously lower than that of the pristine PSPI-1 system (CTE = 56 × 10 –6 /K).