A strategy to construct low temperature curable copolyimides with pyrimidine based diamine

Low temperature curable polyimide (PI) films with outstanding mechanical and thermal properties are highly demanded in advanced packaging. Herein, a flexible diamine with pyrimidine named PMNH2 was synthesized and copolymerized with pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA), which showed significant influences on degree of imidization as well as the mechanical, thermal and dielectric properties of PI films. The copolyimide films have been proven almost complete imidization at 200 °C without any other curing accelerators. Meanwhile, the utilization of PMNH2 improved mechanical properties of copolyimide films greatly without severely sacrificing their thermal stability when curing at 200 °C. Copolyimide films containing 20 wt% and 30 wt% PMNH2 exhibited fairly high tensile modulus of 3.06 and 3.13 GPa, tensile strengths of 120 and 118 MPa, elongations at break of 61.1% and 20.1% and glass transition temperature of 352 and 342 °C, respectively. Moreover, the curable mechanism has been deeply probed by comparing the basicity and electron effects of the different diamine monomers via theoretical calculation, which showed an enlightening result that stronger basicity of pyrimidine-based diamine promoted the low temperature curing process, however the weaker electron-donating ability decreased the reactivity. Hence, both basicity and electron effects are key factors to be considered in designing new diamine monomer for low temperature curable PI films, providing opportunities of application in the advanced microelectronic package.