High-performance low temperature curable copolyimides via multidimensional modulation in alkaline environment and electronic effects of monomers

Abstract

A new design strategy for the preparation of copolyimides (co-PIs) with low curable temperature (200 °C) for IC packages and liquid crystal displays is presented. Firstly, two novel pyrimidine-based diamines named MePMNH₂ and 2MePMNH₂ were designed and synthesized, the electron-donating ability and alkalinity of which were effectively adjusted by the states of the methyl group at the meta- or ortho-/para-position of the N atom in pyrimidine. The co-PI film of ODA and ODPA with 5 wt% 2MePMNH₂, which was cured at a low temperature of 200 °C, showed greatly improved properties with a Young's modulus of 4.1 GPa, elongation of 21.1%, a tensile strength of 163 MPa, and a 5% decomposition temperature (T_d,5%) of 515 °C compared with the reference PI films (without 2MePMNH₂). Moreover, the intrinsic D_k and dielectric loss values of PI-Me_20%-200 cured at 200 °C were reduced to 3.13 and 0.0089 at 10 GHz. The excellent dielectric performance of MePMNH₂ and good solubility were in agreement with its more twisted structures, which was verified by theoretical calculations. Increased alkalinity and electron-donating ability of diamines promoted the imidization process by lowering the energetic barrier and accelerating the cyclization reaction, as evidenced by the reduced curing temperature and improved comprehensive performances. Besides, the structural regulation of diamines also facilitated the cyclization reaction to some degree. This work not only provides efficient monomers based on a pyrimidine building block, but also sheds light on the inherent mechanism between structure and properties, and paves the way to designing highly promising PI films utilized in advanced packaging.