Improvement in Mechanical and Thermal Properties of Transparent Semi‐Aromatic Polyimide by Crosslinking

Both the formation of insoluble salt for aliphatic diamines and the poor reactivity for aliphatic dianhydrides result in semi‐aromatic polyimide (SAPI) with low molecular weight and, thereby, in highly brittle SAPI films in nature. To solve the above problems, a cyclic‐olefin copolymer synthesized from norbornene and maleic anhydride is used as the third monomer (crosslinking agent) to prepare the crosslinked SAPI (cSAPI) films. The relative viscosity of the precursor solution of cSAPI is increased with the loading of cyclic‐olefin copolymer, giving rise to the improvement in mechanical and thermal properties of cSAPI films. The tensile modulus, tensile strength, and elongation at break of cSAPI film with 5 wt% cyclic‐olefin copolymer are increased to 4.49 GPa, 137 MPa, and 14.9% from 3.57 GPa, 111 MPa, and 7.6%, respectively, compared to those of the baseline SAPI film. Meanwhile, the glass transition temperature of cSAPI films is increased to 282 from 270 °C of the SAPI film, which is advantageous to better meet the requirement for high‐temperature manufacturing processes such as lead‐free solder reflowing. In addition, all cSAPI films exhibit good optical transparency in the entire visible region. A cyclic‐olefin alternating copolymer with an aliphatic nature and a rigid and noncoplanar steric structure at the same time is employed as a crosslinking agent to prepare crosslinked semi‐aromatic polyimide films with improved molecular weight, which is expected to overall improve the semi‐aromatic polyimide films’ optical transparency, thermal stability, and mechanical properties.