High-temperature polyimide composites prepared from soluble polymeric and crosslinkable oligomeric precursors: phase demixing and properties

Rigid/crosslinked flexible polyimide composites were prepared from soluble polymeric and oligomeric precursors: poly(4,4'-oxydiphenylene pyromellitamic acid) [PMDA-ODA(PAA)], poly(4,4'-oxydiphenylene pyromellitamic diethyl ester) [PMDA-ODA(ES)], acetylene-terminated isoimide oligomer of 3,3',4,4'-benzophenonetetracarboxylic dianhydride/1,3-bis(3-aminophenoxy)benzene (BTDA-APB) and acetylene-terminated imide oligomer of 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/1,3-bis(3-aminophenoxy)benzene (6FDA-APB). PMDA-ODA(PAA) precursor was found to react with BTDA-APB isoimide oligomer in N-methyl-2-pyrollidone (NMP), resulting in gels. Furthermore, PMDA-ODA(PAA) exhibited a limited miscibility in making its homogeneous solution with 6FDA-APB imide oligomer in NMP, consequently leading to large domains in the softbaked blend films as well as in the resultant composite films. In contrast, for PMDA-ODA(ES) blends with 6FDA-APB and BTDA-APB, homogeneous ternary solutions with a relatively high solids content of less than or equal to 30 wt% were easily achieved in NMP over the entire composition range. Dried blend films were optically clear except a few compositions. For some compositions, the optical transparency in the dried films was retained through the thermal curing process, providing optically transparent semi-interpenatrating network (semi-IPN) composites. However, for the other compositions, phase separation took place through thermal curing, forming domains of submicrometre scales in the resultant composites. Properties of the polyimide composites were investigated by dynamic mechanical thermal analysis (d.m.t.a.), stress-strain analysis and self-adhesion measurement. In particular, the self-adhesion property of PMDA-ODA was significantly improved by the composite formation with 6FDA-APB, but not improved by the composite formation with BTDA-APB. The good self-adhesion in the PMDA-ODA/6FDA-APB composite was characterized by X-ray photoelectron spectroscopy and attributed mainly to the favourable molecular interdiffusion through the 6FDA-APB component segregated on the surface. In addition, the residual stress and relaxation behaviour were measured