The effect of curing conditions and ageing on the thermo-mechanical properties of polyimide and polyimide–silica hybrids

The effect of curing conditions and ageing on the thermo-mechanical properties of polyimide and its ceramers has been studied. Polyamic acid (PAA), a polyimide precursor, was prepared by the well known reaction of pyromellitic dianhydride and oxydianiline in dimethylacetamide as solvent. The silica network was developed in PAA solution by hydrolysis and condensation of tetraethoxysilane. The pure PAA films and those with 25 wt% silica content were imidized by six different curing protocols involving both step-wise and isothermal heatings. The dynamic mechanical thermal (DMTA) and thermal mechanical analyses (TMA) were performed on the pure and ceramer films cured under the different imidization conditions. The data for the mechanical properties were obtained for the same films left over for a year. The variations of storage and loss moduli and the glass transition temperature associated with a-relaxation and the linear thermal expansion coefficient have been explained in terms of degree of imidization and the effect of silica network densification under different conditions as a result of imidization and ageing.