Study On Aggregation Structure And Thermal Properties Of Polyimide/Silica Nano-Hybrid Films

Polyimide has been used in a wide variety of engineering applications such as microelectronic packaging, high temperature adhesives, and composite materials owing to their outstanding physical and chemical properties, including good thermal stability, excellent mechanical strength, and good dielectric insulation etc. The rapid increment of electric power and application of new technologies call much more rigid claims to the polyimide manufactures, demanding high performance polyimide which the pure can't meet to, such as the strength, the water absorbability and thermal properties.It has been proved by many researches that incorporating some inorganic nano-particles including silica, alumina and titania into polyimide matrix resin via sol-gel approach would provide some improvements to its properties, such as increasing dimensional stability, reducing thermal expansibility and improving the heat-resistant property, owing to the excellent physic and chemistry properties of the inorganics. Polyimide incorporated with diverse inorganic nano-particles may obtain distinct properties and can respond to the rigid demands of certain applications. Till now, great attentions have been focused, mainly, on the preparation of polyimide/silica nano-hybrid films through sol-gel route and the influences of silica on the morphologies, thermal decomposition temperature (T_d) and glass transition temperature (T_g) etc. But there were seldom systematical researches about the influences of different incorporating conditions on the aggregation structure and the thermal properties of PI/SiO₂ nano-hybrid films.In present study, polyamic acid/organic-Si solutions were synthesized via sol-gel route and thermal imidization by the hydrolysis and condensation of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) that served as the precursors of the inorganic moiety and produced the silica nano-particles in-situ in the polyamic acid solution (PAA) based on the stepwise polycondensation of pyromellitic dianhydride (PMDA) and 4, 4′-oxydianiline (ODA) in N, N-dimethylacetamide (DMAc). The coupling agent, providing chemical bonding between the polyimide and the silica nano-particles, was added to decrease the extent of phase separation, caused by the great difference in the properties of organic matrix and the silica materials. When using TEOS as the inorganic precursor, the films were recorded with PI/TEOS-SiO₂ nano-hybrid films, and recorded with PI/MTEOS-SiO₂ nano-hybrid films when using MTEOS. The chemical structure, aggregation structure and the thermal properties of the nano-hybrid films were respectively characterized by Fourier Transform Infrared Spectroscope (FT-IR), Atomic Force Microscope (AFM), Scanning Electronic Microscope (SEM), Wide-angle X-ray Diffraction (WAXD), Thermogravimetric Analysis (TG) and Differential Scanning Calorimeter (DSC).All the results indicated that the practical amount of inorganic silica nano-particles, dispersed homogeneously in the polyimide resin via sol-gel route was almost equal to the calculated addition. With increasing silica content, the morphology of inorganic phase transited from small isolated spheric-like particles to IPN structure in PI/TEOS-SiO₂ films, but from small isolated rod-like particles to big micro-agglomerations in PI/MTEOS-SiO₂ films. The incorporation of silica nano-particles not only reduced the glass transition temperature (Tg), postponed the glass transition zone, which resulted the unreadable figure of Tg, but also improved the heat-resistant property of polyimide resin. Contrastively, the heat-resistant property of PI/MTEOS-SiO₂ films was better than that of PI/TEOS-SiO₂ films. The influences of silica precursors, preparation technics and coupling agents on the aggregation structure and the thermal properties of PI/SiO₂ films were also investigated in the study.