Morphology Evolution In Process And Structure-Property Relationship Of PI/SiO₂ Hybrid Materials

Polyimide/silica (PI/SiO₂) hybrid materials are important class of new generation materials which combine desirable properties of organic polyimide (toughness, ductility and processability) and those of a ceramic phase (heat resistance, retention of mechanical properties at high temperature and low thermal expansion). Controlling the physical properties of the hybrid materials highly depends on the improvement of compatibility between the organic and the inorganic phases. One of the primary challenges for preparing the hybrid materials using sol-gel method is the phase separation between the organic and inorganic components. In NMP solvent, silicon oxide under acid-catalyzed conditions and base-catalyzed conditions yield different sol structures. Based on the organic or inorganic structure and the mechanism of phase separation, controlling the morphology of hybrids and in a higher degree to keep the mechanical properties are key problems of solving the application of polyimide/silica hybrids. The studies in this dissertation have focus on the purpose to make clear the formation process of phase separation and the relationship between morphology and properties of hybrids.Several hybrid films composed of copolyimide with hydroxyl group, silica andγ-glycidyloxypropyltrimethoxysilane were prepared by the sol-gel process based on hydrolyzed tetraethoxysilane (TEOS) under acidic condition. The components, morphologies and mechanical properties of the hybrids were investigated by FTIR, UV-vis, SEM, Stress-Strain tests and DMA. The results showed that SiO₂ particle size significantly decreased, fractured cross sections of hybrid were rougher and the surfaces of spherical SiO2 particles were more widely covered by PI component. Moreover, two sizes of silica particles were discovered in BOT and BODT hybrid. The tensile mechanical properties of hybrids increased when adding GOTMS. The critical points of maximum tensile strength and elongation at break move from 11 wtï¼…to 16 wtï¼…SiO₂ content. DMA results showed that the storage moduli of hybrids with GOTMS, when above 260℃, were obviously higher than those without GOTMS; the tan8 transition temperature of hybrid films went up from 317℃to 337℃. It suggests that chemical interaction between CPI-OH and silica is formed and the PI molecular mobility is restricted by the chemical interaction. The surface Si content is much lower than the theoretical bulk content in hybrid films.The mesophase morphology of the PI/SiO₂ hybrid films composed of polyamic acid (PAA) and hydrolyzed TEOS (TEOS-OH) through sol-gel method was investigated by SEM. The double phase separations were discovered in the preparation of the hybrids. When the NMP solvent was removed, the miscibility between TEOS-OH and PAA decreases, and the first phase separation took place. The second phase separation took place due to the imidization reaction which destroyed the interaction between carboxyl group of PAA and hydroxyl group of TEOS-OH. Formation mechanism of the nano-silica particles of second phase separation was confirmed by the FTIR spectra. The hydroxyl group in diamine is introduced in the polyimide side chain, the double phase separation still take place. Decreasing molecular weight of PAA, the silica particle size derived from second phase separation increase from 200nm to 350nm. After introducing GOTMS into hybrid film, amounts of the silica particles from the second phase separation decrease, which is due to the chemical bond between polyimide and silica component. Decreasing the molecular weight of PAA as well as introducing the GOTMS into hybrid, the size of silica particles separated from organic phase increases from 200nm to 650nm. At the same time, the core-shell structural hollow silica particles were discovered. This implied that, increasing the compatibility between organic and inorganic components, amounts of organic component could exist in the inorganic phase. With solvent further evaporated and silane oxide further condensation, the phase separation in inorganic phase takes place and the core-shell structure hollow silica particles appear.According to double phase separation mechanism, a series of PI/SiO₂ hybrid films (8 wtï¼…silica content) were prepared by sol-gel method based on hydrolyzed TEOS and poly(amic acid-imide) with different imidization degree. Imidization degree, as well as corresponding morphology was investigated by FTIR and SEM. The results show that, amount of small silica particles derived from second phase separation decrease and even disappear at higher imidization degree, and the larger size of silica particles formed by solvent evaporated increase from 300nm to 1μm and form ellipsoid at higher imidization degree, simultaneously. This implied that, with decreasing amount of carboxyl group in polymer, the compatibility between inorganic and organic component decrease, and the phase area derived from first phase separation increase. Tensile strain-stress tests, DMA and TMA tests show that, amounts of small silica particles in hybrid films increase, the mechanical properties, Tg and thermal expansion coefficient of the hybrid become higher. The nano-scale silica particles act as physical cross-linking point to improve the mechanical and thermal properties.A series of transparent PI/SiO₂ nanohybrid films were fabricated via sol-gel process using hydrolyzed tetraethoxysilane (TEOS) under base condition. The results show that, when increasing silica content, size of the silica particles remains about 100 nanometers. When silica content reaches 30ï¼…, the silica particle size is almost the same with the size at 7ï¼…silica content. Alter probing into hydrolysis mechanism of TEOS under base condition, it reveals that formation of silica particles is related to clusters of sol particles. The sol particles catalyzed by base contain large amounts of hydroxyl groups, which could interact with the carboxyl groups in PAA and restrict further agglomerate of sol particles, resulting in hybrid films with uniform dispersion of silica particles. Imidization reaction of PAA is catalyzed by the electronegative sol particles. The tensile strain-stress test shows that, below 16 wtï¼…, both the tensile strength and the elongation at break of hybrid films increase with increasing silica content. When the silica content exceeds 16ï¼…, the tensile strength and the elongation at break decrease. DMA test results show that, increasing silica content, the mobility of polyimide chain is restricted by the nano-scale silica particles. The thermal stability decrease when the silica contents increase. That because the electronegative silica particles make the imide ring of polyimide unstable.