Polyimide membranes modified by polyhedral oligomeric silsesquioxane

Four kinds of polyimide(PI)/Si membranes were prepared using PI with octaphenylsilsesquioxane(OPS), octa(aminophenyl)silsesquioxane(OAPS), polyphenylsilsesquioxane(PPSQ) of ladder structure, and nano-silica(SiO₂), respectively. Their morphologies were studied by scan electron miscroscope(SEM) and the mechanical properties were measured by tensile testing and the thermal properties were studied by differential scanning calorimetry(DSC), thermogravimetric analysis(TGA) and limited oxygen index(LOI). The SEM and XRD results of the PI/Si composites proved that there were good compatibility between PI and OAPS because the aminophenyl groups in OAPS could participate in the polymerization process of PI. The mechanical properties of PI/OAPS membrane were nearly the same with that of PI. And the addition of OPS or PPSQ decreased the PI mechanical properties sharply. DSC analysis implied that the additives could increase the glass-transition temperature(T_g) a little and the T_g of PI/OAPS was the highest because the Si-O caged structures existed in the PI main chain. The addition of OAPS, PPSQ or SiO₂ would improve the thermal stability a little according to the TGA results. One interesting result is that the LOI of PI/Si membranes significantly increased compared with pure PI membrane. The LOI for PI/OPS membrane was 57%, while that for PI was 46.5% only. The improvement was caused by the formation of a well covered silicate char structures after burning. And the LOI of the membranes of PI with polyhedral oligomeric silsesquioxanes are higher than PI/SiO₂.