Synthesis, Structures And Properties Of Polyimide/Ladder-Like Polysilsesquioxane Hybrid Films

In the field of high performance polymer, polyimide (PI) has receivedwide attention due to its outstanding properties. Furthermore, PI compositeshave shown very promising properties. Blending silicon element in PI matrixhas been one of the most important research trends, because the compositespossessed higher adhesive properties, lower dielectric constant and lowerwater absorption. Ladder-like polysilsesquioxane (LPSQ) has attractedconsiderable interest because of their excellent chemical, physical andelectrical properties derived from the unique double-chained skeleton.Moreover, the aspect ratio, functional groups and average functionality ofLPSQ are controllable according to diverse requirements. Thus, LPSQs werechosen to incorporate into PI matrix for the first time in order to fabricate highperformance PI. By studying the relationship between structure and propertiesof hybrid films, the performance of PI composite can be controlled ultimately.The main content of this dissertation are shown below.Two kinds of ladder-like polyphenylsilsesquioxane (LPPSQ) have beensynthesized by quilibrium polycondensation and sol-gel method, respectively.They were characterized by gel permeation chromatograph (GPC), Fouriertransform infrared (FTIR), nuclear magnetic resonance (NMR), X-raydiffraction (XRD) and molecular simulation. The results confirmed that the regularity of LPPSQ decreased slightly as the molecular weight increased.Mercapto functionalized ladder-like polysilsesquioxanes (LPMPSQs) havebeen synthesized with different feed ratio. They were characterized by FTIR,29Si-NMR and XRD. The results suggested that the dosage of mercaptomonomer had great effect on the final production. LPPSQ has been modifiedto ladder-like poly(nitrophenyl)silsesquioxane (LPNPSQ) andpoly(aminophenyl)silsesquioxane (LPAPSQ) by nitration and reductionreaction, respectively. They were also characterized by FTIR, NMR, XRD andmolecular simulation. The results confirm the ladder-like structures of LPSQand suggest the decrease of regularity after the chemical modification. Thefunctionality of LPSQ can be controlled by adjusting the time of nitrificationor the amount of reducing catalyzer agent.A series of LPSQs with different molecular weight, functional groups andaverage functionality was incorporated into PI matrix, respectively. ThesePI/LPSQ hybrid films showed different properties owning to the difference ofLPSQs. The structure of PI/LPSQ hybrid film was investigated with XRD,scanning electron microscopy (SEM) and X-ray photoelectron spectroscopic(XPS). The properties were studied using dynamic mechanical analysis(DMA), thermogravimetry (TGA), tensile tests, water contact angles analysisand water absorption test. The main conclusions are shown below.(a) With theincrease of LPMPSQ fraction, the concentration of silicon atom on the surfaceof the hybrid film keeps rising, and the water absorption capacity decreasesaccordingly. However, the doping content had no effect on the thermalstability of PI/LPMPSQ hybrid films. PI/LPMPSQ hybrid films stillmaintained excellent mechanical properties of PI when the doping content wasless than and equal to10wt%.(b) The molecular weight of LPMPSQ also hadno effect on the thermal properties of PI/LPMPSQ hybrid films. Themechanical properties depended on the interaction of the two phases when the doping content was less than and equal to10wt%. With the doping contentfurther increasing, the effect of phase separation became important.Long-chained LPMPSQ was more inclined to aggregation, so the mechanicalproperties of PI/long-chained LPMPSQ hybrid film were weaker thanshort-chained LPMPSQ's.(c) There are hydrogen bonds, physicalentanglement, and chemical cross-link between LPPSQ, LPNPSQ, LPAPSQand PI matrix, respectively. The strong interfacial interactions can improvedispersion of LPSQ significantly, thereby improving the thermal andmechanical properties of hybrid films. However, these strong interactions canalso limit the migration and enrichment of LPSQ to the surface of the film.(d)With the increase of amino concentration in LPSQ, the cross-linking densityof hybrids raised and the interfacial interactions were enhanced, thereby themovement of PI and LPSQ was more restricted. As a result, the surface siliconconcentration decreased, Young's modulus increased and the elongation atbreak dropped. Due to the incomplete imidization, both Tgand tensile strengthdecreased with the increase of amino content.(e) XPS results of PI/LPPSQand PI/LPMPSQ hybrid films showed that the migration and enrichment ofLPSQ to the surface of the film mainly occurred at solvent evaporation stagebefore heat treatment. It is worth mentioning that the polymer chains of PIseem to get absorbed on the surface of LPMPSQ particles after thermal curing.Compared with PI/LPPSQ, all the LPMPSQ particles were in close contactwith the PI matrix, which implied the existence of the relatively stronginteractions between mercapto groups and PI.