Study On The Synthesis, Structure And Properties Relationships Of Polymer Composites Containing Polyhedral Oligomeric Silsesquioxanes

Polyhedral oligomeric silsesquioxane(POSS)is a class of inorganic/organic hybrid cage-like molecule represented by the generic formula of(RSiO_1.5)_n(n=6,8,10 or higher,R is an organic group).POSS molecules contain an inorganic silica-like core,which is covered by organic substituents.Due to the hybrid nature and nanometer-scale feature, they have a good compatibility with most of polymers,and can be easily incorporated into polymers to prepare composites by chemical or physical methods.The incorporation of POSS derivatives into polymer can lead to dramatic improvements in polymer properties which include increases in mechanical properties,thermal stability,oxidation resistance and dielectric properties,as well as reductions in flammability.In the present dissertation,several different kinds of POSS were prepared and characterized by Fourier-Transform Infrared Spectrography(FT-IR),Nuclear Magnetic Resonance Spectrography(NMR)and Element Analysis(EA)etc..The synthesized POSS includes octavinyl polyhedral oligomeric silsesquioxane(OV-POSS),octaphenyl polyhedral oligomeric silsesquioxane(OPS),octanitrophenyl polyhedral oligomeric silsesquioxane(ONPS),octaaminophenyl polyhedral oligomeric silsesquioxane(OAPS), octa(γ-chloropropyl)polyhedral oligomeric silsesquioxane(OCP-POSS)and octa(tetramethylammonium)polyhedral oligomeric silsesquioxane(TMA-POSS).These POSS were respectively incorporated into different polymer systems,including poly(methyl methacrylate),polyimide and bismaleimide to prepare organic-inorganic polymer composites.The morphology of composites were examined by Transmission Electronic Microscopy(TEM),Scanning Electronic Microscopy(SEM)and X-ray diffraction(XRD).The thermomechanical properties were investigated by Differential Scanning Calorimetry(DSC),Dynamic Mechanical Analysis(DMA)and Thermal Gravimetric Analysis(TGA).The relationships between structure and properties of these composites were also discussed.The details in this thesis are described as follows:(1)Poly(methyl methacrylate)(PMMA)containing octaviny polyhedral oligomeric silsesquioxane(OV-POSS)nanocomposites were synthesized by simple solution polymerization.The actual OV-POSS content in the composites and the reaction degree of the vinyl groups in the OV-POSS were calculated on the basis of FTIR,TGA and ¹H NMR respectively.The DSC and TGA results indicate that the incorporation of OV-POSS molecules could improve the thermal properties of PMMA nanocomposites significantly. The improvement of thermal properties mainly depend on the OV-POSS content in composites.The glass transition temperature(T_g)and thermal decomposition temperature (temperature of 5%weight loss)of the nanocomposite with 12.27wt%of OV-POSS were increased by 23℃and 93℃correspondingly.(2)Organic/inorganic hybrids from two different octafunctional cubic silsesquioxane nano-cage cores and poly(methyl methacrylate)(PMMA)were synthesized using atom transfer radical polymerization(ATRP),in which octa(γ-chloropropyl)polyhedral oligomeric silsesquioxane(OCP-POSS)and functional octaaminophenyl polyhedral oligomeric silsesquioxane(OAPS)nano-cages were used as ATRP initiators.GPC data revealed that the resulting hybrids exhibit unimodal and narrow molecular weight distributions indicating well-controlled synthesis.The polydispersity index(PDI)of obtained hybrids are 1.19 and 1.12 respectively.The influence of POSS cages on the thermal property of hybrids was investigated using differential scanning calorimetry-(DSC) and thermal gravimetric analysis(TGA).It was observed that the hybrids exhibit significantly higher glass transition temperature compared with its linear counterpart, because of slow relaxation caused by the star-like architecture.TGA study indicate that there are significant improvement in thermal stability of composites as compared with linear PMMA.(3)Transparent nanocomposites containing POSS were obtained by incorporation OCP-POSS and POSS-triol into PMMA via "physical blending".XRD and TEM studies indicate that two different type of POSS were completely dispersed at nano-scaled in PMMA matrix.The results of SEM show that there are much difference in the fracture surface of PMMA composites containing different POSS.DMA results show that the incorporation of POSS can lead to slight decrease of glass transition temperature of PMMA composites in comparison with neat PMMA.The influence of different POSS structures on storage moudlus of PMMA nanocomposites in glass transition region are different.The PMMA/OCP-POSS nanocomposites display lower storage moudlus compared with pure PMMA.However,the incorporation of POSS-triol can lead to increase of storage moudlus in comparison with pure PMMA,which exhibits the reinforcement of POSS-triol.The TGA results show that the incorporation of POSS molecules could improve the thermal stablity of PMMA composites significantly.But PMMA/POSS-triol composites display much higher thermal decomposition temperature than those of PMMA/OCP-POSS composites. (4)The mesoporous silica was synthesized under alkaline condition,using TMA-POSS as the silicon source and hexadecyltriethylammonium bromide(CTAB)as the template. The results indicate that the synthesized mesoporous silica exhibited a well-ordered hexagonal nano-pore structure and with larger specific surface area.With increaseing of CTAB moler ratio,distance of the crystal plane d₁₀₀is increased.The different effects of the pH on the mesoporous structure in the reaction system with different moler ratio of n(POSS)/n(CTAB)were observed.The polyimide composites were obtained by incorporation mesoporous silica into polyimide via "physical blending".The incorporation of mesoporous silica could decrease significantly dielectric constant of polyimide materials. The TGA results indicate that the incorporated mesoporous silica could improve the thermal stability of polyimide composites.(5)Bismaleimide(BMI)/2,2′-diallylbisphenol A(BA)composites containing POSS-triol were prepared.Curing process of BMI/BA and BMI/BA/POSS-triol curing systems were measured and analyzed by dynamic differential scanning calimetry at different scanning rates.The dynamics parameters of two curing systems were calculated by Kissinger and Flynn-Wall-Ozawa methods.The results show that the incorporation of POSS-triol has little effect on the curing mechanism of BMI/BA system.Dynamic mechanical analysis (DMA)results show that the incorporation of POSS-triol can lead to slight decrease of storage moudlus of bismaieimide composites in glass transition region,which display toughness effect of POSS,but it can exhibit reinforcement role in rubbery region.All BMI/BA/POSS-triol composites have higher glass transition temperature than the pure BMI/BA composite.The glass transition temperature of BMI/BA/POSS-triol composites increased with the increase of concentration of POSS-triol.The use temperature of bismaleimide-based composites are improved further.The TGA results indicate that the polyimide composites with POSS-triol have better thermal stability than pure polyimide material.