Study On The Preparation Of Several Kinds Of Silsesquioxanes And Combustion Properties Of Their Polystyrene Composites

Silsesquioxanes are a large branch of organic-inorganic silicon-containing compounds. They can be represented as (RSiO_3/2)_n, where R can be hydrogen or any alkyl, alkylene, aryl, arylene groups, or organo-functional derivatives of them. Due to its special definite microstructure, good thermal stabilities, excellent dielectric properties, and ideal controllable performance in reactions, silsesquioxanes have attracted considerable research interests since 1960s. In this work, the general synthesis, characterization and applications of ladder-like and cage-like silsesquioxanes have been summarized on the basis of extensive investigation of literatures. The synthesis, side vinyl polymerization, carbonization in vacuum of ladder-like silsesquioxanes and its application in clay modification are studied; the combustion properties and fire retardant mechanism of halogen-free and halogen-containing polyhedral oligomeric silsesquioxanes (POSS)/Ploystrene(PS) system have been investigated and contrasted; POSS-surfactants lamellar hybrids were also prepared in this work, and were used as a special filler for PS in order to improve its combustion properties. In summery, this dissertation is composed of the following parts:(1) The synthesis and thermal properties and other related research of ladder-like polyvinylsilsesquioxanes (LPVS). Ladder-like poly(vinylsilsesquioxanes)(LPVS) with substitutional vinyl which was side-by-side polymerized (LPVS-SP) and contains four parallel chains in the skeleton were synthesized by two steps: first, LPVS was prepared by stepwise coupling polymerization on the basis of amido H-bonding self-assembling template from vinyltrichlorosilane (VTCS); then the vinyl of LPVS was polymerized side by BPO at high temperature. The composition and structure of the products have been characterized by ¹HNMR, ²⁹Si NMR, FTIR, TEM, SEM, XRD and elemental analysis, and then its formation mechanism is suggested. LPVS-SP possesses better thermal stabilities than LPVS for its four-parallel-chain structure and stable Si-0 bond. The thermal crosslinking and pyrolysis of LPVS-SP are studied by heating it in vacuum. X-ray diffraction measurements and weight measurements indicate that the turning point temperature from weight loss to inter-molecules thermal crosslinking is around 700°C. At last, LPVS is introduced to modify montmorillonite (MMT): the intercalated LPVS-OMT nanocomposite was prepared by solution blending, and then it is further treated at 250.0°C for 2 hours in order to eliminate surfactant in OMT.. SAXS and TGA suggest that the nanocomposite retains its intercalated structure and exhibits better thermal stability, which implies potential application of this polymer composite.(2) Synthesis and characterization of Octa(tetramethylammonium) polyhedral oligomeric silsesquioxanes (TMN-POSS) and Poly(bromophenylsilsesquioxanes) (Br-POSS) and their polystyrene composites enhanced combustion properties. TMN-POSS was synthesized from tetraethoxysilane and (CH₃)₄NOH-5H₂O. It is used as fillers to prepare TMN-POSS/PS composites. When compared to pure PS, PS composites with loads of 20% and 30% of TMN-POSS in the composite can decrease the peak heat release rate(HRR) of composites by 31.7% and 54.6%, the average HRR by 17.6% and 41.5%, the peak CO release rate by 55.3% and 66.1%, the peak CO₂ release rate by 23.8% and 53.1%, the concentration of CO₂ by 31.1% and 62.0% respectively. The Limit Oxygen Index (LOI) also can be raised from 16.5 to 22 and 23 respectively.The Octaphenyl POSS (Ph-POSS) is synthesized via the hydrolysis and condensation of phenyltrichlorosilane (PhSiCl₃) and the subsequent rearrangement reaction catalyzed by tetramethylammonium hydroxide. Poly(bromooctaphenylsilsesquioxane) (Br-POSS) are easily synthesized from Ph-POSS via bromination with Br₂/Fe system. Br-POSS together with Sb₂O₃ are used as fillers to prepare (Br-POSS + Sb₂O₃) /PS composites. When (Br-POSS+Sb₂O₃) compositions are as high as 15% and 20%, the peak HRR of composites is decreased from more than 900kW/m² to less than 200kW/m² , and LOI can be raised from 16.5 to 29 and 30 respectively.Improvement of combustion properties of the POSS/polymer composites mainly depends on barrier effect generated from silicon-oxygen cages of POSS, elements or groups of effective flame retardance in organic substituents of POSS, and dispersion of POSS in polymer matrix. There are both similarities and differences between the fire retardant mechanisms of TMN-POSS/PS composites and (Br-POSS+ Sb₂O₃) /PS composites. They contain same silicon-oxygen cages, possess similar dispersions, but have different organic substituents.(3) Studies on POSS-surfactants lamellar hybrids. Two-dimensional multilayered complex of Octabenzenesulphonate Polyhedral Oligomeric Silsesquioxane(ObsPOSS) modified by alkyltrimethylammonium bromide (C_nH_2n+1N(CH₃)₃Br, CnTAB, where n denotes the carbon number in the alkyl chain) via ion-exchange reaction. Lamellar framework was obtained for the different compatibility between the organic part (ObsPOSS core made up of silicon oxide) and the inorganic part (side group of ObsPOSS and surfactant alkyl chain), the lamellar structure with a layer space from 3 to 5 nm can be confirmed by XRD, TEM and SEM. Similar lamellar hybrids can be obtained from octa(y-chloroammoniumpropyl) polyhedral oligomeric silsesquioxanes (OCAP-POSS) dodecyl benzene sulfonic acid sodium salt (C₁₈H₂₉NaO₃S, DBSS). Further research shows that different POSS to surfactant ratios result in different arrangement of layers: when there exists excessive POSS, the layer is arranged in a "Alkyl-POSS-POSS-Alkyl" sequence; while on the contrary, the layer is in "Alkyl-POSS" sequence. For one kind of lamellar hybrids with a given POSS to surfactant ratio, the layer space of changs linearly as a function of the alkyl chain length of surfactants.The synthesized lamellar hybrid has been used as fillers to prepare PS composite with improved combustion properties. A lamellar hybrid content as low as 1% in the composite can cause about a 50.1% decrease of peak HRR, however, a higher content of this hybrid doesn't seem to work.