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- 29Si NMR (3) (entfernen)
The present study contributes to the controversial discussion in the literature whether Si–O–C bonds in wood–silica–gel composites exist. 13C NMR is a suitable method to proof such bonds. Because of the low concentration of 13C isotopes in natural wood, 13C cellulose was used as 13C-enriched substitute. A tailored sol for the impregnation of that cellulose was chosen by liquid 29Si NMR pre-investigations of various sols whose reactivity and stability were time-dependently analysed. It is based on a sub-stoichiometric hydrolysis of tetraethoxysilane (TEOS) with 1 mol water per mol TEOS. Thermal analyses were performed to show a comparability of the thermal behaviour of wood–silica–gel and cellulose–silica–gel composites. There are two strong hints of the existence of stable Si–O–C bonds: (1) by thermal analysis, a shift in the fire behaviour of 100 K can be observed with a change in pyrolysis behaviour of the composite and (2) the proof by REDOR NMR that a dipolar coupling between 1329Si and 13C nuclei exists.
Solid residues of bisphenol A polycarbonate (containing 0.45 wt% poly(tetrafluoroethylene))/silicone acrylate rubber/bisphenol A bis(diphenyl-phosphate) (PC/SiR/BDP) and PC/SiR/BDP/zinc borate (PC/SiR/BDP/ZnB) after thermal treatment were investigated by solid-state and liquid-state NMR, focusing on the role and interaction of SiR with the other components of the polymer blend.
In PC/SiR/BDP, part of the SiR reacts to an amorphous silicate network rather than being completely released in the gas phase. The silicate network consists of Q4 and Q3 groups formed via intermediate D and T groups. The D groups are formed by a reaction of SiR with bisphenol-A units as well as phenyl groups of PC and BDP. In addition a small amount of silicon diphosphate was observed after thermal treatment at temperatures higher than 810 K. The same decomposition products (without SiP2O7) occur in the solid residues of PC/SiR/BDP/ZnB samples. The formation of intermediate D and T groups occurs earlier, at slightly lower temperatures. Any formation of a borosilicate network was excluded. The results also apply for the fire residues of PC/SiR/BDP and PC/SiR/BDP/ZnB and are thus valuable for understanding the impact of SiR on pyrolysis and flame retardancy mechanisms in the condensed phase during the burning of PC/SiR/BDP blends. SiR was found to influence the pyrolysis and the char formed. Beyond the replacement of highly combustible mechanical modifiers, SiR harbours the potential to enhance flame retardancy.
Organically modified cage-like double four-ring spherosilicates have received considerable interest in the construction of nanosized hybrid materials, as well as building units for structural well-defined polymers. This group is extended by perfluoroalkyl ligands containing spherosilicates, synthesized by addition reaction of the octahydridodimethylsiloxyoctasilsesquioxane [H(CH3)2Si]8Si8O20 and heptadecafluorodecyl methacrylate. The resultant liquid spherosilicate substituted with eight terminal perfluoroalkyl groups was characterized by 29Si and 13C NMR spectroscopies and MALDI Time-of-Flight mass spectrometry. Partial substitution of perfluoroalkyl ligands by trimethoxysilyl containing groups provides condensable precursors for the synthesis of hydrophobic and oleophobic materials via the sol-gel process. This new spherosilicate, carrying on average four perfluoroalkyl groups and four trimethoxysilyl groups shows better hydrophobic and oleophobic properties compared with commonly used perfluoroalkyltrialkoxysilanes under identical concentration of perfluoroalkyl chains. In addition a comprehensive literature survey is given on structural well characterized, organically modified cage-like double four-ring spherosilicates.