Resinas de silicona a base de silsesquioxanos: síntese, estudo estrutural, morfológico e avaliação do comportamento térmico

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

In the present work the synthesis, structure, morphology and thermal behaviour of silicone resins prepared from phenyltrichlorosilane, phenyltriethoxysilane, n-octyltriethoxysilane or n-dodecyltriethoxysilane, as sources of T units, and poly(dimethylsiloxane) or 1,3-diethoxytetramethyldisiloxane, as sources of D units were investigated. Two series of resins were prepared: the first one, based on poly(phenylsilsesquioxane) polymers, PPSQ, was prepared under different experimental conditions and the second was composed of T and TD resins obtained through the sol-gel process. In this last series, T, T and T resins were synthesized in the absence of solvents, while the TD ones, containing 20- 50% D units, which present as long or short dimethylsiloxane segments D and D, were obtained in toluene. The appearance of the resins varied from resin-Iike glasses to white powders, in the first series, while the films derived from T were transparent and the others were translucent, except for TD, The molecular structure of these resins was investigated by FT-IR and Si MAS NMR spectroscopies. A dependence of the percent of T and T sites on the experimental conditions used for the preparation of PPSQ samples was observed. The polymers obtained in aqueous medium, in the presence of CTABr or TPABr showed a higher amount of T sites. The PPSQ polymer prepared in organic solvent exhibited a less-organized structure with higher amount of residual silanol groups. The other T and TD resins presented condensation degrees in the range of 81 up to 100%. The organization of PPSQ resins were evaluated by XRD. Two well-defined halos at repeating distances of 12,5 and 4,6 A were assigned to intra-segmental and intersegmental spacing, respectively. The organization of these materiais was dependent on the condensation degree. The resins prepared by the sol-gel process exhibited less-defined halos. An increase in the inter-segmental spacing for the T and T derived resins in relation to PPSQ ones was also observed. Small-angle X-ray scattering, SAXS, curves were also obtained for these materials. A surface-fractal behavior was observed for the PPSQ resins prepared in the aqueous medium, and a surface and mass-fractal curves, for low and high q values, was observed for the sample prepared in the organic solvent. The T and TD resins derived from T and T showed halos associated to intersegmental spacing. The TD material presented a halo indicative of a system containing concentrated scattering centers with some spatial correlation, while TD showed a typical curve of a polydisperse system. T exhibited no scattering, which confirmed the high homogeneity of this material. FEM micrographs of the PPSQ samples prepared in the presence of CTABr showed the presence of spheres dispersed on a dense matrix. Other PPSQ samples, obtained in aqueous medium, showed a morphology characterized by the coalescence of spheres. The morpholgy of the resins obtained through sol-gel method was typical of homogeneous, non-porous materials. The thermal stability of the PPSQ resins was investigated by TGA traces and was lower for the silanol-rich samples. Regardless of the presence of D units, a good thermal stability up to 350°C was observed for resins prepared by the sol- gel method. Whereas the DSC traces of the T derived resins showed a glass-transition at -70°C, the n-dodecyl containing materials exhibited a first-order transition associated with a melting process in the range between -0,7°C and -16°C. The T resins exhibited a broad transition from -150 to 150°C range.

ASSUNTO(S)

sintese inorganica quimica inorganica polimeros inorganicos estrutura quimica

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