Surface and structural properties of porous silica coated with polysiloxanes

A variety of physicochemical techniques such as Fourier transform infrared spectroscopy (FTIR), high resolution thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and adsorption measurements were used to unveil the surface properties and the nature of interactions between the silica surface and siloxane polymers, OV-225 and LCP-83. Thermogravimetric measurements of coated samples with varying degree of coverage indicate a weight change due to the loss of physically adsorbed water, thermo-desorption and/or decomposition of coated polymer, and condensation of silanols. The nitrogen adsorption behavior of all siloxane polymer modified surfaces falls in the category of type IV isotherm with the formation of a monolayer followed by layer-by-layer adsorption and capillary condensation. These isotherms show a decrease in the pore volume and mean pore size with increasing polymer coverage. An uniform coverage of the silica surface by OV-225 polymer is evident from the pore size analysis. Also a steady gradual decrease in the surface heterogeneity is visible by comparing the adsorption energy distributions for the samples with increasing coating level.; The physically coated siloxane polymer on trimethylsilyl-modified silica exhibits distinctly different characteristics in comparing to the coated on bare silica. A reduced thermal stability of the samples reflected by desorption of polymer at lower temperatures (<500°C) and no systematic shift of the pore size distributions towards lower pore sizes were observed for the samples with increasing coating level. Thermogravimetric characteristics of liquid crystal coated silicas is analogus to the samples coated with OV-225 polymer.; The nature of interactions between the siloxane polymer and silica was investigated by FTIR and NMR. The FTIR study was focused on the analysis of the temperature dependence of the nitrile bands. These bands were deconvoluted into three major components, which were attributed to the formation of the hydrogen bonded species on the silica surface, interfacial species, and pure polymer species. Dipolar couplings among the protons, spin-lattice relaxation times, and coherence magnetization transfer between silica surface and siloxane polymer were evaluated from one and two dimensional 29 Si and proton NMR experiments. An enhancement of relaxation rates on the rotating frame of silicon nuclei due to efficient and effective relaxation pathways primarily due to dipolar interactions between protons and silicon atoms was observed. Two-dimensional heteronuclear 29Si- 1H NMR experiments further revealed that aromatic protons are within 0.4 nm range from the surface supporting a parallel orientation of six-membered ring on the silica surface.