On Aggregation Behavior Of Ormosils In Sol-Gel Process

It has been an increasing interest in employing Ormosils (Organically Modified Siloxanes) as precursors to prepare various Organic-Inorganic materials through sol-gel processing. Those materials with chemical inertness, mechanical stability, optical transparency are becoming more and more widely used for optical and electrical materials, chemical/biomedical sensors, catalyst, multifunctional coatings and films, and so forth.Organic component can be introduced in the sol-gel materials by two ways:precursor being organically modified and organic additives being incorporated insol-gels. Different from traditional sol-gel precursors with simple structure and lowermolecular weight, Ormosils present marked substituent effects on sol-gel processdepending on the structure and property of the organic substituent R. In summary, Rgroup has significant influences on reaction rate of hydrolysis-condensation, speciesdistribution of the silonols and silicates, structure evolution and the spatial arrangement,and type of self-organizations. The incorporation of organic additives to sol-gel systemaffects the reaction rate of the precursor undergoing hydrolysis and condensation,modulates the size distribution and dimension of the particles in sol, and controls themorphology of the materials. According to the effects of organic substituent andadditives, the structures and properties of sol-gel materials can be modified,accompanied with the function of composite materials being enlarged. Therefore, it issignificant both in theory and application to systematically investigate the influences ofthe organic moiety on the sol-gel process, which assists in further understanding theorganic modification mechanism in the sol-gel process, controlling the synthesisconditions of organic-inorganic hybrid materials and taking deep insight into propertiesof the final products. With emphasis on the insights above mentioned, we carried out theresearch in this thesis mainly including two parts:(1) For the first time, based on results obtained from fluorescence probe technique and Laser Light Scattering following five hydrolysis-condensation systems derived from different Ormosils with different organic sustituent, the aggregation of hydrolysis products derived from Ormosils with longer substituent into O/W micelle in the aqueous solution was found. The research focuses on organization style of thehydrolysates of Ormosils droved by the organic group with characteristic chemical structure and the effects of such organization style on the evolution of species in condensation process. The in-situ stationary fluorescence spectroscopy with pyrene as a probe was used to follow the hydrolysis and condensation of Ormosils in acid condition. Fluorescence quenching technique was also used to examine the probe behavior in sols. The information obtained from fluorescence spectroscopy shows that at molecular scale, pyrene as a fluorescence'probe could respond sensitively to the organization style and evolution of species produced by Ormosils during the hydrolysis-condensation reaction. When pyrene molecules located in the systems derived from Ormosils with short alkyl group or protonated epoxy group, the fluorescence spectroscopy was similar to that in control solution. It indicates that species of hydrolysates dispersed in the sols and such systems are homogenous, which was also confirmed by obvious fluorescence quenching. In comparison with control solution, when pyrene molecules located in the systems derived from Ormosils with long organic group, a market excimer emission and a decreased fluorescence quenching were observed, implying such systems are inhomogenous. The fluorescence behavior are fitted to the fact that the hydrolysates in these systems are amphiphilic molecules and prone to aggregate like a surfactant to form micelle.Moreover, information from in-situ fluorescence spectra demonstrates that the organization style of the hydrolysates may affect the kinetics of condensation and result in variation of the species. In summary, organic moiety in the precursor with hydrophobicity and steric inhibition restrains the Si—O—Si chain from developing and result in a narrow size distribution of condensation products. Comparatively, a wide distribution of the products in size engenders when the precursor is modified by short hydrophobic group or long hydrophilic group. While oligomers derived from precursor with longer organic chain prefers to precipitate from bulk solution due to strong hydrophobicity.(2) Mechanism on gelation of MAPTMS enhanced by NPG as additives was investigated. A series of techniques including ATR-FT IR, 29SiNMR, *HNMR Raman, UV-Vis, and viscosity have been utilized to follow the sol-gel process of MAPTMS in presence and absence of NPG Moreover, TGA-DSC, XRD, TEM were employed to characterize final solid-state material. Based on the experimental results, the mechanism on the gelation of MAPTMS sol-gel system enhanced by NPG was proposed.hi earlier stage of the reaction for MAPTMS hydrolysis-condensation system, hydrolysate—silanols with long hydrophobic organic tail R=—CH2CH2CH2O(CO)C-(CH3)CH2 and hydrophilic head Si-OH resemble surfactant and aggregate into W/O micelle at high precursor/water ratio. NPG molecule exhibits hydrophobic phenyl and hydrophilic -NH- and -COOH. As a result, a H-bond interaction occurs between Si-OH in hydrolysate and -NH- or -COOH in NPG, hydrophobic interaction and 71-71 stacking between R of hydrolysate and phenyl of NPG make NPG molecules insert aggregates palings formed by silanols and for co-aggregation. An "isolation effect" restrains the condensation between silanols and enhances hydrolysis of Si-OCH3 in the precursor. With increase in surfactant-like silanols, a concentrated microemulsion system engenders. With the reaction going on, decrease in water and increase in methanol lead to abatement of H-bond interaction. Consequently, the microemulsion conformation is destroyed and instead of it, a gel emulsion system with network structure forms. Subsequently, a large number of Si-OH condensate quickly into silicates with high branched and cross-linked structure, and gel emulsions finally transform into solid-state gel.