The Preparation Of Silica/organic Silicone Composites By The Sol-gel Process Of Tetraethylorthosilicate

Organically modified silicas have been widely applied in mechanical, chemical protection and optic fields. Among these silicas, organic silane modified silicas became an attractive and intensive research area, which are also our focus in this study. On lots of home and abroad literatures basis, the study status, including the silica preparation from the TEOS sol-gel process, surface silanols of silica, composite system of silica with organic silicone, the reaction of TEOS with DDS, is analyzed and summarized detailedly. And the development trend of silica-organic silicone composites is brought forward. According to these concepts, the main idea and the study process are given.The dissertation consists of four parts:1. The dynamic laser scattering (DLS) investigation on the hydrolytic polycondensation of tetraethylorthosilicate(TEOS)Sol-gel process usually take place during the hydrolytic polycondensation of tetraethylorthosilicate(TEOS), in which the prepared condition greatly affects the structure and property of resulting materials. The hydrolytic polycondensation of siloxanes involves a cascading series of chemical reactions where the reaction products become the reacting species in an ever-changing molecular size and morphology. At different stages of the acid-catalyzed sol-gel process, the components had great differences, thus possesing different reactive potential.Dynamic laser scattering was employed to investigate the size and growth kinetics of polymeric species formed prior to gelation and to obtain information about the structure of these species. The infrared spectrometry was also used to follow the sol-gel process.The colloidal particle size observed during the sol-gel process has a rising and falling period prior to the sharp rises at the gelation point, indicating the behaviour of the reactive colloidal silica. At the same time, the Si-O network expands following the sol-gel process. These are very guideful for us to prepare nanosilica composites.2. Effects of dimethyldiethoxysilane addition on the sol-get process of tetraethylorthosilicateDimethyldiethoxysilane (DDS) and methytriethoxysilane (MTES) are common alkyl-substituted ethoxysilane monomer. Their mutual reactions are always ways to incorporate the silicon-carbon bond into inorganic silica network, thus lead to new materials. So sol-gel process of TEOS with MTES, DDS, or other organic silicone was widely investigated.In this part, we investigated the effects of DDS addition on the sol-gel process of TEOS by varying the addition-time of DDS to acid-catalyzed TEOS solution in different water contents (the molar ratio of H₂O: Si-OR was 0.5, 1.5, 2.0, 4.0). The objective of this part is to investigate experimentally how DDS addition-time would influence the sol-gel transition behavior as well as the properties and structure of the final polymer.Scanning electronic microscope-Energy dispersive X-ray analysis(SEM-EDX), Thermogravimetric analysis(TGA), and infraed spectrum(IR) were carried out to characterize the result polymers.SEM analysis proved that:Solutions with lower water content (the molar ratio of H₂O: Si-OR is 0.5) have slightest difference in appearance for simultaneous, 10min, or 50min delay of DDS addition. They appeared homogeneous and lead to gels.While solutions with higher water content (the molar ratio of H₂O: Si-OR is 2.0) undergo phase separation. The input-time delay makes DDS react more often with TEOS oligomers than with TEOS monomers. So we can see that phase separation rate becomes more rapid, and the sizes of hydrophobic particles also rise.In solutions with medium water content (the molar ratio of H₂O: Si-OR is 1.5), the simultaneous input of TEOS and DDS leads to gels. But the DDS delay leads to phase separation during which irregular and serious aggregated particles also formed. When DDS is added at the end of sol-gel process, the gelling is also accelerated as that in low water content.Thermogravimetric analysis and the energy dispersive X-ray(EDX) analysis of particles show that: Different result samples with DDS addition in our study have different components and thermo properties. 10min delay of DDS addition makes the result polymer(â…¢B10) have the lowest total weight loss and high content of CH₃ groups.At the end of this part, we discuss why in different water content, the addition-time delay of DDS have different effects on the acid-catalyzed TEOS sol-gel process and how the result polymers are formed.Under acid conditions, the structure of the polymers results primarily from the distribution of silanols produced during the hydrolysis reactions and not from the condensation process.In the case of an excess of water (the molar ratio of H₂O: Si-OR is 2.0) where hydrolysis is more rapid and completely compared with condensation, more reactive silanols are produced on the surface of the reacting species which make it inclined to condensate with any functionlized group such as OR or OH that exist not only in TEOS but also in DDS. The input-time delay makes DDS react more often with TEOS oligomers than with TEOS monomers. Since DDS has a more rapid hydrolysis rate than TEOS, the reaction between DDS and TEOS oligomers is based on complete hydrolysis and OR-free species.In a low water content (the molar ratio of H₂O: Si-OR is 0.5) and acid condition where hydrolysis is more slowly and incomplete accompanied with condensation, polymer growth among oligomeric intermediates containing both OR and OH groups results in extended linear or randomly branched polymers. Sols prepared by co-condensation of DDS-TEOS exhibit a gelation machanism similar to that of TEOS itself. As the input time of DDS is delayed, the degree of copolymerization of DDS-TEOS decreases and the homogeneity of sols lessens, thus rapidly hydrolyzed DDS accelerate the linkage among oligomeric TEOS, which contribute to the faster gelation time.In a medium content of water (the molar ratio of H₂O: Si-OR is 1.5 ), the hydrolyzed DDS is inclined to form DDS-TEOS bonds with oligomeric intermediates of TEOS containing both OR and OH groups. So the amount of surface reactive silanols and the morphology of TEOS oligomer determine the result polymer. Co-condensation of DDS-TEOS also exhibits a gelation mechanism similar to that of TEOS itself. When the input time of DDS is delayed, the formation of DDS-TEOS bonds is suppressed and the heterogeneous reaction is promoted.3. Investigation of reaction parameters in the TEOS-DDS hydrolytic-condensation processIn this section we discuss different effects of various parameters such as the water addition time, the concentation of hydrochloric acid(HCl), the amount of ethanol(C₂H₅OH), et al., on the TEOS-DDS hydrolytic-condensation process.Water addition is a most important factor for TEOS sol-gel process, thus determines its importance in the TEOS-DDS hydrolytic-condensation process. When the molar ratio of H₂O: Si-OR is 2.0, the second water-ethanol addition decide the result polymer of the process.The concentration of hydrochloric acid(HCl) is inquisive of the TEOS-DDS hydrolytic-condensation process. The higher concentration of HCl leads to short period of process from particles with serious aggregation to smoothly spherical particles, and then to rod-like particles.The molar ratio of TEOS:DDS influences the character of result material. When the molar ratio of TEOS:DDS is 1:0.5, the result material shows serious aggregation and irregular apperance. When the molar ratio of TEOS:DDS is 1:1, the result materials are spherical particles. But when the molar ratio of TEOS:DDS is 1:1.25, the result materials are particles with different sizes.The amount of ethanol enhance the homogeneity of the reaction system, which also act as the diluent, thus retards the gel process and reaction rate.4. Investigation of the TEOS-DDS-DPDS hydrolytic-condensation processDiphenyldiethoxysilane(DPDS) is another alkyl-substituted ethoxysilane monomer. The phenyl in the molecular decide the thermal stability of the result material. So incorporating the DPDS into inorganic silica network must lead to new materials with good thermal stability. In this part, we investigated the effects of dimethyldiethoxysilane(DDS) and diphenyldiethoxysilane(DPDS) addition on the sol-gel process of tetraethylorthosilicate(TEOS) by varying reaction parameters to acid-catalyzed TEOS solution in certain water contents (the molar ratio of H₂O:Si-OR is 2.0).It was found that when TEOS:DDS:DPDS=1:0.5:0.25, effectively control of the process will lead spherical particles with smooth surface and slightly aggregation. The sizes of the particles are closely related to the DDS addition time and the second water-ethanol addition following the DPDS addition.Scanning electronic microscope-Energy dispersive X-ray analysis(SEM-EDX), Thermogravimetric analysis(TGA), and infraed spectrum(IR) were carried out to characterize the spherical particles. Results showed that the TEOS-DDS-DPDS spherical particles realize bondly incorporation of TEOS with DDS and DPDS, which has the lowest gravity loss under high temperature compared with the TEOS gels and the TEOS-DDS particles.At the end of this part, we discuss how the TEOS-DDS-DPDS result polymers are formed. The process may be divided into three steps. First, the formation of TEOS oligomers based on complete hydrolysis and OR-free species under high water content. Second, the formation of spherical TEOS-DDS skeleton with partly occupied reactive surface silanols. Third, the formation of TEOS-DDS-DPDS spherical particles through the bonding conjunction of surface silanols with hydrolated DPDS monomer.The investigation above must be helpful for us to exploit the new silica-organic silicone composit materials.Further studies should be focused on the new process in some particular conditions of silica-organic silicone preparation(ultrasonic method, introduction of some alkyl-substituted ethoxysilane monomer, replacement of TEOS hydrolytic-condenstion species by industrial nanosilica and so on). More application should be carried out by using the TEOS-DDS and TEOS-DDS-DPDS particles in some matrix.