The Effect Of Hydroxyl Radicals On The Hydrolysis And Condensation Reaction Of Organosiloxanes

With organically modified siloxane as precursors,sol-gel method is one of the most important methods to prepare the organic-inorganic hybridized materials.Silica based hybrid materials have attracted intensive attention in the field of materials science and engineering due to their stable chemical properties,excellent mechanical properties and optical transparency as well as many other advantages of organic-inorganic hybrid materials.At present,this kind of materials have been widely used in functional coatings,biological sensing,catalytic materials,medical materials,optics and thin film materials,etc.Siloxanes with different organically modified groups can not only directly provide the corresponding material properties and functions but also make further modification after the sol-gel reaction according to the organic functional groups.However,the sol-gel reaction possesses some complexity and specialization.In this dissertation,the new strategy for promoting the hydrolysis and condensation of organosiloxanes was proposed based on ultraviolet(UV)irradiation.In addition,the influence of UV irradiation on the hydrolysis and condensation of organosiloxanes was studied;moreover,the mechanism was further clarified.The aim of this study is to provide a new method for the preparation of organic/inorganic hybrid materials by using organosiloxanes as precursors through sol-gel method,and to reveal the mechanism of the effect of UV irradiation on the sol-gel process of organosiloxanes to enrich the theory of organosiloxane sol-gel system.According to the above research objectives,the following research work was carried out:First,a new method for promoting rapid gelation of 3-methacryloxypropyltrimethoxysilane(MAPTMS)under UV irradiation was established without introducing any other additives.The results showed that:UV irradiation can effectively accelerate the gelation reaction in the sol-gel process of MAPTMS,in addition,the terminal methyl methacrylate groups in the product was kept intact;The products containing silanol at both ends were produced by ring-opening of cyclic condensation product in the presence of UV irradiation,further,promoting the gelation of MAPTMS by cross-linking reaction with linear condensation products in the system;According to the results of experiments,the mechanism of acceleration of the gelation of MAPTMS under UV irradiation was proposed.Secondly,y-glycidoxypropyltrimethoxysilane(GPTMS)was used as the precursor to reveal the influence of UV irradiation on its hydrolysis and condensation reaction.The results show:(1)UV light irradiation can promote the hydrolysis of GPTMS in acidic and alkaline environment.(2)Under acidic and alkaline conditions,the epoxy groups in GPTMS have ring-opening effect,and the UV light plays a great role in promoting the ring opening of epoxy groups,and the ring-opening effect is more likely to take place under alkaline condition.(3)Under alkaline condition,ring-opening products of epoxy groups are prone to condensation,and both open and closed systems have a significant effect on the distribution of hydrolyzed condensation products.Methanol produced from the hydrolyzation of siloxane can promote the condensation of silanol and the condensation of diol(epoxy ring opening product),thus promoting the formation of cross-linking structure of organic and inorganic network and promoting the gelation in this system.Despite the structures of gel sample formed were similar under the open light irradiation conditions and non-irradiated alkaline conditions,but the light irradiation plays a significant role in promoting the formation of gel.Thirdly,the hydrolysis system of typical organosiloxanes was tracked using pyrene as a fluorescence probe,and the influence of UV light on the clustering behavior of different groups in the process of hydrolysis condensation of the organosiloxanes such as MAPTMS with hydrophobic and sterically hindered substituents,octyltrimethoxysilane(OTMS)with hydrophobic long-chain alkyl substituents,and propyltrimethoxysilane(PTMS)with short alkyl chain substituents were revealed.The behavior of the molecular clusters was investigated by the hydrolysis of the different precursors under UV irradiation,indicating that:(1)although the length of the substituents and their steric hindrance were different,leading to the homogeneity of the hydrolyzates was different in the three solutions,the fluorescence spectra of pyrene probe showed that UV light could obviously promote the hydrolysis of all organosiloxane.(2)Substituent structure showed a significant effect on the clustering behavior of the precursor hydrolyzate.MAPTMS containing longer rigid hydrophobic substituents has a strong molecular aggregation behavior,and the hydrolysis and condensation reaction have a clearer reaction process.The organosiloxane with flexible long hydrophobic chain substituents shows a strong molecular clustering behavior,and the organosiloxane with short hydrophobic chains had no obvious molecular clustering behavior.(3)Fluorescence probe pyrene is an effective method to detect the molecular clustering behavior of organosiloxane hydrolysis and condensation reaction system.Although the spectral characteristics of fluorescent probe pyrene cannot directly reflect the change of molecular structure,it is very sensitive to the microenvironment change caused by the change of molecular structure,and its rich structures such as fine structure,dimer and excimer complex peak The experimental results show the feasibility of fluorescence probe pyrene for the detection of organosiloxane hydrolysis and condensation system.These conclusions have positive implications for deepening the effect of UV light on the hydrolysis of organosiloxanes,and are of great significance for photophysical methods in the detection of organosiloxane sol-gel process.Fourth,with MAPTMS as an organosiloxane precursor,the mechanism of hydrolysis and condensation under UV irradiation was revealed.The effect of hydroxyl radical scavenger and different saturated gases on the hydrolysis and condensation of MAPTMS and the effect of hydroxyl radical scavenger on the hydrolysis,condensation reaction of MAPTMS,photolysis in the hydrolysis of hydrolysis system,hydroxyl radical detection of fluorescent probe,UV-induced Si-O-Si bond of alcohol,gel composition and structural characterization were studied.It is proved that:(1)hydroxyl radicals promote the gelation process in the system of MAPTMS hydrolysis and condensation under UV light.Also,the general effect of hydroxyl radicals on the hydrolysis and condensation of organosiloxanes is revealed;(2)UV irradiation can cause the alcoholysis of siloxane hydrolysis condensation products in the presence of methanol.To sum up,in this study,we first discovered and revealed the influence of UV irradiation and hydroxyl radicals on the hydrolysis and/or condensation reaction of organosiloxanes.The results obtained have important theoretical and practical value for the enrichment of organosilicon hydrolysis and condensation chemistry and the construction of a new method for the hydrolysis and condensation products of siloxanes based on UV irradiation.