Organik-inorganik hibrit malzemelerin sentezi ve karakterizasyonu

Organic–inorganic hybrids that combine the advantages of both kinds of materials, such as mechanical strength and thermal stability with the processability and flexibility of an organic polymer matrix, exhibit multifunctional characteristics. Hybrid materials with the desired properties can be obtained in many forms such as bulk, powder, nanocomposites, coatings, glasses, fibres, foams etc., depending on the forming process and a wide variety of components of hybrids such as metal oxides, alloys, ceramics, clay, rubber, resins, elastomers, natural materials. Through the combinations of different inorganic and organic components with appropriate processing methods hybrid materials can be developed with new properties for electrical, optical, biomedical, structural, or related applications. In principle two different approaches are used for the formation of hybrid materials: First approach, well-defined preformed building blocks are applied that react with each other to form the final hybrid material in which the precursors of blocks still at least partially keep their original integrity and second approach, one or both structural units are formed from the precursors that are transformed into a novel (network) structure as in-situ method. There are mainly two classes of hybrid materials; the one with weak interactions between the two components of hybrid materials, such as van der Waals, hydrogen bonding or weak electrostatic interactions and no strong interaction such as covalent bonds and the second having strong chemical interactions between the components of the hybrid materials such as covalently bonding. Silica has attracted much interest due to their low toxicity, ease of formation in a wide range of sizes and morphologies, high stability, and the surface that can be further functionalized. This thesis based on the study of incorporation of silica nanoparticles and silicon dioxide network domains, as the inorganic components, with organic polymeric structures leading the formation of hybrid materials and, the characterization of the such materials. The interactions between the inorganic silicon dioxide and organic components were based on the strong chemical interaction as covalent bonding. Silica nanoparticles leading to an extreme increase in interfacial area have been considered as a challenging reinforcement with a wide range of properties for hybrid materials. Their efficiency in polymeric matrices requires uniform dispersion and strong interfacial bonding between two component of the hybrids. Common strategies are being developed to improve the poor dispersion of nanoparticles in polymer matrices and also organic solvents for advanced interfacial bonding of nanoparticles and matrices. Coupling reactions of silica nanoparticles with silane coupling agents having functional groups is one of the most common approaches for the modification of the surface of the silica particles. As the first step of the coupling reaction, silane coupling agents hydrolyze to form silanols and during the hydrolysis step, condensation can also take place between silanols resulting in ormation of siloxane bridges (Si-O-Si). The condensation between the silanol groups of coupling agents decrease the number of free silanols of silane coupling agents that reduce the rate of possible condensation with the silanol groups of the silica particles. Hence, the formation of a siloxane network layer on the surface of the silica nanoparticles may also results in a variety of concentration of functional groups of silane coupling agents. In the second step of coupling reaction of silica particles, the possibility of different type of condensation reactions between free silanol groups of coupling agents and silanol groups of silica nanoparticles may also result in inadequate concentration of functional groups grafted on the surface of silica particles. Seemingly effective coupling reactions of silica nanoparticles over the silane coupling agents may become challenging due to the such disadvantages. Direct functionalization of the surface of silica particles with reactive organic moieties can overcome such problems of silane coupling reactions and the limitation of high concentration of functional groups on the surface of silica particle. 2,4-toluene diisocyanate (TDI) having two reactive isocyanate groups takes the place of being a very effective organic moiety for reacting with the hydroxyl groups of silica particles resulting in surface grafting and also for bringing the ability for further reactions on the surface of silica particles. Grafting of end-functional polymers on the surface of silica nanoparticle as "grafting to" method has been described as one of the main approaches and the "grafting from" method, where polymers are grown from either monomer functionalized or initiator functionalized surfaces of silica nanoparticles, has been described as another approach. By a free radical manner, in most of silica nanoparticles functionalized with the organic monomers, the polymerization proceeds both by surface monomers of nanoparticles and by the free monomers existing in polymerization medium resulting a high ratio of ungrafted polymer formation. For this reason, initiator grafted nanoparticles may be considered for the higher grafting ratios of polymers onto the nanoparticles surfaces. In this study, firstly, well defined, mono-dispersed silica nanoparticles within desired size range was synthesized according to Stöber method as depicted in Figure 1. Physical and chemical structure of the nanoparticles such as particle size, specific surface area and hydroxyl number characterized clarifying their surface properties for the modification of silica nanoparticles. Figure 1 : Synthesis of silica nanoparticles by Stöber method. After the definition of the silica nanoparticles, surface modification of the nanoparticles was achieved with the reaction over isocyanate groups of toluen di-isocyanate (TDI) in order to gain both improved dispersion in organic phase and further attachment possibility of benzoin photoinitiator moieties onto the surface of silica nanoparticles as represented in Figure 2. Figure 2 : Grafting of TDI and benzoin moeities onto the surface of silica nanoparticles In the second stage, photopolymerization of methyl methacrylate (MMA) was achived over the benzoin photo-initiator attached succesfully onto the surface of silica nanoparticles. Since it was discovered by Dupont, photoinitiated polymerization has become an important industrial process. The main positive attributes of photochemical processes are that they offer a rapid conversion of formulated reactive liquids to solids by radical or cationic means. The photopolymerization of MMA by grafting from method was simply performed under UV radiation in the presence of benzoin functionalized silica macroitiators. Well-defined, spherical silica nanoparticles grafted with TDI was also incorporated into preformed epoxy-acrylate resin over the covalent bonding between hydroxyl groups of epoxy resin and free isocyanate groups of Si-TDI by the formation of urethane linkage. Epoxy-acrylate resin forming networks with chemically incorporated silica naoparticles (EA-Si hybrid resin) was cured under UV treatment in the form of film. The effect of uniform, well-dispersed, covalently incorporated silica nanoparticles on the thermal, morphological and mechanical behavior of cured EA-Si hybrid film was also investigated. In this thesis, the design of alkoxysilane precursor of aromatic amide-urethane structure was also aimed for the formation of silicon dioxide network domains and polymeric matrix together as hybrid material. For this purpose, preformed terephthalic acid chloride was reacted with m-amino phenol yielding aromatic amid containing dihydroxy monomer, N1,N4-bis(3-hydroxyphenyl), and the reaction of the synthesized monomer with amino propyl trimethoxysilane (IPTES), formed alkoxysilane containing aromatic amide-urethane macromonomer, over the urethane linkage. Synthesized macromonomer (Figure 3) characterized by 1HNMR indicating the high purity, has the potential to form hybrid materials by owning the organic and inorganic precursor in the same macromonomer. Aromatic amide-urethane alkoxy silane based sol-gel was prepared and incorparated into epoxy acrylate resin formulations following UV curing process. The resulting epoxy acrylate hybrid material was determined and thermal and morphological properties of the hybrid material were characterized. Figure 3 : Alkoxysilane modified aromatic amide-urethane precursor.