Study On Polydimethylsioxane/Silica Hybrid Network & Epoxy/Polydimethylsiloxane Interpenetraitng Polymer Network

An interpenetrating polymer network (IPN) is a composition of two or more chemically distinct polymer networks held together exclusively by their permanent mutual entanglements. Three-dimensional cross-linked structures that contain entangled polymeric segments generate Synergistic effects induced by forced compatibility of the components and thus can be used for toughening, reinforcing, and damping, etc.As one of the most important engineering polymeric materials epoxy resin possesses excellent properties. However, the low crack growth resistance restricts application. Polydimethylsiloxane (PDMS) was generally selected as a modifier to improve the thermal stability, hydrophobicity, and flexibility at low temperatures of epoxy resin. Unfortunately, PDMS is immiscible with epoxy resins. In order to improve the miscibility of epoxy and PDMS, some approaches have been tried including functional group capping, block inserting, as well as grafting. However, the above approaches have some disadvantage, especially reducing the modulus of epoxy resin. It is interesting to notice that the IPN technique constituted an effective method to inhibit the decreasing of modulus.A novel semi-simultaneous, semi-sequential methodology wasdesigned to prepare a (diglycidylether of bisphenol A / epoxy modifiedsilica)/ PDMS IPN [(DGEBA/e-silica)/PDMS IPN] in this paper. Firstcrude silica was modified using silsisquioxane to introduce epoxy groupsonto the surface of silica. The vinyl-terminated PDMS (vinyl-DPMS) andhydrogen-containing PDMS(H-PDMS) were pre-cured throughhydrosilylation to some degree of crosslinking, into which DGEBA ande-silica were subsequently introduced as a solution in trichloromethane.The e-silica played a role as a compatibilizer: it co-cured with DGEBA andgenerated H-bonding with the oxygen atoms on the PDMS, and thus thecuring-induced phase separation was inhibited. Due to the formation ofH-bonding depending on time, the mixing of PDMS system with DGEBAsystem was carried out after an adequate pre-curing, consequently themethod was denoted as "semi-simultaneous, semi-sequential". In thesubsequent reactions, the curing reactions of DGEBA and the PDMSoccurred independently and simultaneously, an IPN structure was generated.The IPN sample was characterized using differential scanning calorimetry(DSC), (Transmission electron microscopy) TEM, and scanning electron microscopy (SEM). DSC indicated three glass transitions on the spectrum, with two for the parent polymers displaced toward the center of the spectrum, the third one indicating the intermolecular interactions between the components. TEM indicated that silica was dispersed in matrix in nanometer level. SEM showed that the interpenetrating of PDMS with DGEBA in form of networks caused a brittle-ductile transition of the latter resulting in better ductility and toughness. The tensile strength of the IPN exhibited a maximum as the fraction of PDMS network and e-silica increased. The elongations at break and impact strength kept increasing with increasing content of PDMS. Thermal stability increased with increasing content of PDMS, and a better thermal stability was obtained than the neat DGEBA.DGEBA/PDMS IPN with domains of about 5 nm diameter was prepared via a simultaneous approach in a common solvent of DGEBA and PDMS. The common solvent supplied the initial miscibility between DGEBA and PDMS. The comparable gelling rates of the two systems before gelation are critical to prepare IPN near molecular level. Compared with plain DGEBA/PDMS blends, the domains size of the IPN is obvious smaller and thus fewer reduction in storage modulus. DSC indicated that broadened and displaced glass transitions for the segments of DGEBA resin which revealed a three-dimensional interlock structure of the two components. SEM indicated that the surface of IPN was smooth and no obvious phase boundary. Scanning electron microscopy-Energy dispersive X-ray (SEM-EDX) showed that the dispersion of elements is not different between domains 20μm and 1μm. Atomic force microscopy (AFM) conformed that the domain size in the IPN ranged from 5-10 nm.A novel methodology of reinforcement of PDMS through formation of inorganic-organic hybrid network was proposed. Silica was modified with vinyltriethoxysilane through a dry method. The vinyl-modified silica (vi-silica) containing vinyl groups was employed to reinforce PDMS by participating in the hydrosilation reaction. An inorganic-organic hybrid network was formed to reinforce PDMS due to chemical bonding between vi-silica and PDMS. The gel contents resulting from various vinyl/hydrogen mole ratios and different atmospheres confirmed that vi-silica participated in the hydrosilation. The inorganic-organic hybrid network provided better mechanical properties.The influence of pre-treatment temperature on gel time and mechanical properties of modified silica (m-silica) filled PDMS system was studied. Silica was modified with hexamethyldisilazane and hexamethyltrisiloxane (D3) via dry method. When the pre-treatment temperature was increased, the gel time of m-silica filled PDMS system increased but mechanical properties decreased. However, when the pre-treatment temperature was too low, the gel time decreased and the mechanical properties increase. For this reason the pre-treatment should be carried out at an optimal temperature.In summary, the reinforcement and toughening approaches and mechanisms for various networks of DGEBA/PDMS/silica were explored in this dissertation. The discoveries made in this work will be valuable for the further research and practical production.