EPOXY-BASED ORGANIC-INORGANIC HYBRID MATERIALS BY SOL-GEL METHOD: CHEMICAL TAILORING AND MULTI-SCALE CHARACTERIZATION

The epoxy resins are organic matrices with excellent heat, moisture, and chemical resistance and good adhesion to many substrates, therefore they are mostly applied in the field of coatings, adhesives, casting, composites, laminates and encapsulation of semiconductor devises. However, due to their low mechanical properties and high coefficient of thermal expansion value compared with inorganic materials, the epoxy resins cannot meet all the requirements, especially for the electrical and structural applications such as epoxy molding compounds. Thus organic/inorganic materials are frequently employed in order to overcome this limitation. Two separated routes can be followed in order to prepare these hybrid/nanocomposite materials, either the addition of preformed inorganic particles, i.e. layered silicates montmorillonite (MMT), or the in situ growth of siloxane clusters, since both MMT and silica particles are commonly used for the reinforcement of epoxy matrix to lower shrinkage on curing, to decrease coefficient of thermal expansion, to improve thermal conductivity and barrier properties, and to meet mechanical requirements. In order to prepare epoxy based hybrids/nanocomposites materials, the sol-gel method is widely used either to modify preformed nanoparticles (i.e. MMT) or to synthesize siloxane clusters. Therefore, in this study, both organo-siloxane clusters and silylated MMT by sol-gel method were used to prepare epoxy-based hybrids/(nano)composites. Considerable attention was given to the use of coupling agents to make compatible the organic matrix and the inorganic particles and improve the interfacial interactions providing chemical bonds between them. In details the surface modification of Na-MMT was done by the silylation reaction with three different aminosilanes, namely 3-aminopropyltriethoxysilane (A1100), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (A1120) and 3-[2-(2-aminoethylamino)ethylamino]-propyl-trimethoxysilane (A1130). The effects on the Na-MMT d-spacing of three aminosilanes with different chain length were studied in details by combining experimental and computational techniques. Additionally, different routes in terms of reaction temperature and aminosilane concentration was followed to the aim to correlate the final d-spacing between silicates layers and the process parameters. Therefore, with silylated A1100 and A1120 MMT several epoxy-based composites were prepared employing two different dispersion methods, namely the sonication (S) and a combination of sonication and high energy ball-milling (SB). The effect of both the silylation reaction parameters and the dispersion method on the mechanical and thermal properties of composites was evaluated. It was found that the silylation reaction of Na-MMT with aminosilanes is a valuable approach to enhance the interactions between the epoxy matrix and the fillers by means of both the covalent bonds due to the cross-linking reaction and the hydrogen bonding with the hydroxyl groups of opened oxirane rings. In fact, the silylated MMTs provide composites with improved mechanical properties with respect the pristine Na-MMT in terms of increased Tg and elastic modulus in the rubbery region. This improvements is more evident in the sonicated composites since the combination of sonication and ball milling makes compact the interlayer spacing and partially destroys the original layers structures. Additionally the silylated clay composites highlighted an increased fire resistance compared to the pristine epoxy resin as well as to the Na-MMT composite. The effect of both the coupling agent and sol-gel process parameters on the organo-siloxane domains morphology and mechanical properties of epoxy-based hybrids was evaluated. It was found that the use of large amount of γ-Glycidoxypropyltrimethoxysilane (GOTMS) as coupling agent represents an available route to tailor the mechanical and thermal properties of epoxy-based hybrids samples. In details, we demonstrated that a suitable choice of functionalized siloxane monomers, amine hardener and reaction conditions leads to the formation of nano-heterogeneous networks with well-organized cage-like structures, up to nearly homogeneous bicontinuous systems. In fact under particular process parameters GOTMS molecules are able to spontaneously arrange to form structures similar to polyhedral oligomeric silsesquioxane POSS units with well established architecture. Therefore, as the siloxane amount increases the number of cages become high enough to make them bonded with the amine hardener Jeffamine D230. Thus the distance between two neighbouring cages will be determined only by the length of the amine hardener which links them. This ordered arrangement highlighted as distance correlation peak in the SAXS patterns profile of hybrids samples at high siloxane content becomes responsible of the improved thermal and mechanical properties of hybrids samples. In particular, the co-continuous organic-inorganic structure is demonstrated with the achievement of films instead of powders in the pyrolysis experiments. It also affects the viscous-elastic behaviour causing both the Tg and the elastic modulus to increase. Moreover the symmetric shape of the loss factor peak speaks in favour of siloxane structure homogenously dispersed throughout the organic matrix. Whereas the increase of the Tg’s value highlights the strong evidence of hindrance the polymeric chains movements during the glass transition. Hence, only the high siloxane content assures the clusters to be bonded highlighting the improved mechanical properties. To the best of our knowledge it is the first time such cage-like clusters bonded by Jeffamine D230 molecules could be detected assuring the Tg to increase without any phase separation. Moreover, the effect of two amine hardeners, namely MXDA and Jeffamine D230 on the inorganic network morphology and then on the mechanical and thermal properties of hybrid samples was also evaluated. It was found that MXDA is an organic cross-linker faster than the Jeffamine D230, therefore in the MXDA-based hybrids the organic grows more rapidly than the inorganic network, leaving out the GOTMS siloxanes monomers, as the high content of T0 units in MXDA-based hybrids proved. The presence of T0 units is detrimental in the cured hybrids samples, since it causes a dramatic reduction in Tg values. On the contrary, in the Jeffamine-based samples the Tg is markedly increased with respect to the neat epoxy due to their particular organic-inorganic morphology. In fact the Jeffamine is able to bond two neighbouring siloxane cages building up a co-continuous organic-inorganic structure, as the siloxane amount increase