DENACIM · Design of Nanocomposites with Controlled Interfacial Morphology

Final Activity Report Summary - DENACIM (Design of Nanocomposites with Controlled Interfacial Morphology)

Some of the most important type of materials studied in recent years has been organic / inorganic hybrid nanocomposites. These systems, based upon the incorporation of nanoscale clusters (i.e. nanoparticles) of metals or metal oxides into a polymeric matrix, offer the possibility of superior mechanical properties, mainly controlled by the nature of the load-transfer through the polymer-particle interface. The interface in such polymer-based nanocomposites will have a significant effect on the properties of the nanocomposite, depending on the strength of the interactions between the filler particles and the polymer matrix. Despite the technological importance of this class of materials, the exact nature of the interactions between nanoscale inclusions embedded in polymer matrices are not well established, fact which contributes to the inconclusive experimental data regarding the influence of the nanoscale constituents on the materials properties. Our efforts have, therefore, been directed to develop a quantitative correlation between the structure of this interfacial region and the resulting mechanical properties of the nanocomposites, a correlation that could be used as a design parameter in the engineering of such materials. In order to achieve the stated goals, we concentrated on the synthesis of several types of nanocomposites and studied ways to allow us the characterisation of the interfacial structure generated as a result of the interfacial interactions between the polymer matrix and the inorganic nanoparticle fillers. Then, we extracted the relevant parameters with which it would be possible to correlate the various measured mechanical properties of the materials. We developed two independent novel methods to characterise the structure of the interfaces formed in inorganic-polymeric nanocomposites. As the interfacial interactions between the polymer matrix and the nanoparticles lead to the formation of close contacts, i.e. 'anchors' or 'hooks,' between the segments in the polymer chains and the surface atoms of the nanoparticles, the density of such contacts per unit area of nanoparticle surface (designated as d) should constitute the sought-after parameter that would be correlated to the properties of the material. The mechanical properties of the new nanocomposites were probed by various tests, commensurate with different material length scales. Nanoindentation probed localised properties at the nanoscale, dynamic mechanical analysis probed the impact of the nanoparticles on the viscoelstic response of the materials and tensile testing provided a macroscale measure of elasticity and strength. A similar trend was observed with all three measurement methods (albeit with different values), which indicates that the interfacial phenomena that dominate the intimate vicinity between the nanoparticles and the adsorbed polymer layer propagate their impact through all length scales into the bulk. This is the first time that we were able to actually point to a correlation between the structure of the interphase and the resulting mechanical properties. It seems that the dependence on one of the main properties (the elastic modulus) on the interfacial structures scales as d^(1/3). Based on this initial correlation, we have designed and synthesised a polymer matrix consisting of a polymer that was modified so that it would have pendant ionic groups. The resulting polymer is characterised by a fixed interval of between the ionic groups along the polymer chain, thus generating fixed intervals between highly-reactive potential anchoring points. Indeed, the previously-obtained non-linear correlation between the relative elastic moduli of the nanocomposites with the corresponding anchoring density was upheld, albeit with a slightly different scaling parameter, i.e. d^(2/5). These polymer-based hybrid nanocomposites demonstrate the possibility of controlling the bulk properties of the composite materials by pre-designing the interfacial properties at the nanoscale.

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