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Polymer / metal nanoparticles composites with enhanced non-linear optical properties

Final Report Summary - COMPONLO (Polymer / metal nanoparticles composites with enhanced non-linear optical properties)

The aim of this project was to synthesize and study polymer/metal nanoparticles (nps) composite materials with non-linear optical (NLO) properties. In this context, two families of copolymers were synthesized as net polymers and as polymers/Au nps composites. The first family combined dispersed red 1 methacrylate (MDR) with dimethylamino ethyl methacrylate (DMAEMA) and gave polymers with glass transition temperature (Tg) near room temperature (ca. 30 0C); while the second family combined MDR with hydroxyl propyl methacrylate (HPMA) and gave polymers with higher Tg (ca. 70 0C). All synthesized materials were characterized using complimentary techniques including size exclusion chromatography, UV-Vis, infrared and NMR spectroscopy, light scattering and transmission electron microscopy.
A surprising non-linear optical behavior was recorded for polymers in the low-Tg MDR- DMAEMA family, since the as-prepared polymers films were found to exhibit second harmonic generation (SHG) before thermal-electric poling (corona poling). The SHG signal was found to decrease upon heat treatment and/or corona poling, but it is recovered after ageing the film at room temperature. The incorporation of spherical Au nps in polymeric films influences greatly their NLO activity, as the measured SHG signal of the composite materials was found much stronger than that of the pristine polymers.
Pristine polymers in the higher-Tg MDR- HPMA family were found to exhibit NLO response only after corona poling. Incorporation of Au nps in such polymers was shown to affect strongly the NLO response of the polymer-Au nps composite films as they exhibit SHG activity even before poling. However, the composite films give lower SHG signal upon corona poling when compared to the signal of the pristine polymer, independently of the polymer to Au nps ratio. This behavior suggests that the presence of Au nps does not promote further orientation of the chromophore groups upon poling, probably because the interaction of metal nps with the polymer “locks” the configuration of the macromolecular chains.
The findings of this project demonstrate clearly the role of Au nps in inducing polymer-chain configurations where the chromophore groups are better oriented than in the case of the corresponding pristine polymers. However, the surface plasmon resonance of Au nps was found to play no crucial role regarding the strength of the SHG activity. Neither the size of spherical Au nps nor the shape of Au nanostructures (nanorods or nanoshells) seem to improve the non-linear optical properties of the composite polymeric films. To the contrary, the development of Au nanorod or Au nanoshell structures was found to have a rather negative effect on the NLO activity, mainly due to scattering of the SHG-produced light by the over-sized gold particles.
This project gave the Fellow the opportunity to work in a new scientific area for the host institute. Following the implementation of the project, the Fellow was offered by the host institute a new research-associate contract because of his new skills and scientific knowledge in the field. This advancement paves the road for a successful career development for the Fellow.