A study of polymer chain diffusion by surface enhanced Raman: effects of plasmonic substrate topology.

We report on a new methodology to track chain interdiffusion between polymer slabs based on Raman enhanced by plasmonic substrates. Diffusion is studied in a deuterated-polystyrene/polystyrene (dPS/PS) polymer pair, designed to provide a well-characterized diffusion behavior. The bilayer, 160 nm thick in total, is supported on a plasmonic substrate that provides local amplification of Raman signals in sample regions of close proximity to it. Gold-based substrates with structures of inverted pyramids, spherical nanoparticles and tipped pillars were investigated. Interdiffusion between dPS and PS is promoted upon annealing and followed in situ by dynamic spectral acquisition. A simple model that describes the coupling between the sampled region arising from the plasmonic effect and the diffusion process is employed to interpret spectral evolution data. It is shown that a highly regular topology and surface continuity are key features of the plasmonic substrate in order to provide reliable results. With pyramidal substrates, the most suitable substrates for this application, data are consistent with diffusion coefficients in the range 10-13-10-15 cm2 s-1 and dimensions of sampled regions below 40 nm. The strategy provides a reliable labeling-free technique to investigate polymer interdiffusion on the nanoscale.