Nanoparticle Diffusion in Complex and Dynamic Environments

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CBET - 1706014PI: Winey, Karen I.Many polymeric materials are modified with nanoparticles to provide special mechanical, electronic, or optical properties. Controlling the distribution of the nanoparticles throughout the polymeric material is essential for achieving these desired properties. Nanoparticles can diffuse through polymers or gels, and in some cases diffusion during processing or during use of the material leads to a maldistribution of nanoparticles and nanoparticle aggregation that degrade product performance. This award will support research into the diffusion of nanoparticles in polymers and gels. It will focus specifically on the regime where the nanoparticle sizes are comparable to the size of mesh formed by entanglements in polymers and by crosslinks in gels. In this regime, nanoparticle diffusion depends on the both the movement of the nanoparticles and fluctuations in the polymer mesh through which the nanoparticles diffuse. Effects of the nanoparticle shape, interactions between the nanoparticles and the polymeric matrix, and spatial variations in the structure of the polymer matrix will also be examined and compared with existing theories. Results from this award will be disseminated to industrial practitioners through an annual symposium organized by the researchers. In addition, the research team will lead hands-on demonstrations of nanoscience for the public at Philly Materials Day and the University of Pennsylvania's annual Nano Day.This award will support research to measure nanoparticle diffusion in hydrogels and entangled polymer melts that are characterized by their mesh size and tube diameter, respectively. The research will explore systems where the nanoparticle diameter and mesh size or tube diameter are comparable, because models predict strong deviations from conventional Stokes-Einstein diffusion in these cases. Single particle tracking methods will be used to measure nanoparticle diffusion in tetra-poly(ethylene oxide) and polyacrylamide hydrogels. Tetra-poly(ethylene oxide) hydrogels form a model network with nearly monodisperse mesh size. Polyacrylamide hydrogels form a network with a mesh size and heterogeneity that can be manipulated by varying solvents. Rutherford Backscattering Spectrometry will be used to measure nanoparticle diffusion coefficients for phenyl-capped, brush grafted, and hydroxyl-terminated nanoparticles (spherical and cylindrical) in polystyrene and poly(2-vinylpyridine) to explore both athermal and attractive nanoparticle-polymer interactions. Quantitative comparisons will be made with models and theories across a critical range of the ratio of nanoparticle to mesh size, as defined as the tube diameter in the reptation model. Finally, one nanoparticle-polymer system has been specially designed to facilitate measuring nanoparticle diffusion by both single particle tracking and Rutherford backscattering methods to compare complementary information provided by individual nanoparticle motion and ensemble averages from a single system.