(247d) Entropic and Enthalpic Driving Forces on Morphology in Polymer Grafted Particle Filled Nanocomposites: Integral Equation Theory and Molecular Simulations

Polymer nanocomposites are a class of materials that consist of a polymer matrix embedded with nanoscale fillers or additives that enhance the inherent properties of the matrix polymer. To engineer polymer nanocomposites for specific applications with target macroscopic properties (e.g. photovoltaics, optics, photonics, parts of automobiles, etc.) it is important to have design rules that relate molecular features to morphologies of the composite. Using theory and simulation, we studied the effects of tuning the properties of grafted and matrix chains in polymer grafted nanoparticle filled composites in order to tune the entropic and enthalpic driving forces for particle dispersion and aggregation. Initially, we studied polymer nanocomposites with homopolymer grafted particles in a homopolymer matrix, at high grafting density and with chemically identical graft and matrix polymers. This allowed us to probe how entropic driving forces affect nanocomposite morphologies. We found that increasing the polydispersity in grafted chain lengths or decreasing the graft and matrix chain flexibility stabilizes the dispersed phase of polymer nanocomposites due to increased wetting of the grafted layer by matrix chains. We further showed that this increased wetting of the grafted layer was primarily driven by increased graft-matrix mixing entropy and decreased conformational entropy penalty of the matrix wetting the grafted layers. In more recent work, we explored composites with chemically different graft and matrix polymers which have, in addition to the entropic driving forces described above, enthalpic driving forces that drive the system towards maximizing attractive graft-graft, matrix-matrix, and graft-matrix interactions. We found that the sharp particle dispersion-aggregation transition appears to be distinct from the continuous wetting-dewetting transition, which is in direct contrast to chemically identical graft/matrix systems where dispersion-aggregation and wetting-dewetting are treated synonymously. In this poster I will describe the above theoretical and computational results, along with experimental results that have confirmed our predictions.