Self-Assembly of Polystyrene on Graphene Nanoplatelets Surfaces

Polymer nanocomposites (PNCs) with a reinforced filler, are significant in engineering applications to enhance functional properties. The key parameter that influenced the final performance is the dispersion of the fillers, the shape of the fillers, and polymer-fillers interfacial properties. Thus, it is crucial to investigate the dynamics of the polymer nanocomposites in the melts to understand the behavior of the PNCs. Here, we are trying to design a polymer blend nanocomposite with a better affinity polymer to the Graphene nanoplatelets (GNPs) and a lower affinity polymer to GNPs to create a percolation pathway.<br/>We probed the relative affinity for GNP in PS and PMMA. Affinity for the nanoparticle inclusion is evidenced by adsorption of the polymer chains which decreases the internal tracer diffusion coefficient . Following the method developed by Hu et al for-silicate clays, we performed the following experiments to determine the impact on the internal chain mobility.[1] We made the following four sets of bilayer samples to measure the tracer diffusion coefficient; PS on PS/dPS, PS on dPS/GNP, PS/GNP on dPS/PS, and PS/GNP on dPS/GNPs, where in all cases the DPS and GNP concentrations were 20 and 2 weight percent, respectively. All the samples were annealed at 150oC for 24 hours then the concentration of deuterium, carbon, hydrogen, and Si ions within the sample were profiled using time-of-flight Secondary Ion Mass Spectrometry (TOF-SIMS). The tracer diffusion coefficients for the four types of samples were, 2.8 x10-17cm2/s, 6 x10-18cm2/s, 3.8 x10-17cm2/s, and 1.1 x 10-17cm2/s, respectively. From the data we see that the largest decrease in the coefficient occurred when the GNP was in both upper and lower layers, indicating adsorption of the PS chains to the GNP. A similar study will be performed for PMMA/dPMMA. It is interesting to note that the diffusion coefficient was higher than the control when dPS was migrating towards an HPS layer containing GNP, while the coefficient was lower than the control when the GNP and the DPS were in the same layer. We, therefore, hypothesized that in addition to the adsorption of GNP to PS, preferential adsorption existed between dPS and GNP relative to the PS GNP interaction. This was further investigated by measuring the contact angle between dPS or PS and a graphene-coated Si surface. From the contact angle and the known surface tension, we obtain work of adhesion of 40.42 mN/m and 60.71 mN/m confirming the higher interfacial energy. DFT calculations were performed by placing PS molecules onto graphene surfaces where the energy for different conformations was calculated. In each case, attractive interactions were obtained confirming the diffusion results. Comparing the dPS to HPS calculations we obtain a measurable difference -0.022 eV for fully adsorbed dPS molecules in agreement with the difference in the work of adhesion measured. Comparing the outcome of this study with that of ref 1, we find that the relative difference in attraction between HPS and dPS has to be considered in the interpretation of the results, and dPS cannot use an indiscriminate tracer molecule.<br/><br/>1 Hu, X. S.; Zhang, W. H.; Si, M. Y.; Gelfer, M.; Hsiao, B.; Rafailovich, M.; Sokolov, J.; Zaitsev, V.; Schwarz, S., Dynamics of polymers in organosilicate nanocomposites. Macromolecules 2003, 36 (3), 823-829