Macromolecules, Vol.52, No.10, 3825-3838, 2019
Structure and Conformation of Stereoregular Poly(methyl methacrylate) Chains Adsorbed on Graphene Oxide and Reduced Graphene Oxide via Atomistic Simulations
A detailed analysis of the structure and conformation of stereoregular and atactic poly(methyl methacrylate) (PMMA) chains confined between oxidized graphene sheets is provided through long-time atomistic molecular dynamics simulations. Low-molecular-weight isotactic-, atactic-, and syndiotactic-PMMA chains confined between graphene oxide (GO) and reduced graphene oxide (RGO) sheets have been simulated at different temperatures ranging from 520 to 580 K. The interfacial properties of PMMA/pristine graphene (PG) are also discussed. GO and RGO structures have been generated based on the Lerf-Klinowski structural model of graphite oxide with carbon-to-oxygen ratios of 3 and 10, respectively. The interfacial packing and adsorption of PMMA chains on PG, RGO, and GO model surfaces are studied through the calculation of interfacial mass density profiles and distribution of monomer/surface distance. Furthermore, the arrangement of PMMA atoms in the vicinity of functional groups of nanosheets and their hydrogen bond formation are investigated. The conformations of adsorbed chains, that is, chains with at least one adsorbed monomer, are analyzed in detail as trains, loops, and tails. It is observed that the number of adsorbed monomers and the average size of trains, that is, consecutive adsorbed monomers of a chain, increase with the concentration of functional groups of the nanosheets. This is related to the strength of the polymer/substrate interactions and the increase of the roughness of model nanosheets which enhances the probability of polymer/surface contacts. The tacticity-dependent adsorption of PMMA chains is also examined in detail. Isotactic-PMMA chains, compared to atactic and syndiotactic ones, exhibit a better interfacial packing and form longer trains. i-PMMA chains are stiffer and, moreover, become more extended in the vicinity of model surfaces. The formation of longer trains by isotactic stereoisomers is found to be consistent with their higher stiffness, that is, higher characteristic ratio and gyration radius. Results reported here suggest a clear correlation between chain dimensions, size of trains, and interfacial packing of the adsorbed PMMA chains.