Gradient of Segmental Dynamics in Stereoregular Poly(Methyl Methacrylate) Melts Confined Between Pristine or Oxidized Graphene Sheets

Segmental dynamics in unentangled isotactic, syndiotactic, and atactic poly(methyl methacrylate) (i-, a-, and s-PMMA) melts confined between pristine graphene, reduced graphene oxide, or graphene oxide sheets is studied at different temperatures, via atomistic molecular dynamics simulations. The segmental dynamics is studied through the analysis of backbone torsional motions. Upon decreasing temperature, the ratios of the interfacial segmental relaxation times to the respective bulk relaxation times increase, revealing a stronger temperature dependence of the interfacial segmental dynamics relative to the bulk dynamics. The alteration of the segmental dynamics at different distances, d, from the surfaces is described by a temperature shift, △T_seg(d) (roughly speaking, shift of a characteristic temperature). Next to a given nanosheet, i-PMMA has a larger value of △T_seg(d) than a-PMMA and s-PMMA. This trend correlates with the better interfacial packing and longer trains of i-PMMA chains. The backbone torsional autocorrelation functions are shown in the frequency domain and are qualitatively compared to the experimental dielectric loss spectra for the segmental α-relaxation in polymer nanocomposites.