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Abstract
Segmental dynamics in unentangled isotactic, syndiotactic, and atactic poly(methyl methacrylate) (i-, a-, and s-PMMA) melts confined between pristine graphene, reduced graphene oxide, RGO, or graphene oxide, GO, sheets is studied at various temperatures, well above glass transition temperature, via atomistic molecular dynamics simulations. The model RGO and GO sheets have different degrees of oxidization. The segmental dynamics is studied through the analysis of backbone torsional motions. In the vicinity of the model nanosheets (distances less than ≈2 nm), the dynamics slows down; the effect becomes significantly stronger with increasing the concentration of the surface functional groups, and hence increasing polymer/surface specific interactions. Upon decreasing temperature, the ratios of the interfacial segmental relaxation times to the respective bulk relaxation times increase, revealing the stronger temperature dependence of the interfacial segmental dynamics relative to the bulk dynamics. This heterogeneity in temperature dependence leads to the shortcoming of the time-temperature superposition principle for describing the segmental dynamics of the model confined melts. The alteration of the segmental dynamics at different distances, d, from the surfaces is described by a temperature shift, (roughly speaking, shift of a characteristic temperature). Next, to a given nanosheet, i-PMMA has a larger value of 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 -relaxation in polymer nanocomposites. The (analogous of dielectric loss, , for torsional motion) curves of the model confined melts are broader (toward lower frequencies) and have lower amplitudes relative to the corresponding bulk curves; however, the peak frequencies of the curves are only slightly affected.
Highlights
The temperature dependence of the interfacial dynamics for the s-Poly(methyl methacrylate) (PMMA)/pristine graphene (PG), syndiotactic PMMA (s-PMMA)/reduced graphene oxide (RGO), and s-PMMA/graphene oxide (GO) systems are provided in Figure S3; it seems that the temperature dependence of s-PMMA containing interfacial systems is qualitatively similar to that of isotactic PMMA (i-PMMA) containing ones; the results for the former cases are rather scattered and their temperature dependency is not as clear as the results of i-PMMA systems
To summarize the surface–chemistry and tacticity effects, we present in Table 1 the layer resolved values of ∆Tseg for i, a, and s-PMMA melts confined between PG, RGO, GO nanosheets; the data of Table 1 are calculated based on the simulations at 580 K
Segmental dynamics in unentangled i, a, and s-PMMA melts confined between PG, RGO, or GO nanosheets was studied at different temperatures (520–580 K), well above Tg, through atomistic molecular dynamics simulations
Summary
Few representative works concerning the simulation of the dynamics of chemically realistic (atomistic) models of interfacial polymer systems (mainly containing carbon-based solid surfaces) include the following: Harmandaris et al [17] studied the segmental and chain dynamics in a thin film of polyethylene adsorbed on graphite. Paul and coworkers [18,19,20] analyzed the dynamics of 1-4-Polybutadiene confined between graphite walls using molecular dynamics simulations They discussed the slow process of mass exchange between layers close to the walls and analyzed dielectric relaxation, segmental reorientation, and local incoherent intermediate scattering function near the confining surfaces. Particular attention is paid to qualitatively comparing the simulation results to the available experimental dielectric spectra of the segmental α-relaxation in polymer nanocomposites and thin film and to provide some insights related to dielectric spectroscopy experiments.
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