Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets

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, ΔTseg(d) (roughly speaking, shift of a characteristic temperature). Next, to a given nanosheet, i-PMMA has a larger value of ΔTseg 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. The εT′′(f) (analogous of dielectric loss, ε′′(f), 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 εT′′(f) curves are only slightly affected.

Evaluation on Structural Properties and Performances of Graphene Oxide Incorporated into Chitosan/Poly-Lactic Acid Composites: CS/PLA versus CS/PLA-GO

“…In this regard, the appropriate functionalization of the GO surface becomes a critical factor in achieving compatible interfaces that facilitate good compatibilizer dispersion into the PLA matrix, resulting in improved final product properties. Since GO contains numerous hydrophilic groups, such as hydroxyl, carboxyl and epoxy groups [ 18 ], it is simple to graft macromolecules onto its surface through covalent or noncovalent functionalization. In biomedical applications, functionalizing GO with a biocompatible polymer will significantly reduce the cytotoxicity of GO to cells and animals [ 16 ].…”

Coupling between Polymer Conformations and Dynamics Near Amorphous Silica Surfaces: A Direct Insight from Atomistic Simulations

“…Notice that these local regions where the surface is uneven are not distributed regularly, as is the case of the crystalline form of silica or coarse-grained models of rough surfaces [ 27 , 28 , 49 ]. Such a nanoscale roughness is also likely to be found in systems containing functionalized graphene nanofillers, such as graphene oxide [ 42 , 43 , 44 ], where the nanowrinkling of the graphene contributes significantly to the heterogeneous character of the solid filler in addition to the hydroxyl and epoxide groups attached to the surface. Contrary to 2D materials where the roughness can be estimated as the deviation of “valleys” and “hills” from the optimal plane [ 46 ], finding the position of the optimal plane in the irregular structure of the amorphous silica slab studied here is not trivial.…”

“…The measured properties are then averaged for both surfaces. Note that analogous algorithm based on the equidistant layers was also implemented in nanocomposites containing graphene-based nanofillers [ 43 , 44 , 66 ] and in polymer/silica nanocomposites [ 59 ].…”

“…In other words, as our analysis scheme allows for the motion between the layers (i.e., we assigned the layer in the beginning of the measurement), the dynamics of the monomers placed around 0.6–1 nm and measured by our protocol may be a combination of dynamics of “just-desorbed” monomers with those that are about to be adsorbed and, thus, coming from a slightly faster region. Recently, the same analysis protocol was applied in the systems of poly(methyl methacrylate) confined between pristine graphene, and the authors also reported a non-monotonic dependence of the stretch exponent on the distance from the graphene [ 44 ]. Moreover, comparing the two chain lengths, the non-monotonic tendency of seems to be more marked in the cPB30/Sil system.…”

“…This technique captures the chemical details and, thus, describes chemistry-specific polymer–solid interactions. Polymer/graphene or polymer/graphite hybrid materials can serve as a good example of hybrid materials containing flat surfaces [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. In these studies, the properties are analyzed as a function of the distance from the flat surface, quantifying in such a way the extension of the interphase region, i.e., extension of the region of the altered properties with respect to expected bulk-like behavior of the given polymer.…”