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

• Main Authors: Alireza Foroozani Behbahani, Vagelis Harmandaris
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Polymers
• Subjects: segmental dynamics; α-relaxation; confinement; interfaces; poly(methyl methacrylate); tacticity
• Online Access: https://www.mdpi.com/2073-4360/13/5/830

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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>≈</mo><mn>2</mn></mrow></semantics></math></inline-formula> 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, <i>d</i>, from the surfaces is described by a temperature shift, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>Δ</mi><msub><mi>T</mi><mi>seg</mi></msub><mrow><mo>(</mo><mi>d</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> (roughly speaking, shift of a characteristic temperature). Next, to a given nanosheet, i-PMMA has a larger value of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>Δ</mi><msub><mi>T</mi><mi>seg</mi></msub></mrow></semantics></math></inline-formula> 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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-relaxation in polymer nanocomposites. The <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><mo>′</mo><mo>′</mo></mrow></msup></msubsup><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> (analogous of dielectric loss, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>ε</mi><msup><mrow></mrow><mrow><mo>′</mo><mo>′</mo></mrow></msup></msup><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, 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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><mo>′</mo><mo>′</mo></mrow></msup></msubsup><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> curves are only slightly affected.