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, RGO, or graphene oxide, GO, sheets is studied at various temperatures, well above glass transition temperature, via...
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| Format: | Article |
| Language: | English |
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MDPI AG
2021-03-01
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| Series: | Polymers |
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| Online Access: | https://www.mdpi.com/2073-4360/13/5/830 |
| _version_ | 1797542166581477376 |
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| author | Alireza Foroozani Behbahani Vagelis Harmandaris |
| author_facet | Alireza Foroozani Behbahani Vagelis Harmandaris |
| author_sort | Alireza Foroozani Behbahani |
| collection | DOAJ |
| description | 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, <i>d</i>, from the surfaces is described by a temperature shift, <inline-formula><math display="inline"><semantics><mrow><mo>Δ</mo><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 display="inline"><semantics><mrow><mo>Δ</mo><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 display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-relaxation in polymer nanocomposites. The <inline-formula><math display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><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 display="inline"><semantics><mrow><msup><mi>ε</mi><msup><mrow></mrow><mrow><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 display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><mo>″</mo></mrow></msup></msubsup><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> curves are only slightly affected. |
| first_indexed | 2024-03-10T13:25:49Z |
| format | Article |
| id | doaj.art-208dd0ad239b494f8c94913500f8b50b |
| institution | Directory Open Access Journal |
| issn | 2073-4360 |
| language | English |
| last_indexed | 2024-03-10T13:25:49Z |
| publishDate | 2021-03-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Polymers |
| spelling | doaj.art-208dd0ad239b494f8c94913500f8b50b2023-11-21T09:36:57ZengMDPI AGPolymers2073-43602021-03-0113583010.3390/polym13050830Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene SheetsAlireza Foroozani Behbahani0Vagelis Harmandaris1Institute of Applied and Computational Mathematics, Foundation for Research and Technology-Hellas, GR-71110 Heraklion, GreeceInstitute of Applied and Computational Mathematics, Foundation for Research and Technology-Hellas, GR-71110 Heraklion, GreeceSegmental 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, <i>d</i>, from the surfaces is described by a temperature shift, <inline-formula><math display="inline"><semantics><mrow><mo>Δ</mo><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 display="inline"><semantics><mrow><mo>Δ</mo><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 display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-relaxation in polymer nanocomposites. The <inline-formula><math display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><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 display="inline"><semantics><mrow><msup><mi>ε</mi><msup><mrow></mrow><mrow><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 display="inline"><semantics><mrow><msubsup><mi>ε</mi><mi mathvariant="normal">T</mi><msup><mrow></mrow><mrow><mo>″</mo></mrow></msup></msubsup><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> curves are only slightly affected.https://www.mdpi.com/2073-4360/13/5/830segmental dynamicsα-relaxationconfinementinterfacespoly(methyl methacrylate)tacticity |
| spellingShingle | Alireza Foroozani Behbahani Vagelis Harmandaris Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets Polymers segmental dynamics α-relaxation confinement interfaces poly(methyl methacrylate) tacticity |
| title | Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets |
| title_full | Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets |
| title_fullStr | Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets |
| title_full_unstemmed | Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets |
| title_short | Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets |
| title_sort | gradient of segmental dynamics in stereoregular poly methyl methacrylate melts confined between pristine or oxidized graphene sheets |
| topic | segmental dynamics α-relaxation confinement interfaces poly(methyl methacrylate) tacticity |
| url | https://www.mdpi.com/2073-4360/13/5/830 |
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