Does Graphene Change T-g of Nanocomposites?
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2014 American Chemical Society. The effect of the addition of graphene on the glass transition temperature (Tg) of polymers was investigated, first with poly(methyl methacrylate) and then with an extensive literature review. Isotactic (i-PMMA) and atactic PMMA (a-PMMA) were blended with pristine graphene (PG) and thermally reduced graphene (TRG). A Tg increase was found for a-PMMA nanocomposites made via in situ polymerization with TRG but not when a-PMMA was solvent blended with TRG. However, a Tg increase was found for TRG solvent blended into i-PMMA and a smaller increase for PG with i-PMMA. Nearly all the increase occurred at the lowest loading, 0.25 wt %, with little change at increased graphene concentration. Tg increases due to interfacial interactions between matrix polymers and fillers. Physical blending such as solvent processes cannot provide enough interaction at the interfaces, whereas chemical blending processes such as in situ polymerization can yield strong covalent bonds. However, i-PMMA molecules can align on graphene sheets at the interface, creating more interaction between i-PMMA and graphene than a-PMMA. Also, the Tg of i-PMMA is 60 C lower than a-PMMA, meaning that hydrogen bonds are stronger at the lower temperature. The Tg increase of TRG/i-PMMA is higher than that of PG/i-PMMA due to more oxygen functionalities on TRG than on PG to act as interfacial interaction sites. A broad literature survey agrees with our PMMA results. We found no changes in Tg for graphene/polymer nanocomposites synthesized via physical blending processes such as solvent or melt blending, except for blending with strongly polar polymers. In contrast, chemical blending processes such as in situ polymerization or chemically modified fillers yielded significant Tg increases in graphene/polymer nanocomposites.
