Graphene/poly(ethylene glycol) nanocomposites as studied by molecular dynamics simulations

• Suppression of polymer crystallization and absence of polymer intercalated structures under the examined conditions
• Detection of a weak thermal transition in the composite materials well above the nominal polymer crystallization temperature
• Molecular-weight-dependent changes in graphene dispersion and in polymer conformations close to graphene
• Strong slowing down and change in the temperature dependence of polymer dynamics in the composites

Fully atomistic molecular dynamics simulations were utilized to examine melt dispersions of graphene nanosheets in a poly(ethylene glycol) (PEG) matrix, in a wide temperature range and for two different polymer sizes. The resulted mixtures were characterized by the organization of graphene in oligomeric clusters. No indications for polymer crystallization were noted in the examined time window. Instead, a weak thermal transition was observed upon cooling, at a temperature higher than the nominal glass transition and the melting point of the polymer. This was found to be associated with the formation of a kinetically-arrested graphene network within the polymeric matrix. Close and below the transition region PEG chains in the composites exhibited frozen-in configurations with an enhanced degree of conformations bearing short end-to-end distances, with those of the higher molecular weight characterized by more distorted shapes close to graphene planes. No polymer intercalation between graphene sheets was observed in the examined samples, while only a weak interaction between the two components was noted. Polymer dynamics in the mixtures were slowed down both in local and in global length scales, accompanied by a distinctly different temperature dependence of the pertinent relaxation rates, compared to those in the pristine macromolecular systems.

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