Macromolecular Structure and Vibrational Dynamics of Confined Poly(ethylene oxide): From Subnanometer 2D-Intercalation into Graphite Oxide to Surface Adsorption onto Graphene Sheets.

Abstract

In this work, high-resolution inelastic neutron scattering (INS) has been used to provide novel insights into the properties of confined poly(ethylene oxide) (PEO) chains. Two limits have been explored in detail, namely, single-layer 2D-polymer intercalation into graphite oxide (GO) and surface polymer adsorption onto thermally reduced and exfoliated graphite oxide, that is, graphene (G) sheets. Careful control over the degree of GO oxidation and exfoliation reveals three distinct cases of spatial confinement: (i) subnanometer 2D-confinement; (ii) frustrated absorption; and (iii) surface immobilization. Case (i) results in drastic changes to PEO conformational (800-1000 cm-1) and collective (200-600 cm-1) vibrational modes as a consequence of a preferentially planar zigzag (trans-trans-trans) chain conformation in the confined polymer phase. These changes give rise to peculiar thermodynamic behavior, whereby confined PEO chains are unable to either crystallize or display a calorimetric glass transition. In case (ii), GO is thermally reduced resulting in a disordered pseudo-graphitic structure. As a result, we observe minimal PEO absorption owing to a dramatic reduction in the abundance of hydrophilic groups inside the distorted graphitic galleries. In case (iii), the INS data unequivocally show that PEO chains adsorb firmly onto the G sheets, with a substantial increase in the population of gauche conformers. Well-defined glass and melting transitions associated with the confined polymer phase are recovered in case (iii), albeit at significantly lower temperatures than those of the bulk.