Dynamics and Structure of Monolayer Polymer Crystallites on Graphene

TL;DR

This study uses molecular dynamics simulations to explore the rapid structural and dynamic changes of a polyethylene chain on graphene, revealing ultrafast melting and ordering phenomena relevant for nanotechnology applications.

Contribution

It provides detailed molecular insights into the ultrafast melting and reordering of polymers on graphene, a novel understanding of non-equilibrium processes in hybrid systems.

Findings

Polymer crystallites on graphene can be reversibly melted within picoseconds.

The system exhibits a transient floating phase during ultrafast melting.

Structural ordering and disordering are linked to ultrafast phase transitions.

Abstract

Graphene-based nanostructured systems and van-der-Waals heterostructures comprise a material class of growing technological and scientific importance. Joining materials with vastly different properties, polymer-graphene heterosystems promise diverse applications in surface- and nanotechnology, including photovoltaics or nanotribology. Fundamentally, molecular adsorbates are prototypical systems to study confinement-induced phase transitions exhibiting intricate dynamics, which require a comprehensive understanding of the dynamical and static properties on molecular time and length scales. Here, we investigate the dynamics and the structure of a single polyethylene chain on free-standing graphene by means of molecular dynamics simulations. In equilibrium, the adsorbed polymer is orientationally linked to the graphene as two-dimensional folded-chain crystallites or, at elevated temperatures, as a floating solid. The associated superstructure can be reversibly melted on a picosecond time scale upon quasi-instantaneous substrate heating, involving ultrafast heterogeneous melting via a transient floating phase. Our findings elucidate time-resolved molecular-scale ordering and disordering phenomena in individual polymers interacting with solids, yielding complementary information to collective friction and viscosity, and linking to recent experimental observables from ultrafast electron diffraction. We anticipate that the approach will help in resolving non-equilibrium phenomena of hybrid polymeric systems over a broad range of time and length scales.