  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.
