title: Heads or tails: how do chemically substituted fullerenes melt?
abstract: We address the question as to whether the melting of chemically substituted fullerenes is driven by the dynamics of the fullerene moiety (the head) or the substituted sub-unit (the tail). To this end, we have performed quasielastic neutron-scattering experiments and classical molecular-dynamics simulations as a function of temperature on the prototypical fullerene derivative phenyl-C61-butyric acid methyl ester. To enable a direct and quantitative comparison between experimental and simulation data, dynamic structure factors for the latter have been calculated from atomic trajectories and further convolved with the known instrument response. A detailed analysis of the energy- and momentum-transfer dependence of this observable in the quasielastic regime shows that melting is entirely driven by temperature-activated tail motions. We also provide quantitative estimates of the activation energy for this process as the material first enters a plastic-crystalline phase, followed by the emergence of a genuine liquid at higher temperatures.
authors.affiliation: Chemical Physics Section, Department of Chemistry, Imperial College London, London SW7 2AZ, UK and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK. jeff.armstrong@stfc.ac.uk felix.fernandez-alonso@stfc.ac.uk.
authors.forename: Jeff
authors.initials: J
authors.lastname: Armstrong
authors.affiliation: Chemical Physics Section, Department of Chemistry, Imperial College London, London SW7 2AZ, UK and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK. jeff.armstrong@stfc.ac.uk felix.fernandez-alonso@stfc.ac.uk.
authors.forename: Sanghamitra
authors.initials: S
authors.lastname: Mukhopadhyay
authors.affiliation: Chemical Physics Section, Department of Chemistry, Imperial College London, London SW7 2AZ, UK and Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway.
authors.forename: Fernando
authors.initials: F
authors.lastname: Bresme
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK. jeff.armstrong@stfc.ac.uk felix.fernandez-alonso@stfc.ac.uk and Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK.
authors.forename: Felix
authors.initials: F
authors.lastname: Fernandez-Alonso