Three Dimensional Picture of the Local Structure of 1,4-Polybutadiene from a Complete Atomistic Model and Neutron Scattering Data

Structure and dynamics of a 1,4-polybutadiene melt in an alumina nanopore: A molecular dynamics simulation

We present results of Molecular Dynamics simulations of a chemically realistic model of 1,4-polybutadiene (PBD)confined in a cylindrical alumina nanopore of diameter 10 nm. The simulations are done at three different temperaturesabove the glass transition temperature T g . We investigate the density layering across the nanopore as well as theorientational ordering in the polymer melt, brought about by the confinement, on both the segmental and chain scales.For the chain scale ordering, the magnitude and orientation of the axes of the gyration tensor ellipsoid of single chainsare studied and are found to prefer to align parallel to the pore axis. Even though double bonds near the wall arepreferentially oriented along the pore walls, studying the nematic order parameter indicates that there is no nematicordering at the melt-wall interface. As for the dynamics in the melt, we focus here on the mean-square-displacement ofthe monomers for several layers across the nanopore as well as the movement of the chain center of mass which bothdisplay a slowing down of the dynamics in the layer at the wall. We also show the strong adsorption of the monomersto the pore wall at lower temperatures.

The Use of Scattering Data in the Study of the Molecular Organisation of Polymers in the Non-Crystalline State

Scattering data for polymers in the non-crystalline state, i.e., the glassy state or the molten state, may appear to contain little information. In this work, we review recent developments in the use of scattering data to evaluate in a quantitative manner the molecular organization of such polymer systems. The focus is on the local structure of chain segments, on the details of the chain conformation and on the imprint the inherent chemical connectivity has on this structure. We show the value of tightly coupling the scattering data to atomistic-level computer models. We show how quantitative information about the details of the chain conformation can be obtained directly using a model built from definitions of relatively few parameters. We show how scattering data may be supplemented with data from specific deuteration sites and used to obtain information hidden in the data. Finally, we show how we can exploit the reverse Monte Carlo approach to use the data to drive the convergence of the scattering calculated from a 3d atomistic-level model with the experimental data. We highlight the importance of the quality of the scattering data and the value in using broad Q scattering data obtained using neutrons. We illustrate these various methods with results drawn from a diverse range of polymers.

Empirical potential structure refinement of semi-crystalline polymer systems: polytetrafluoroethylene and polychlorotrifluoroethylene

Empirical potential structure refinement (EPSR) simulations are performed on total neutron scattering data from powder samples of polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE), both at 300 K. Starting from single strands of polymer consisting of between 30 and 60 monomers of tetrafluoroethylene and chlorotrifluoroethylene in each case, hexagonal simulation cells are constructed consisting of an array 25 (5×5) such strands placed on a hexagonal lattice. Allowed simulation moves are polymer translation moves along all three Cartesian axes, whole polymer rotations about the polymer axis, and individual atom moves within each polymer. For PTFE a number of Bragg peaks are visible in the scattering data and these are found to be consistent with a lattice spacing a(=b) = 5.69(1) Å with a dihedral angle along the (helical) chain of 166° which gives a repeat distance along the chain (c-axis) of ~19.6 Å. The positions of the Bragg peaks are well reproduced by this model, although there is a mismatch in the amplitudes of some of the higher order reflections between simulation and data. For PCTFE there is only one visible Bragg peak (100) which is well reproduced by a hexagonal lattice of atactic parallel polymers with a spacing of a(=b) = 6.37(1) Å. In this case the absence of distinct reflections along the polymer c-axis makes characterization of the internal dihedral angle difficult, but a model with a dihedral angle of 166° was less successful at fitting the diffuse scattering than a model where this angle was set to 180°, giving a nearly straight trans (zig-zag) structure. For PCTFE little change in structure could be discerned when the material was heated to 550 K, apart from a slight increase in lattice spacing. In both cases there is substantial diffuse scattering between the Bragg peaks, and this is correctly replicated by the EPSR simulations.