Nikolai K. Balabaev

Computer simulation of rearrangements in chains of glassy polymethylene subjected at low temperature inelastic deformation

Molecular dynamics simulation of glassy polymethylene (PM) plastic deformation is performed up to ɛ = 30% in uniaxial compression regime at a temperature of 50 K, which is ∼140 K below Tg of the polymer. All atoms of PM chains are represented explisitly (all-atom model). Calculations were performed for two series of samples with different molecular mass distribution of chains: Samples have average degree of polymerization DP ≈ 212 with Mn ≈ 3000 and Mw ≈ 9500 (the first series) and DP ≈ 350, Mn ≈ 5000 and Mw ≈ 9500 (the second series). Each sample contains 12288 -CH2- monomeric units per computational sell. Nonaffine displacements of carbon atoms and conformational rearrangements in chains during deformation are visualized and analyzed. The transformation of relatively fragments of chains up to 16–20 monomer units length are basic structural units, non-conformational displacements of which controls plastic process. Relatively large nonaffine displacements are observed even in the range of low strains, which are usually interpreted as Hookean strains. In the range of yield tooth and steady plastic flow, the number of these displacements increases along with their amplitude. Conformational set of PM chains does not show a serious change during deformation. Analysis had shown that the number of conformational rearrangements of trans-gauche type in PM chains during deformation is small and such rearrangements do not play decisive role in the considered range of PM plasticity, even at ɛ > 15%, at the stage of the developed plastic flow.

Analysis of local rearrangements in chains during simulation of the plastic deformation of glassy polymethylene

A molecular-dynamics simulation of the low-temperature (∼100 K below T g) plastic deformation of glassy polymethylene (PM) was conducted. A model system consisting of 64 chains containing 100 CH2 groups (the united-atoms approach) in each computational cell with periodic boundary conditions was considered. The behavior of 32 such cells was considered. Each cell was subjected to an active isothermal uniaxial compression at a constant temperature of T def = 50 K to a strain of ɛ = 30%. An analysis showed that the inelastic deformation of glassy PM proceeded via nonaffine displacements (“gliding”) of chain fragments comprising 11–13 sites -CH2-. These displacements are correlated and directed mainly along chain axes. Only a small number of conformational rearrangements occur in chains during the deformation of the material. Conformational transitions add only small additional displacements to nonaffine atomic transformations. A free-volume analysis using Voronoi-Delaunay tessellation in the deformed polymer did not show its relation to local plastic rearrangements.

Anisotropy of diffusion in a liquid crystalline system of semi-flexible polymer chains

Molecular dynamic simulations are reported for semi-flexible systems consisting of rod-like linear molecules. The molecules are composed of eight tangent isotropic soft spheres, connected by continuous elastic springs into a linear chain. Rigidity is introduced by additional springs between each sphere along the chain. The elasticity of the springs is used to tune the flexibility of the molecule. The formation of only a nematic LC phase is shown for all systems considered. Persistence length dependences of the jump of the order parameter and boundary volume fractions in isotropic and nematic phases at LC transition agree well with predictions of the Khokhlov–Semenov theory and with available simulation data. The effect of the flexibility on the translational and rotational diffusion in the nematic phase is studied. The anisotropy of translational diffusion was observed. For the estimation of the rotational diffusion coefficient for molecules in a nematic phase the wobbling-in-a-cone model was applied. Anisotropy of translational diffusion and ratio of translational and rotational diffusion coefficients show universal dependences on the order parameter.