Molecular Dynamics Simulation of Low-Temperature Deformation of Glassy Polymethylene

摘要

Molecular dynamics simulation of glassy polymethylene (PM) plastic deformation is performed up to ε = 30% in uniaxial compression and 60% in tensile regimes at a temperature of 50 K, which is ～140 K below T_g of the poly mer. All atoms of PM chains are represented explicitly. Calculations were performed for two series of samples with different molecular mass distribution of chains. Each sample contains 12 288 -CH_2 -monomeric units per computational sell. Contribution from various interactions to potential energy of the system was investigated. Nonaffine displacements of methylene groups 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, nonconformational 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 ε > 15%, at the stage of the developed plastic flow.