Molecular Dynamics Simulation Study Of The Polymer Blend, Crystallization And The Melting Process Of N-Heptane Layers On Graphite Surface

Molecular dynamics (MD) simulation method solves the classical equations of motion (Newton equation) for a system of N atoms interacting according to a force field. In this way, an MD simulation generates a trajectory that describes the change of dynamics variables with time, from which the macroscopic properties (energy, pressure, etc.) can be calculated. MD simulation can give apparent images of theoretical models at atomic and molecular level, and has been developed very quickly in the past decades. Now it has become powerful tools in the researches of Material Science, Chemistry, Physics and Biology. In this thesis, MD simulation method has been used to investigate the polymer blend, crystallization and the melting process of n-heptane on graphite surface. The main results of the thesis are as follows: (1) By means of full atomistic molecular dynamics simulations, the solubility parameters for two pure polymers poly(3-hydroxybutyrate) (PHB) and poly(ethylene oxide) (PEO) are calculated and the results are in agreement with the literature values. Furthermore, the volume-temperature curve of PHB/PEO blend system (1:2 blends in terms of repeated units) is simulated by employing the united atom approximation. From the V-T curve, the glass transition temperature of the blend system is obtained (about 258K). The glass transition temperature is compared well with the experimental results. Solubility parameters of PHB and PEO obtained from the simulations have similar values. And only one glass transition is obtained for the blend system. From above two points, it can be concluded that the blend system is miscible. Energy components such as dihedral torsion energy and non-bonded energy play important roles in the glass transition process of the blend system, as indicated by the plots against temperature from above T gand below T g. It seems that MD simulation is a useful method to determine solubility parameter, V-T curve and the glass transition temperature of polymers. Moreover, the simulated results may be helpful to predict the miscibility of polymer/polymer systems. (2) Molecular dynamics simulations have been used to study the crystallization mechanism of 22,8-polyurethane which contains hydrogen-bonding units (urethane). The process can be characterized by three stages: (a) The extended chain collapses to a globule random coil; (b) The random coil reorganizes into an ordered lamellar structure; (c) Accompanied with the segments clustering due to the hydrogen bond formation, the lamellar develops with local defects. There are two kinds of hydrogen-bond (N-H···O=C and N-H···O) in 22,8-polyurethane, and the hydrogen-bond N-H···O=C is the predominant one. The hydrogen-bonds have influence on the crystallization behavior of 22,8-polyurethane. The stage (c) is owing to the reorganization of the hydrogen-bond, which does not exist in the crystallization process of PE and b-PE. Temperature plays a complicated role in the 22,8-polyurethane crystallization process due to the formation of hydrogen-bond. The lower the temperature is, the slower the crystallization...