A Molecular Dynamics Calculation On The Viscosity Of Polymer-Gas Suspension

The microcellular plastics is the foam plastics which is the microcellular diameter in micron size. The size of the microcellulars are smaller than the flaws in material,so the presence of the microcellular will not reduce the strength of the material, instead, it is advantageous to passivating the nib of the flaw, and prevent the crack extension under stress, improve the impact strength, ductility and endurance life of the material,in the same time the microcellular plastics has the functions such as heat insulation, sound insulation, shock absorption and so on. U.S.Trexel Company developed the technology of injection molding process for preparation of microcellular plastics in 2000.The principle is when the homogeneous phase of polymer and super critical fluid (N2 or CO2) is injected into the mold cavity quickly, the pressure drop in suddenly,so the supercritical gas will escape, forming a large number of microcellulars, with the cooling of the microcellulars and polymer melt in cavity, and acquire the microcellular plastics with uniform size and distribution. The presence of microcellular have a importance influence for forming process.Therefore, it is a great significance for the study on viscosity of polymer melt/microcellular to determinate a reasonable injection molding process, in order to get a good quality microcellular plastic products.This paper based on linear response theory, using equilibrium molecular dynamics method and make use of correlation function calculated the viscosity of pure polymer melt and polymer/gas mixture system respectively. In the simulation, we using the canonical ensemble (NVT),Gaussian caldarium method for controlling the temperature, and using Velocity-Verlet Algorithm for solving discrete equations.Main contents and results, including:(1)Coarse-grained polymer chain referred to as beads-rod chains, and all the polymer beads Interaction through the Lennard-Jones potential energy function, the viscosity of the pure polymer melt was study based on the molecular dynamics model.(2) According to the different constructed systems, we regard as the microcellular composed from beads witch be bound together and characterization for gas.The polymer chains were still coarse-grained as beads-rod chains, the influence of microcellulars volume fraction to polymer/microcellular mixed system was study.(3) The study shows that the microcellulars wihch in polymer melt can reduce the viscosity of the mixture system, and the viscosity was falling with the increases of the volume fraction of microcellular.