Molecular Dynamics Modelling of Polymer Materials

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Abstract
This paper describes novel molecular dynamics simulations of a model amorphous polymer which resembles polyethylene. Methylene groups are represented as single interaction sites and both torsional and angle bending potentials are included. New parameters for both inter- and intramolecular potentials are used as compared to previous work on alkane chains. A previously used torsional potential was found to have a serious deficiency. Dense samples of amorphous polymer were prepared using a self avoiding random walk in three dimensions carried out within the confines of periodic boundaries. For the molecular dynamics an adaptation of the method of Berendsen et al is presented in which changes in the shape and size allow the prepared samples to relax out significant internal stresses. This same method can be used to examine the response of a polymer sample to an externally applied stress field. An example is given of a sample undergoing uniaxial tension, and the results are discussed in terms of the elastic strain, yielding and plastic flow. The latter effect is compared with neck formation, a phenomenon which is observed in real polymeric materials.

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