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Molecular Simulations of Elastomeric Networks

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The use of molecular simulations for predicting and understanding the behavior of crosslinked elastomeric networks is discussed in a new [URL= http://polymerfem.com/polymer_files/molsim04.pdf ]pdf-file[/URL]. The title of the document is: Deformation of Elastomeric Networks: Relation between Molecular Level Deformation and Classical Statistical Mechanics Models of Rubber Elasticity.

[I]Abstract[/I]: In this work a specialized molecular simulation code has been used to provide details of the underlying micromechanisms governing the observed macroscopic behavior of elastomeric materials. In the simulations the polymer microstructure was modeled as a collection of unified atoms interacting by two-body potentials of bonded and non-bonded type. Representative Volume Elements (RVEs) containing a network of 200 molecular chains of 100 bond lengths are constructed. The evolution of the RVEs with uniaxial deformation was studied with molecular dynamics techniques. The simulations enable observation of structural features with deformation including bond lengths and angles as well as chain lengths and angles. The simulations also enable calculation of the macroscopic stress-strain behavior and its decomposition into bonded and non-bonded contributions. The distribution in initial end-to-end chain lengths is consistent with Gaussian statistics treatments of rubber elasticity. It is shown that application of an axial strain of +/- 0.7 (a logarithmic strain measure is used) only causes a change in the average bond angle of -/+ 5 degrees indicating the freedom of bonds to sample space at these low-moderate deformations, the same strain causes the average chain angle to change by -/+ 20 degrees. Randomly selected individual chains are monitored during deformation, their individual chain lengths and angles are found to evolve in an essentially affine manner consistent with Gaussian statistics treatments of rubber elasticity. The average chain length and angle are found to evolve in a manner consistent with the eight-chain network model of elasticity. Energy quantities are found to remain constant during deformation consistent with the nature of rubber elasticity begin entropic in origin. The stress-strain response is found to have important bonded and non-bonded contributions. The bonded contributions arise from the rotations of the bonds toward the maximum principal stretch axis(es) in tensile(compressive) loading.

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