2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

Filler-induced deformations of amorphous polyethylene chains, nanoscopic-size effects, corresponding reinforcement, and comparisons with experiment

M.A. Sharaf, A. Kloczkowski, K.I. Jacob and J.E. Mark
Helwan University, EG

Monte Carlo simulations and methods, rotational isomeric states, RIS, Monte Carlo-rotational isomeric states, MC-RIS, nanoparticles and inclusions, nanocomposites, chain confinement and stiffening, reinforcement, elastomers

Abstract Estimates were made of chain deformations caused by the presence of spherical nano-size filler particles that were randomly dispersed in an amorphous polyethylene matrix. Attention was first focused on the dependence of the size distributions on the particular spatial arrangements of the filler inclusions. Specifically, Monte Carlo rotational isomeric state (MC-RIS) simulations were carried out to predict the effects of the volumes excluded by the filler particles on the conformational and configurational distribution functions for the chains, including conformations corresponding to chain segments being flattened onto the particle surfaces. These simulations are considered more reliable than alternative approaches since the detailed internal structure of the chain units was not neglected, as is generally the case in coarse-grained simulations. The results show a significant increase in chin dimensions (chain stiffening) with a maximal effect at low loadings, presumably from the very large interfacial areas between the fillers and the polymer matrix. Reducing the size of the inclusions at constant loading caused a substantial increase of the interfacial area with an accompanying reduction in the average wall-to-wall distance between inclusions. Hence, in this particular regime, a significant fraction of the chains experience conformational and spatial modifications of chain trajectories that are quite different those in the bulk. There is a significant decrease in chain dimensions after increase in the concentration of very small nanoscopic inclusions interpretable in terms of the volumes excluded to the chains by the inclusions with account of the nature of the dispersion and aggregation. These predictions and the observed changes were in excellent qualitative agreement with results from neutron scattering experiments, unlike the predictions from dense-system simulations. Some elastomeric properties of the filler-polymer composites were then computed from these distributions, and the resulting stress-strain isotherms in elongation were qualitatively compared with some relevant experimental results.

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