Monte Carlo simulations on filler-induced network chain deformations and elastomer reinforcement from oriented oblate particles

Abstract

Although the filler particles typically used to reinforce elastomers are at least approximately spherical, prolate (needle-shaped) or oblate (disc-shaped) particles have been used in some cases. The fact that anisotropic structures and properties can be obtained in these cases has encouraged a number of experimental and theoretical investigations. The present study extends some earlier Monte Carlo simulations on prolate particles in an amorphous polyethylene matrix, but now focuses on oblate particles. The particles were placed on a cubic lattice, and were oriented in a way consistent with their orientation in composites that were the subject of an experimental investigation by one of the authors. Rotational isomeric state representations of the chains were then generated to model the elastomeric network in the presence of the filler particles. The chain end-to-end distributions were found to be non-Gaussian, and to depend significantly on the excluded volumes of the particles. The particle-induced deformations of the network chains were consistent with results of some other relevant simulations and with recent neutron scattering results. Specifically, the chain dimensions were found to decrease with increase in the axial ratios characterizing the oblate shapes. As anticipated, the chain dimensions became anisotropic, with significant differences parallel and perpendicular to the direction of the particle axes. In general, the network chains tended to adopt more compressed configurations relative to those of prolate particles having equivalent sizes and aspect ratios. Use of these distributions in a standard molecular model for rubberlike elasticity gave values of the elongation moduli, and these were found to depend on the sizes, number, and axial ratios of the particles, as expected. In particular, the reinforcement from the oblate particles was found to be greatest in the plane of the particles, and the changes were in at least qualitative agreement with the corresponding experimental results.