Monte Carlo simulations on ®ller-induced network chain deformations and elastomer reinforcement from oriented oblate particles q M.A. Sharaf a, * , J.E. Mark b a Department of Chemistry, Helwan University, Ain-Helwan, Cairo 11795, Egypt b Department of Chemistry and Polymer Research Center, The University of Cincinnati, Cincinnati, OH 45221-0172, USA Abstract Although the ®ller 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 ®ller particles. The chain end-to-end distributions were found to be non-Gaussian, and to depend signi®cantly 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. Speci®cally, 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 signi®cant differences parallel and perpendicular to the direction of the particle axes. In general, the network chains tended to adopt more compressed con®gurations 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. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Monte Carlo methods; Rotational isomeric states RIS); Reinforcement 1. Introduction There is relatively little molecular understanding of the reinforcement of elastomers, even though this phenomenon is of extraordinary importance in the utilization of rubber- like materials [1±3]. The most common used reinforcing ®llers are carbon black and silica, and they are typically separately prepared and then mechanically blended into an elastomer. Their importance is due to the fact that they can give large increases in the elastomeric modulus at a given strain, and improvements of various technically important properties such as tear and abrasion resistance, and resili- ence. The primary unagglomerated particles of the ®ller are almost always spherical by virtue of the way they are formed, for example in combustion processes or in precipi- tations from solution. There have now been several studies showing that reinforcement of elastomers can also be achieved through the in-situ polymerization of monomers such as styrene to yieldsphericalparticlesofaglassypolymer,inthiscasepoly- styrene PS) [4±6]. One advantage of such glassy particles is the fact that they can be deformed into non-spherical shapes through the deformation ofthe hostelastomer above the glass transition of the PS, and then cooling the material under the applied deformation, before letting it relax [4±6]. In this way, biaxial deformations can be used to obtain oblate disk- shaped) particles [6]. The same effect could be achieved by the use of uniaxial compression but with much greater dif®- culties in achieving signi®cant uniform deformations. Such disc-like ®llers could provide an interesting parallel to the claysplateletsrecentlyusedtoreinforceavarietyofpolymers, including elastomers. The method described, however, also lines up the particles in the direction of the deformation used to distort the particles into their ellipsoidal shapes. In the case of oblate particles, the primary axis is perpendicular to the Polymer 43 2002) 643±652 0032-3861/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0032-386101)00545-6 www.elsevier.com/locate/polymer q This paper was originally submitted to Computational and Theoretical Polymer Science. Following the incorporation of Computational and Theo- retical Polymer Science into Polymer, this paper was consequently accepted for publication in Polymer. * Corresponding author. Present address: School of Textiles and Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA. Tel.: 11-404-894-4117; fax: 11-404-894-8780. E-mail address: sharafma@yahoo.com M.A. Sharaf).