Filler Size Effect on the Structure of Polymer-Filler Interface in Polymer-Based Nanocomposites: Entropic and Energetic
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Filler Size Effect on the Structure of Polymer-Filler Interface in Polymer-Based Nanocomposites: Entropic and Energetic Effects R.C. Picu and M.S. Ozmusul Department of Mechanical, Aerospace and Nuclear Engineering Rensselaer Polytechnic Institute, Troy, NY 12180 ABSTRACT A study of the polymer structure in the vicinity of an interface with a rigid, curved wall is presented. The study is performed by means of lattice Monte Carlo simulations in melts of low and high density. Both purely entropic and energetic systems are considered. The configurational entropy of the chains as well as the cohesive interactions in the bulk polymer lead to the formation of a low-density layer in the close neighborhood of the wall. This layer is about one monomer thick in the purely entropic case, and has a thickness of about three radii of gyration in presence of energetic interactions. Increasing the wall curvature leads to an increase in density in the depleted layer, the effect being more pronounced in the energetic system. Chain end segregation at the wall is observed in all cases. This effect increases with increasing chain length and decreases with increasing wall curvature. The bonds are preferentially oriented in the direction tangential to the wall. The degree of orientation decreases with increasing wall curvature and is independent of chain length. Finally, the evolution of the density, of the segregation effect and of the bond preferential orientation with temperature is investigated. The density at the wall decreases with decreasing temperature in the melt state, while the segregation becomes more pronounced. The volume of polymer in which the structure is affected by the presence of the wall increases with decreasing temperature. INTRODUCTION An impenetrable interface induces structure in the polymer located in its immediate vicinity. This structure is observed on various length scales and is due to both energetic and entropic interactions of the chains. The presence of the wall restricts the number of configurations a chain might take and hence reduces the entropy of chains in its vicinity. The polymers are therefore pushed by an entropic force away from the rigid wall. This effect leads to the formation of a depleted layer next to the wall. If the density is high, the packing entropy becomes important and the density next to the interface increases. Considering energetic interactions, either between the polymers or between polymers and the wall, further complicates the picture. The cohesive energy in the polymer bulk leads to a reduction of the density in the interface, while an attractive interaction between polymer chains and the wall causes the opposite trend. These issues have been studied extensively for the flat rigid interface over the last years [e.g. 1-5]. The topological constraints in the vicinity of the interface require that the bonds are preferentially oriented in the direction parallel to the wall. The degree of alignment depends on the details of the energetic interactions with the wall. A similar alignm
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