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43154-AC7
Molecular Origin of Reversible Changes and Hysteresis in Surfaces and Hidden Interfaces of Polymers
Polymers are one of the most important classes of materials used by mankind. Many of the most important synthetic macromolecules are polymeric hydrocarbons that are synthesized from small molecules derived from petroleum. Often the physical properties of these polymers can be modified in useful, and cost-effective, ways by the addition of filler particles. The current interest in anything "nano" has led to specific interest in composites prepared by the addition of nano-sized filler particles to well-known polymers. The mechanisms by which the useful properties of polymers are improved by the addition of the nano-sized filler particles is a matter of much current discussion. The types of mechanisms proposed have been based on energetic interactions between the filler and the polymer matrix, or alternatively, on entropic changes in the polymer matrix in response to the filler particles. Our contribution to this discussion utilizes a simple model system that completely removes the preferential energetic interaction, and thereby reveals the entropic effects produced on the polymer due to the presence of the nano-sized filler particle.
Our simulation starts from a polymer melt. The chains in the melt need not all have the same degree of polymerization, but they are all constructed from the same chemical repeat unit. A subset of the chains is selected for the formation of the particles. Particle formation is produced by enhancing the attractive part of the intra- (and only the intra-) molecular Lennard-Jones potential for the repeat units in these selected chains. When this enhancement is tuned appropriately, the selected chains collapse intramolecularly, to globular structures with radii of gyration equivalent to that expected for a uniform sphere with the mass and partial specific volume of that chain. The extent of collapse is captured by a parameter 'c' defined in the equation (see TOC graphic and nugget) where 's' denotes the radius of gyration of the chains that form particles, the subscript '0' denotes the unperturbed (pure melt) state before collapse, the subscript 'filler' denotes the value in the filled composite, 'M' is the molecular weight of the particle, and ρ is the reciprocal of the partial specific volume.
After equilibration, we examine the influence of these collapsed chains (the nanoparticles) on the remaining uncollapsed chains which represent the polymer matrix. Since all components of the system are subject to exactly the same inter-molecular Lennard Jones potential, preferential energetic intermolecular interactions are impossible.
We have demonstrated that the matrix chains usually expand in response to the formation of the nanoparticles. The expansion can be quite dramatic in some cases, with the mean square radius of gyration increasing by more than a factor of two.
