Dynamics of Linear Entangled Polymers Reinforced with Nanoscale Rigid Particles

Recent molecular dynamics simulations [1-3] suggested that the polymer-surface interactions can be the dominant factor in the rheology of polymer systems filled with nanoparticles. These interactions include the short-range forces between the surfaces and the polymer segments. These forces can be responsible for the suppression of the mobility of the polymer segments at the surfaces and even result in the formation of an immobilized glassy layer at the surfaces. Dionne et al. [3] studied the structure and dynamics of an amorphous polyethylene (PE) melt containing homogeneously distributed spherical nanoparticles. The PE chains were simulated using both molecular dynamics and Monte Carlo methods. The chain dynamics were monitored by computing the Rouse relaxation modes and the mean-square displacement (MSD). The most notable observation they pointed out, was the slowing down in the Rouse dynamics seen on all subsections of the chain no matter how small the subsections were, meaning that on average every monomer feels the confinement of the neighboring particles, slowing the relaxation of every chain subsection. They also showed that the slowing down due to polymer-particle energetic interaction was similar for all relaxation modes, independent of their wavelength. Taking advantage of these insights, a reptation-based model, that incorporates transient polymerparticle surface interactions, is proposed to describe the dynamics and rheological behaviors of linear entangled polymers filled with isotropic rigid nanoscale particles. Dispersed nanoparticles are sufficiently small such that even at low volume fractions, the average particle wall-to-wall distance is on the order of the chain size. Using the theory of the activation process, it is shown that the polymerparticle interactions give rise to an exponential increase of the characteristic time, g, for the detachment of all trapped monomers in the chain, from nanoparticle surfaces, i.e.,