Equilibrium molecular dynamics simulations on amorphous polymers filled with solid nanoparticles show that mechanical reinforcement results from the formation of a long-lived transient polymer−particle network only over a narrow range of parameter space. In these cases it is necessary that (i) the interfacial zone occupy significantly less volume than the bulk region and (ii) particle−polymer interactions must be strong enough that the relaxation time for the small fraction of adsorbed monomers is much longer than that characterizing the neat polymer. In all other cases, reinforcement will appear to be particle driven since there is no clear demarcation between the adsorbed segments and the bulk polymer. However, the apparent size of the particle will be larger because of the adsorbed segments. Both reinforcement mechanisms occur for systems that do not easily equilibrate, leading us to stress the importance of starting states and processing history which is reminiscent of glassy systems.

Over the past few decades, research studies have established that the mechanical properties of hydrogels can be largely impacted by the addition of nanoparticles. However, the exact mechanisms behind such enhancements are not yet fully understood. To further explore the role of nanoparticles on the enhanced mechanical properties of hydrogel nanocomposites, we used chemically crosslinked polyacrylamide hydrogels incorporating silica nanoparticles as the model system. Rheological measurements indicate that nanoparticle-mediated increases in hydrogel elastic modulus can exceed the maximum modulus that can be obtained through purely chemical crosslinking. Moreover, the data reveal that nanoparticle, monomer, and chemical crosslinker concentrations can all play an important role on the nanoparticle mediated-enhancements in mechanical properties. These results also demonstrate a strong role for pseudo crosslinking facilitated by polymer–particle interactions on the observed enhancements in elastic moduli. Taken together, our work delves into the role of nanoparticles on enhancing hydrogel properties, which is vital to the development of hydrogel nanocomposites with a wide range of specific mechanical properties.

The effect of nanoscale fillers on the mechanical properties of polymers has been extensively studied. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] While there are many interesting aspects to this problem, here we focus on the mechanical behavior of nanocomposites, especially the mechanical reinforcement that is afforded when nanoparticles are added to polymers. The cumulative knowledge in this field points to two primary mechanisms responsible for mechanical reinforcement. The first mechanism, which is essentially particledriven, 7,15 occurs at relatively high particle volume fraction, beyond the particle percolation threshold, and may be referred to as "jamming". The second, termed the "network reinforcement" mechanism, occurs due to the formation of a long-lived percolating polymer network with the particles as the "network nodes". 11,14,[16][17][18] Recent experiments on pure entangled polymer melts, 19,20 polymers filled with platelet fillers, 3,8,10,[21][22][23] nanotubes, 24 and spherical nanoparticles 16 show that, regardless of reinforcing mechanism, the startup of shear flow is accompanied by stress overshoots. After a certain time, which appears to be related to the "aging" time of the system and the applied strain rate (but not the total strain), 16 the stress recovers to a welldefined plateau value. The viscosities derived from these longterm stress plateaus suggest that these materials shear thin over the whole range of accessible frequencies. Both of these results (i.e., stress overshoots and shear thinning) have been empirically attributed to network structures which exist in the quiescent state, but which are disrupted on the application of shear. While the variation of viscosity with shear flow has been studied through simulations, 6,7,11 simulations have not been used to understand shear stress overshoot on the startup of shear flow. 3,15,16,21 To gain simulation derived insights into the origins of stress overshoots in the nonlinear rheology of nanocomposites, we perform nonequilibrium molecular dynamics simulations on two different realizations of a quiescent nanocomposite structure possessing the same spatial distribution of nanoparticles. In one case there is a percolating polymer network mediated by the particles, while in the other there is no percolating network present. The transient responses of these two structures to an applied shear are very different and unequivocally show that the percolating structure results in stress overshoots. Detailed analysis demonstrates that only the polymer strands which "bridge" the particles to form a percolating network experience stress overshoots, with these overshoots disappearing when the network is broken. While the time needed to break this transient network increases with decreasing strain rate, the net strain at the maximum stress overshoot value is approximately constant with strain rate. Finally, we make connections between these results and those obtained from solutions of entangled polymers and suggest that the origins of stress oversho...

We use molecular dynamics simulations of a solid Brownian particle in an explicit solvent to analyze the velocity field generated by a stochastic motion of a particle. The simulation data demonstrate that the amplitude of the velocity field around a Brownian particle decays much faster than the velocity field around a particle moving with a constant velocity. However, the time-integrated response of the velocity field around a Brownian particle has exactly the same distance dependence as the velocity field around a particle moving with a constant velocity. This finding elucidates the validity of an assumption used in theoretical descriptions of Brownian particles dynamics in confined geometries and in colloids; namely, that viscous drag forces can be computed as if the particles move with constant velocities.