Diffusion and viscosity in a supercooled polydisperse system

Abstract

We have carried out extensive molecular dynamics simulations of a supercooled polydisperse Lennard-Jones liquid with large variations in temperature at a fixed pressure. The particles in the system are considered to be polydisperse in both size and mass. The temperature dependence of dynamical properties such as the viscosity (eta) and the self-diffusion coefficients (D(i)) of different size particles is studied. Both viscosity and diffusion coefficients show super-Arrhenius temperature dependence and fit well to the well-known Vogel-Fulcher-Tammann equation. Within the temperature range investigated, the value of Angell's fragility parameter (D approximately 1.4) classifies the present system as a very fragile liquid. The critical temperature for diffusion (T(D(i))(o)) increases with the size of the particles. The critical temperature for viscosity (T(eta)(o)) is larger than that for diffusion, and sizable deviations appear for the smaller size particles, implying a decoupling of translational diffusion from viscosity in deeply supercooled liquids. Indeed, the diffusion shows markedly non-Stokesian behavior at low temperatures where a highly nonlinear dependence on size is observed. An inspection of the trajectories of the particles shows that at low temperatures the motions of both the smallest and largest size particles are discontinuous (jump type). However, the crossover from continuous Brownian to large length hopping motion takes place at shorter time scales for the smaller size particles.

Thermorheological properties near the glass transition of oligomeric poly(methyl methacrylate) blended with acrylic polyhedral oligomeric silsesquioxane nanocages

“…The POSS cages need larger void spaces through which to translate when compared with the equally large but linear oligomer chains, which can diffuse by segmental Rouse motions. The higher free volume of the POSS at T g leads to the greater enthalpy relaxation observed in the DSC curves of Figure 4, whereby smaller T 8 POSS cages maintain a higher degree of mobility due to their greater ability to diffuse into the remaining pockets of free volume than the larger cages (Murarka and Bagchi 2003).…”

“…Materials with molecular structures of similar shape to that of the POSS (e.g. polydisperse ensembles of Lennard-Jones spheres or hyperbranched dendrimers) fall near the fragile liquid limit (Dantras et al 2002;Murarka and Bagchi 2003). Dendrimers, for example, have been shown to exhibit enhanced enthalpy relaxation in DSC measurements (Dantras et al 2002; see Figure 4) and also have relatively high free volumes at the glass transition temperature with 0.034 ≤ f g /B ≤ 0.042 (Dorgan et al 2003;Tande et al 2003).…”

The effect of hydrogen bonding on the diffusion of water in n-alkanes and n-alcohols measured with a novel single microdroplet method

“…1 However, since the equation was originally derived for a relatively large solute in a continuum solvent, it is not applicable to the case we wanted to test, where the solute is smaller than the solvent ͑as in our case of water in the homologous series of alcohols and alkanes͒. 9 One modification to the Stokes-Einstein equation for small solutes is based on a reduced viscosity assumption, 1 which states that D ϰ −␣ , where ␣ lies between 2/3 and 1.…”

Energy Landscape, Antiplasticization, and Polydispersity Induced Crossover of Heterogeneity in Supercooled Polydisperse Liquids

“…Because these systems exist in the glassy phase over a wide range of polydispersity, they offer an opportunity to test many of the theories and ideas developed in this area in recent years. We find that polydispersity introduces several unique features to the dynamics of these systems not present in the binary systems usually employed to study dynamical features in supercooled liquids and glasses.In this Letter we particularly investigate how polydispersity influences the potential energy landscape, fragility, and heterogeneous dynamics of polydisperse LennardJones (LJ) systems in the supercooled regime near the glass transition [8]. The polydispersity in size is introduced by random sampling from a Gaussian distribution of particle diameters, .…”