Spatial Dependence of Non-Gaussian Diffusion of Nanoparticles in Free-Standing Thin Polymer Films

Abstract

The addition of nanoparticles (NPs) to a free-standing polymer film affects the properties of the film such as viscosity and glass transition temperature. Recent experiments, for example, showed that the glass transition temperature of thin polymer films was dependent on how NPs were distributed within the polymer films. However, the spatial arrangement of NPs in free-standing polymer films and its effect on the diffusion of NPs and polymers remain elusive at a molecular level. In this study, we employ generic coarse-grained models for polymers and NPs and perform extensive molecular dynamics simulations to investigate the diffusion of polymers and NPs in free-standing thin polymer films. We find that small NPs are likely to stay at the interfacial region of the polymer film, while large NPs tend to stay at the center of the film. On the other hand, as the interaction between a NP and a monomer becomes more attractive, the NP is more likely to be placed at the film center. The diffusion of monomers slows down slightly as more NPs are added to the film. Interestingly, the NP diffusion is dependent strongly on the spatial arrangement of the NPs: NPs at the interfacial region diffuse faster and undergo more non-Gaussian diffusion than NPs at the film center, which implies that the interfacial region would be more mobile and dynamically heterogeneous than the film center. We also find that the mechanism for non-Gaussian diffusion of NPs at the film center differs from that at the interfacial region and that the NP diffusion would reflect the local viscosity of the polymer films.

Relative Chain Flexibility Determines the Spatial Arrangement and the Diffusion of a Single Ring Chain in Linear Chain Films

“…As the ring monomer approaches the film surface, m increases by up to 40%, suggesting that ring monomers are quite mobile at the film surface. The presence of such mobile surfaces is consistent with previous studies. − This suggests that the lateral diffusion of the relatively flexible ring chain at the film surface would be facilitated due to the high mobility at the film surface.…”

“…The transport of polymer chains in thin polymer films has been an issue of interest because the transport of thin polymer films is different from that of bulk polymer melts. There were many reports that the mobility of chains increased at the film surface compared to that at the film center. − In addition, Si et al showed that the density of entanglements of linear chains was reduced at the surface of films. Also, it is known that the structure of ring chains at the film surface is different from that in bulk melts. , Therefore, one may expect that the ring diffusion in linear polymer chain films should depend on the spatial arrangement, the extent of threading, the compressed ring conformation at the surface, and the mobile film surface.…”

Temperature Dependence of Conformational Relaxation of Poly(ethylene oxide) Melts

“…Here, r i denotes the position vector of the center of mass of a strand i at time t. We also investigate the self-part of the van Hove correlation function (G s (r, t) = δ(r − |r i (t) − r i (0)|) ) of each strand. If PEO chains were to follow the conventional Fickian diffusion, G s (r, t) is expected to be Gaussian [56][57][58]. In order to estimate how much the diffusion of strands deviates from being Gaussian, we calculate the non-Gaussian parameter (α 2 (t)) of strands of PEO chains as follows;…”

The effects of defects on the transport mechanisms of lithium ions in organic ionic plastic crystals

“…But the notion of solid electrolytes is somewhat contradictory: systems in their solid states are characterized by the extremely high viscosity (which makes the molecular transport very slow), high ion conductivity and high transport properties still need to be guaranteed for desired electrolytes. In order to achieve the high ion conductivity in the solid state of high viscosity, the system should be dynamically heterogeneous: 3,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] some molecules undergo slow (either translationally or rotationally) diffusion while other molecules of the same kind undergo fast diffusion. The slow diffusion of the molecules is mostly determined by the high viscosity of solid systems.…”