Simulational study of anomalous tracer diffusion in hydrogels

In this article, we analyze different factors that affect the diffusion behavior of small tracer particles (as they are used e.g.in fluorescence correlation spectroscopy (FCS)) in the polymer network of a hydrogel and perform simulations of various simplified models. We observe, that under certain circumstances the attraction of a tracer particle to the polymer network strands might cause subdiffusive behavior on intermediate time scales. In theory, this behavior could be employed to examine the network structure and swelling behavior of weakly crosslinked hydrogels with the help of FCS.Comment: 11 pages, 11 figure

Directional wetting in anisotropic inverse opals

Porous materials display interesting transport phenomena due to the restricted motion of fluids within the nano- to micro-scale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy

Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy

We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.