Abstract
In this paper we examine the polymer density distribution of gel particles and its effect on solvent diffusivity through the polymer network. In order to access the inner particle regions, external polymer layers were removed by plasma etching, thus reducing them from the outside. Higher polymer densities after erosion showed internal heterogeneity, with the density increasing towards the center of the particles. An exponential decay polymer density model is proposed, and the spatial relaxation length measured. The diffusion of solvent through the particles, before and after the plasma oxidation, revealed a correlation between the diffusion coefficient and the internal density.
Highlights
Hydrogels are 3D polymer networks that absorb or expel water in response to environmental conditions
Hydrogels at equilibrium are described by the Flory-Rehner theory [5], which considers the balance between osmotic pressure due to polymer-solvent mixing and the elastic pressure due to the cross-links
We studied polymer density distribution into minigels, which are gel particles in the range of several tens of micrometers
Summary
Hydrogels are 3D polymer networks that absorb or expel water in response to environmental conditions. A correlation was established among this polymer distribution and the solvent diffusivity across the polymer network during particle swelling The study into this length-scale may add more universality to the knowledge of the natural inhomogeneity of hydrogels. At low polymer solubility (χ > 0.5), the effect of the mixing contribution depends on the polymer density; the gel tends to swell for high polymer concentrations and to collapse for even higher Φ values. The network elasticity always contributes to gel de-swelling It is set only by the network cross-link density Nc (the network’s number of chains, which is assumed constant in the first approximation, along the whole network with homogenous density distribution) and modulated by the polymer volume fraction Φ, as πe. This rearrangement is the mechanism responsible for polymer gel swelling
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