Molecular dynamics simulations of small molecule diffusion in hydrogels

Solute diffusion across polymer matrices is important in several technological applications such as controlled drug delivery, gel electrophoresis and gas separation. Our primary objective is to investigate the effect of size-exclusion on solute diffusion in polymer matrices with a focus on crosslinked hydrogel networks. The diffusion co-efficient of a solute in a hydrogel depends on the free-volume available as determined by the solute size and the polymer concentration. However, hydrogels have a highly disordered pore structure due to the random nature of the crosslinking techniques used in their preparation. The characterization of the free-volume of this pore structure by experimental techniques or mathematical modeling is not straightforward. Consequently, this precludes a direct testing of the correlation between diffusion and free-volume. In this thesis, we prepare simple, defect-free, end-crosslinked polymer networks via computer simulation. We choose a coarse-grained bead-spring representation for our polymer chains. In addition to crosslinked networks, we also prepare other host matrices: polymer solutions composed of flexible chains and fibrous matrices composed of interpenetrating rods. The free-volume available to a solute of a given size in these host matrices is characterized by calculating the so-called accessible volume fraction. Subsequently, the solute diffusion coefficients are computed by performing Molecular Dynamics (MD) simulations. Our general finding is that the solute diffusion coefficients in these host matrices correlate well with the accessible volume fraction, which collapses onto one single variable the twin effects of solute size and polymer concentration. In the case of a crosslinked network with a given crosslinking density, solute diffusivities are a unique function of the accessible volume fraction. We also conclude that the mobility of the chains in the polymer matrix has a significant effect on solute diffusion, even at high accessible volume fraction. We have also investigated the effect of attractive solute-polymer interactions on solute partitioning and diffusion in crosslinked networks. A solute with specific interactions diffuses faster than an inert solute of the same size. We attribute this to the active partitioning of solutes across the bottlenecks in the network that the solute encounters during diffusion.