Abstract
The selective solute partitioning within a polymeric network is of key importance to applications in which controlled release or uptake of solutes in a responsive hydrogel is required. In this work we investigate the impact of cross-links on solute adsorption in a swollen polymer network by means of all-atom, explicit-water molecular dynamics simulations. We focus on a representative network subunit consisting of poly(N-isopropylacrylamide) (PNIPAM) and N,N'-methylenebisacrylamide (BIS/MBA) cross-linker types. Our studied system consists of one BIS-linker with four atactic PNIPAM chains attached in a tetrahedral geometry. The adsorption of several representative solutes of different polarity in the low concentration limit at the linker region is examined. We subdivide the solute adsorption regions and distinguish between contributions stemming from polymer chains and cross-link parts. In comparison to a single polymer chain, we observe that the adsorption of the solutes to the cross-link region can significantly differ, with details depending on the specific compounds' size and polarity. In particular, for solutes that have already a relatively large affinity to PNIPAM chains the dense cross-link region (where many-body attractions are at play) amplifies the local adsorption by an order of magnitude. We also find that the cross-link region can serve as a seed for the aggregation of mutually attractive solutes at higher solute concentrations. Utilizing the microscopic adsorption coefficients in a mean-field model of an idealized macroscopic polymer network, we extrapolate these results to the global solute partitioning in a swollen hydrogel and predict that these adsorption features may lead to non-monotonic partition ratios as a function of the cross-link density.
Highlights
In the zoo of constituents, thermoresponsive hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) are among the most intensively investigated systems, since their volume phase transition at about room temperature as well as a high water content promise good biocompatibility[5,11] and make them convenient to handle
The aim of this work is to investigate the effects of crosslinking on solute adsorption in swollen hydrogels made up of PNIPAM and BIS (below the volume phase transition temperature (VPTT)) by utilizing all-atom, explicit water molecular dynamics (MD) simulations of a minimal polymer network setup
We investigated the effects of cross-linking on solute adsorption in swollen PNIPAM hydrogels by means of explicit-water MD simulations at T = 290 K, i.e., below the PNIPAM collapse transition temperature
Summary
In the zoo of constituents, thermoresponsive hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) are among the most intensively investigated systems, since their volume phase transition at about room temperature as well as a high water content promise good biocompatibility[5,11] and make them convenient to handle. BIS has chemical similarity to PNIPAM (compare Fig. 1), is non-degradable, has a very high reactivity, and retains PNIPAM’s LCST.[13,14,15,16,17] Besides these morphological properties, the degree of crosslinking influences solute uptake and partitioning. The partition ratio is the ratio of the solute concentrations inside and outside the gel and is a crucial parameter controlling device functionality especially for drug delivery or catalytic systems. For the latter, for instance, metal nanoparticles inside hydrogels
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