Cross-linker effect on solute adsorption in swollen thermoresponsive polymer networks

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.

Tuning the selective permeability of polydisperse polymer networks

“…69 The bonded polymer parameters were determined via coarse-graining from explicit-water, all-atom simulation results of cross-linked PNIPAM chains, utilizing a force-field from our group's work. 120 Since the cross-linker connects monomers of four polymer chains, the network is tetra-functional, and in addition to the m-m-m bending, there are six bending potentials for the m-xlink-m arrangement. Therefore, we have nine different bonded (7 bending (angles), 2 stretching (bonds)) potentials in total and we determined eighteen bonded parameters K ij r , r ij 0 , K ijk y , and y ijk 0 by fitting harmonic potentials to the free energies obtained from the all-atom simulations.…”

Atomic scale investigation of the volume phase transition in concentrated PNIPAM microgels

“…To keep up with the large number of experimental and theoretical results, the role of numerical simulations has recently become more and more crucial for a better understanding of microgels behavior [35]. In particular atomistic simulations have helped evaluating the molecular features that affect the lower critical solution temperature of the polymer and thus the related VPT of microgels [36][37][38][39][40][41][42][43][44][45][46][47] and they provided useful insights into the molecular mechanism that drives the process [48][49][50][51].…”

Modeling of stimuli-responsive nanoreactors: rational rate control towards the design of colloidal enzymes

“…6.ii, mimics the cross-linker unit of a hydrogel network, and thus lends itself to study the influence of cross-linkers on adsorption of molecules. In our previous study, 112    …”

“…In the following, we will review our recent efforts to understand partitioning and diffusion of solutes in swollen and collapsed PNIPAM hydrogels by all-atom MD simulations. [111][112][113][114]…”

“…2,4,83,93 We have studied partitioning and permeability of polymer networks and PNIPAM polymers recently on the molecular level by coarse-grained 109,110 as well as all-atom molecular dynamics computer simulations. [111][112][113][114] Hence, in the last couple of years many quantitative concepts have emerged both on the continuum and the microscopic level that will eventually lead to a more fundamental understanding of nanoreactors in the future. The possibility arises to optimize responsive nanoreactors to reach the high recognition, selectivity, and feedback control as found for enzymes, 115,116 to create "colloidal enzymes".…”