Effect of Monomeric Sequence on Mechanical Properties of P(VP-co-HEMA) Hydrogels at Low Hydration

Abstract

We have used molecular modeling of both random and blocky hydrogel networks of poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) with VP:HEMA=37:13 composition to investigate the effect of the monomeric sequence on the mechanical properties. The degrees of monomer sequence randomness for the random and the blocky copolymers were 1.170 and 0.104, respectively, and the degree of polymerization was set as 50. The equilibrated density of the dry gel network was 0.968+/-0.007 and 0.911+/-0.007 g/cm3 for the random and the blocky sequences, respectively. In the partially hydrated state with 10 wt % water content, the effect of the monomeric sequence causes more distinct differences in density of 1.004+/-0.007 and 0.916+/-0.009 g/cm3 for the random and the blocky copolymer network, respectively. We observed that in such networks, the water molecules are associated more closely with the N-vinyl-2-pyrrolidone than with the hydroxyethyl methacrylate moieties, which is consistent with results from quantum mechanical solvation free energy calculations. By simulating a compressive deformation of the dry gels up to 80% strain, we found that the random sequence network develops higher stress levels than the blocky network. We also found that stress reduction occurs in the random sequence network due to the hydration, which is not evident in the blocky sequence network. This difference in stress reduction between the random and the blocky sequence networks is due to the difference in the structural rearrangement of monomers in the presence of water during deformation. The random sequence network is able to undergo much more efficient rearrangement of HEMA units than in the blocky sequence network.

Effect of monomeric sequence on transport properties of d-glucose and ascorbic acid in poly(VP-co-HEMA) hydrogels with various water contents: molecular dynamics simulation approach

“…The CNs from qg NðVPÞÀOðascorbic acidÞ and qg NðVPÞÀOðdÀglucoseÞ show larger values than those of qg XðHEMAÞÀOðascorbic acidÞ and qg XðHEMAÞÀOðdÀglucoseÞ for both monomeric sequences, indicating that the guest molecule has a greater accessibility to the VP units than the HEMA units in both monomeric sequences. We expect that hydrophilic guest molecules, such as ascorbic acid and D-glucose, may tend to be more associated with the VP units because the VP units are more hydrophilic than the HEMA units, as evaluated by previous experiments [26,28,34,76,77] and simulations [54,55]. The guest molecules can be confined within the polymer network in low hydration conditions, as distinctly shown for a water content of 20 wt.…”

“…1f, we placed MBA at one end of each P(VP-co-HEMA) chain and then arranged all chains to participate in the network structure through periodic boundary conditions. It should be noticed that the topology of our current model is very ideal, having 6 chain-ends connected without any structural variations such as free dangling ends and self-looping, which is deliberately chosen to remove the topological diversity in our modeling study as used in the previous studies [54,55,57,58], while experimental systems have 2-4 chain-ends connected via one cross-linker with structural variations. We think that the current theoretical topology provides the same topological condition to both sequences to investigate the effect of monomeric sequence.…”

“…Since such initial structure has nonphysical strained conformation (Fig. 1g), we applied a molecular mechanics (energy minimization) and annealing procedure to relax the system to a physically meaningful structure as employed in our previous studies [54,55,[57][58][59][60][61][62]. The annealing procedure accelerates the equilibration by driving the system repeatedly through sequential thermal (between 300 and 600 K) and pressure (between 50 and 110 % of the expected density) annealing cycles.…”

“…Throughout this simulation, the hydrogel systems were described using the generic DREIDING force field [63] and F3C water force field [64]. These force fields have been successfully used for studies on various hydrated polymeric network systems such as polymer membranes [59][60][61][62][65][66][67] and hydrogels [54,55,57,58]. The total potential energy is given as follows:…”

“…Therefore, our primary objective in this study is to elucidate the effect of the monomeric sequence on the transport properties of P(VP-co-HEMA) hydrogel with various water contents. For this, we simulated the blocky and random P(VP-co-HEMA) hydrogel systems as reported in our previous studies [54,55] with guest molecules of D-glucose and ascorbic acid (Fig. 1h, i).…”