Molecular dynamics simulation study of P (VP-co-HEMA) hydrogels: Effect of water content on equilibrium structures and mechanical properties

Poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) (P(VP-co-HEMA)) hydrogel system with a composition of VP:HEMA=37:13 was studied using molecular dynamics simulations in order to investigate the effect of the water content on the equilibrium structures and the mechanical properties. The degree of randomness of the monomer sequence for the random and the blocky copolymers, were 1.170 and 0.104, respectively, and the degree of polymerization was fixed at 50. The equilibrated density of the hydrogel was found to be larger for the random sequence than for the blocky sequence at low water contents (<40 wt%), but this density difference decreased with increasing water content. The pair correlation function analysis shows that VP is more hydrophilic than HEMA and that the random sequence hydrogel is solvated more than the blocky sequence hydrogel at low water content, which disappears with increasing water content. Correspondingly, the water structure is more disrupted by the random sequence hydrogel at low water content but eventually develops the expected bulk water-like structure with increasing water content. From mechanical deformation simulations, stress-strain analysis showed that the VP is found to relax more efficiently, especially in the blocky sequence, so that the blocky sequence hydrogel shows less stress levels compared to the random sequence hydrogel. As the water content increases, the stress level becomes identical for both sequences. The elastic moduli of the hydrogels calculated from the constant strain energy minimization show the same trend with the stress-strain analysis.