Kinetic study of swelling-induced surface pattern formation and ordering in hydrogel films with depth-wise crosslinking gradient

Abstract

Hydrogels undergo extensive three-dimensional volume changes when immersed in water, the degree of which is determined by the network chemical composition and degree of crosslinking. When the hydrogel is attached to a rigid substrate, it swells preferentially perpendicular to the substrate. This anisotropic swelling generates a compressive stress, which drives the formation of surface patterns when exceeding a critical stress value (σ ≥ σ_c). In order to develop an indepth understanding of the mechanism of surface pattern formation in hydrogels, we investigated the dynamic evolution of surface patterns in photocured hydrogel films from poly(2-hydroxyethyl methacrylate) (PHEMA) crosslinked with different concentrations of ethylene glycol dimethacrylate (EGDMA, 0–3 wt%). During curing in the presence of oxygen, a modulus gradient along the film depth was generated due to oxygen inhibition of the radical polymerization near the film surface. The swelling-induced wrinkling pattern formation followed Fickian-type kinetics (λ ∼ t^1/2) at early stages, which was independent of the final pattern morphology. The onset of wrinkling was found at a linear expansion of α_c ≈ 1.12, which remained constant with increasing EGDMA concentration but decreased with increasing film thickness, indicating an increase in critical stress with crosslinker concentration. In contrast, the equilibrium linear expansion value, α_e, decreased significantly (from 2.55 to 1.20) with increasing crosslinker concentration (from 0 to 3 wt%), resulting in transition from random patterns to highly ordered hexagonal structures.