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 ≥ σc). 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 ∼ t1/2) at early stages, which was independent of the final pattern morphology. The onset of wrinkling was found at a linear expansion of αc ≈ 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, αe, 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.