Solvent induced transition from wrinkles to creases in thin film gels with depth-wise crosslinking gradients

We investigated solvent induced transition of surface instability from wrinkles to creases in poly(2-hydroxyethyl methacrylate) (PHEMA) gels with depth-wise crosslinking gradients. The mode of surface instability and morphology of surface patterns was found to be dependent on the equilibrium linear expansion, which was a function of crosslinker concentration and the solvent-polymer interaction. The maximum linear expansion was obtained when the PHEMA film was swollen in a good solvent, which had the Hildebrand solubility parameter (δ_s) close to that of PHEMA gels, 26.6 to 29.6 MPa^1/2. In a relatively poor solvent (e.g. water), wrinkling patterns were obtained and the morphoplogy was determined by the concentration of the crosslinker, ethylene glycol dimethacrylate (EGDMA). In a good solvent, such as alcohol and alcohol/water mixture, the equilibrium linear expansion ratio increased significantly, leading to the transition from wrinkling to creasing instability. In an ethanol/water mixture, we systematically varied the ratio between ethanol and water and observed the transition from wrinkling to creasing when gradually adding ethanol to water, and the reverse transition when adding water in ethanol. The onset of the linear expansion ratio for creasing (α_c,c) was again found dependent on EGDMA concentration: α_c,c ≈ 2.00 and 1.3, respectively, for gels with 1 and 3 wt% EGDMA. Finally, we demonstrated confinement of the creases by combining swelling and photopatterning.