Evolution of surface morphology in 2-hydroxyethyl methacrylate copolymer exposed to γ radiation

Buckling patterns were observed on the outer surface of HEMA copolymers irradiated in air and subsequently immersed in solvents. The structure parameters of these buckling patterns on the irradiated copolymers were obtained via image analysis; these parameters include pattern size, pattern uniformity, and undulation amplification. The pattern size increased with time due to the release of elastic constraints and then reached a constant morphology, which persisted upon attaining equilibrium swelling due to a radiation-induced concentration gradient in carboxyl groups along the thickness direction. The pattern size increased as the dose rate decreased. Solution pH also influenced the surface morphology of irradiated samples. Increases in swelling accompanied by increasing the pH of the solvent resulted in patterns with morphologies, which shifted from noodlelike to needlelike grains. All of this is explained by the changes in osmotic pressure and radiation-induced cross-link density gradients along the thickness direction of the samples.