Periodic structures with a variety of geometries in nano- or micro-scale are of interest for applications, including photonic and phononic crystals, color displays, memory and logic layouts, and tunable surfaces. Recently, pattern deformations of 2D periodic arrays from elastomeric (e.g. PDMS) or elasto-plastic (e.g. SU-8) materials triggered by mechanical instabilities, including mechanical compression, solvent-induced swelling, and polymerization followed by solvent evaporation, have been studied and exploited for different applications. Among them, the swelling and shrinkage of hydrogels in response to different external stimuli are intriguing given the complexity and functionality of the materials. Here, we study the pattern transformation behaviors in 2D periodic arrays from poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogels, including pure PHEMA, poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm), and poly(2-hydroxyethyl methacrylate-co-acrylic acide) (PHEMA-co-PAA) systems. The PHEMA membrane undergoes (1) a breathing mode (i.e. the hole reduces size but retains the shape) when exposed to deionized-water; (2) a phase transition to a diamond plate pattern with mutually perpendicular ellipses driven by capillarity during drying process; and (3) a recovery upon re-exposure to water. In comparison to the pure PHEMA, PHEMA-co-PNIPA and PHEMA-co-PAA are sensitive to temperature and pH, respectively. Therefore, their pattern transformation and recovery, as well as grain boundary are strongly influenced by the dynamic interactions between the polymer matrix and water molecules under different temperatures and pH buffer solutions. Here, we present our investigation on the pattern transformation mechanisms and the kinetics in different gel systems upon the swelling and drying stages both experimentally and theoretically. We believe that the study of the dynamic behaviors of hydrogel membranes at the microscopic level toward different environmental changes will offer new insights of harnessing hydrogels for design of energy efficient systems to control light, heat and sound transmission and insulation.

Understanding pattern transformation mechanisms in different responsive hydrogel membranes

242nd National Meeting (2011)

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