Buckling into single-handed chiral structures from pH-sensitive hydrogel membranes

We investigate swelling induced pattern transformation of pH-sensitive hydrogel membranes made of poly(2-hydroxyethyl methacrylate)-co-poly(acrylic acid) (PHEMA-co-PAA) with periodic pores in both square and oblique arrays. The hydrogel membranes are fabricated by replica molding from a poly(dimethylsiloxane) (PDMS) mold with square arrayed pillars, or from the same mold that is mechanically pre-stretched at a very small strain (5%) at different angles (15 degrees, 30 degrees, 45 degrees, 60 degrees, and 75 degrees) relative to the lattice axis. When swollen in different pH buffer solutions, the membrane undergoes buckling of pores to slits at pH 3 (achiral, swelling ratio similar to 2.0) to onset of symmetry breaking of slits at pH 4.7 (swelling ratio similar to 2.3), to the formation of twisted, S'' shaped slits at pH 6.4 (chiral, swelling ratio similar to 5.8). While swelling of the square array hydrogel membranes leads to similar to 50/50 mixture of right- and left-handed chiral structures, predominant single handedness with enantiomeric excess fraction >95% (i.e. the absolute difference between the mole fraction of each enantiomer) is obtained from the oblique array of porous membranes. Single left-handedness is obtained from the ones with low pre-stretching angles (15 degrees and 30 degrees), while nearly exclusive right-handedness is obtained from the ones with larger pre-stretching angles (60 degrees and 75 degrees). (C) 2015 Elsevier Ltd.