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°, 30°, 45°, 60°, and 75°) 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 ∼2.0) to onset of symmetry breaking of slits at pH 4.7 (swelling ratio ∼2.3), to the formation of twisted, “S” shaped slits at pH 6.4 (chiral, swelling ratio ∼5.8). While swelling of the square array hydrogel membranes leads 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° and 30°), while nearly exclusive right-handedness is obtained from the ones with larger pre-stretching angles (60° and 75°).