Controlling the roll-to-helix transformation in electron-beam-patterned gel-based micro-ribbons.

Abstract

Helix formation has been of ongoing interest because of its role in both natural and synthetic materials systems. It has been extensively studied in gel-based ribbons where swelling anisotropies drive out-of-plane bending. In contrast to approaches based on photolithography or mechanical bilayer construction, we use electron-beam patterning to create microscale ribbons at ∼1-100 μm length scales in pure homopolymer precursor films of poly(acrylic acid) (PAA). The radiation chemistry creates a ribbon comprising a crosslinked hydrophobic top layer and a hydrophilic gel bottom layer with a continuous through-thickness variation in between. The classic roll-to-helix transition occurs as the ribbon aspect ratio increases. Notably, we see examples of single-loop rolls, multi-loop rolls, minimal-pitch helices, plus a transition structure comprising both helical and roll-like features. Finite-element modelling recapitulates key aspects of these conformations. Increasing the pH from below to above the PAA p<i>K</i>_a increases the out-of-plane bending to the extent that the ribbons plastically deform and nonminimal-pitch helices form across a wide range of aspect ratios and irradiation conditions. The nonminimal pitch is caused by an in-plane anisotropy associated with the plastic deformation. We mimic this anisotropy by patterning ribbons comprising micro-tiles separated by gaps which receive electron exposure due to proximity effects. We observe a transition from roll to helix to tube with increasing gap angle. The chirality is completely determined by the gap orientation (±<i>θ</i>). However, in contrast to established approaches to generate in-plane anisotropies based on mechanical properties, finite-element modelling indicates that anisotropic through-thickness swelling of the gap material plays a dominant role in helix formation and suggests that this micro-composite ribbon behaves like a rigid origami metamaterial where deformation at the creases (the gaps) between structural elements controls the shape shifting.