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

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