Thermoresponsive Colloidal Chains Collapse to Form Helices

Colloids linked to form linear chains represent model systems for polymers. Chains are prepared using fluorescent micron sized colloidal polystyrene beads that are coated with 100 nm thermoresponsive PNIPAM microgels. Microgel-coated PS beads are lined up in an AC electric field and are crosslinked through amine groups in the microgels, to form thermoresponsive colloidal chains. Here, we demonstrate that these chains exhibit thermally-induced reversible transitions. Interestingly, we show that chains characterized by intermediate flexibility spontaneously organize into helices on heating. In contrast, stiffer chains exhibit a modest decrease in size, without a qualitative change in shape. More flexible chains exhibit a dramatic decrease in size but do not form an ordered helical structure. Thus, in chains where there is no directional specificity to the interactions between monomeric beads, we obtain helical structures when chains with intermediate flexibility collapse. Our work suggests that a purely mechanical instability for semiflexible filaments can drive helix formation, without the need to invoke any directional interactions.