Rigidity Dictates Spontaneous Helix Formation of Thermoresponsive Colloidal Chains in Poor Solvent

The
formation of helical motifs typically requires specific directional
interactions. Here, we demonstrate that isotropic interparticle attraction
can drive self-assembly of colloidal chains into thermo-reversible
helices, for chains with a critical level of backbone rigidity. We
prepare thermoresponsive colloidal chains by cross-linking PNIPAM
microgel-coated polystyrene colloids (“monomers”), aligned
in an AC electric field. We control the chain rigidity by varying
cross-linking time. Above the LCST of PNIPAM, there is an effective
attraction between monomers so that the colloidal chains are in a
bad solvent. On heating, the chains decrease in size. For the most
rigid chains, the decrease is modest and is not accompanied by a change
in shape. Much less rigid chains form relatively compact structures,
resulting in a large increase in the local monomer density. Unusually,
chains with intermediate rigidity spontaneously assemble into helical
structures. The chain helicity increases with temperature and plateaus
above the collapse transition temperature of the microgel particles.
We simulate a minimal model that captures the spontaneous emergence
of the helical conformations of the polymeric chain and provides insight
into this shape transition. Our work suggests that a purely mechanical
instability for semiflexible filaments can drive helix formation,
without the need to invoke directional interactions.