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

Abstract

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.

Transient helix formation in charged semiflexible polymers without confinement effects

“…where u i is the unit vector of U i = r i × r i+1 . A simple uncharged, semi-flexible polymer chain shows H2 values ≈ 0 (or negative values of H2) as expected for a chain locally bent due to thermal fluctuations, while calculating H2 over helical segments of a polymer chain would lead to high positive values of H2 [14,34].…”

“…Thus, the helical structure is ubiquitous in the macro-molecular world. This has spurred a number of studies that investigate their emergence in several natural systems [4,[6][7][8][9][10][11][12][13][14][15]. Furthermore, helical motifs in synthetic molecules have been explored for their key technological ramifications [16] such as the synthesis of NEMs devices [17][18][19][20][21][22], piezoelectric devices [23] and other optical materials [24][25][26].…”

“…These beads can be molecular units of a heteropolymer, or groups of amino acid sequences with length scale of the size of the beads, or even larger units of molecules such as nucleosome complex in the DNA dressed by suitable (charged) proteins, or even polymeric chains where the monomeric units are micron sized colloids. In this case, the colloidal surface can be dressed by microgel particles which have been cross-linked together as shown by [14,35]. The colloidal particles then form a polymeric chain held together by cross-links with tunable semiflexibility (persistence length).…”

Polarization-induced nanohelixes of organic cocrystals from asymmetric components with dopant-induced chirality inversion

“… 2 The strain computed from the approximated elastic energy of such deformations helps us to distinguish their predominant driving mechanism by considering the possible causes, such as screw dislocations, thermal/mechanic/compositional fields, and heteromeric stress. 3 Organic chiral molecules can be assembled into twisted arrangements in one direction by symmetry breaking, 4 and chiral additives can further enhance the chiral expression in the form of helixes and/or preferably handedness, even in achiral systems. 5 The surface charge ultimately accumulated from electrostatics and piezoelectricity is, in a few specific instances, another way to create a twisted arrangement in a material.…”

Chain Assembly Kinetics from Magnetic Colloidal Spheres

“…Analogous to molecular subunits within polymers, colloidal particles can also be used as the self-repeating monomeric precursors, linking into 1D colloidal chains. To achieve this, various strategies have been proposed for the fabrication of colloidal chains with distinct architectures, lengths, and compositions including chemical bonding, , electrostatic interactions, ,, hydrogen bonding, directional interaction patches, ,, coordination chemistry, heat, optical traps, UV light, and external fields. ,,− We have recently demonstrated the fabrication of chains that were chemically linked from beads assembled under a 1D magnetic field and have also shown that these colloidal chains can be actuated via different mechanisms upon magnetic field application. , …”