Mesoscopically helical order in chiral block copolymers

Mesoscopically helical order in chiral block copolymersSupermolecular, helical assemblies are a common structural motif exploited in far-ranging biological contexts, from the flagellar appendages of swimming microorganisms to the protein coats that sheath rod-like viruses.  The screw-like structure of these biological assemblies is a consequence of the chiral subunits (proteins and amino acids) from which they are built, and this chirality imbues the structures with a right- or left-handed sense or twist.  Grason and Russell, working collaboratively at the Materials Research Science and Engineering Center at UMass Amherst, demonstrate that a similar strategy may be employed in synthetic polymers, which are seen to self-assemble into helical structures.  In general, block copolymers are assemble into array of mirror-symmetric, nano-structured arrays of ordered spherical, cylindrical and layered domains.  This work uncovered a novel theoretical tool to both model and predict the self-assembled structures that emanate from constructing one of the constituent polymeric blocks from chiral segments.  The theory shows that interactions between chiral segments “twist” the trajectories of the self-organized chain molecules.  This twist is transmitted through the tension carried along the chains to the entire assembly, driving otherwise cylindrical assemblies to “buckle” into nanohelices organized in a hexagonal array.  These results confirm the notion that mesoscopic, helical structures observed on tens of nanometer length scales in melts of the chiral block copolymer, poly-L-lactide-b-polystyrene, stems from the chiral nature of inter-molecular forces taking place at the nanometer lengthscale.  Going forward, this opens a door to a new class of complex meso-chiral structures formed in chiral block copolymer materials yet to be discovered.