Solution self-assembly of poly(3-hexylthiophene)–poly(lactide) brush copolymers: impact of side chain arrangement

Abstract

We exploit the crowded intramolecular environment of brush copolymers and π-π interactions of poly(3-hexylthiophene) (P3HT) side chains to produce tailorable nanostructures by self-assembly in solution. A series of brush copolymers consisting of regioregular P3HT and amorphous poly(d,l-lactide) (PLA) side chains grafted on a poly(norbornene) backbone are synthesized via ring-opening metathesis polymerization (ROMP) of norbornenyl-functionalized P3HT and PLA macromonomers. Three P3HT-PLA brush random copolymers and three brush block copolymers are prepared to create pairs of brush random and block copolymers containing comparable composition ratios of P3HT and PLA side chains. The relative volume fraction of P3HT and PLA side chains in the brush copolymers dictates thermal properties and crystallinity with little dependency on the side chain arrangement. However, the nanoscale morphologies of brush copolymers in a selective solvent are significantly altered by the side chain arrangement as well as copolymer composition. The different self-assembly behaviors in solution are attributed to the molecular design: in the brush block copolymers, self-assembly is driven by P3HT crystallization through both intra- and intermolecular π-π interactions, but intramolecular π-π interactions are largely suppressed in the brush random copolymers. Thus, tailoring brush copolymer architecture during synthesis enables additional levels of control over π-π interactions between P3HT side chains that are not present in conventional linear P3HT-based copolymers. The ability to use macromolecular chain topology as a way to access or tailor π-conjugated nanostructures may be beneficial in the context of controlling morphology at the nanoscale or producing patterned thin films for optoelectronic applications.