Solvent Tunable Self-Assembly of Amphiphilic Rod-Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments: Polymeric Nanostructures with Variable Shapes and Sizes

The nanoscale self-assembly of four amphiphilic rod-coil di- and triblock copolymers with chiral, rodlike poly(N-1-phenethyl-N'-methylcarbodiimide) (PPMC) segments and random coil, hydrophilic PEG blocks has been investigated using dynamic light scattering (DLS) and tapping-mode atomic force microscopy (AFM). This self-assembly proved to be highly tunable simply upon altering the concentration and chemical structure of the hydrophilic selective solvent and/or blending the copolymers with polycarbodiimide homopolymer. When spin-coated from dilute (c = 0.5 mg/mL) THF/H2O solutions, these interesting polymers adopted either simple spherical micelles or spherical polymersomes depending on the relative amount of H2O used for dissolution. Switching selective solvent from H2O to MeOH induced changes in aggregation behavior, as evidenced by DLS and AFM, with interesting nanoworm and nanomaggot micelle assemblies observed when spin-coated from dilute THF/MeOH solutions. Blending high-MW PPMC homopolymer with the block copolymers and spin-coating from dilute THF/25 vol % MeOH solutions resulted in the formation of long, interconnected nanofibers with several different observed tangling pathways including parallel packing, perpendicular wrapping, and helical twisting of nanofibers. Additionally, a large number of toroid nanostructures were also identified by AFM when spin-coated from these conditions. Finally, spin-coating copolymer/homopolymer blends from THF/25 vol % EtOH induced the nanoscale formation of long, bundled superhelical nanofibers with defined helical structures depending on the homopolymer copolymer chiral pairing (i.e., (R)-(R) pairing formed P superhelical nanofibers and M superhelix for (S)-(S) pairing). The highly tunable nature of these polymeric nanostructures offers new opportunities for the formation of nanoparticles with variable shapes and sizes simply upon altering the solvent combinations opening up new applications as biological mimics and drug delivery agents.