Self-seeding of block copolymers with a π-conjugated oligo(p-phenylene vinylene) segment: a versatile route toward monodisperse fiber-like nanostructures

Three well-defined crystalline-coil diblock copolymers of oligo(p-phenylenevinylene)-b-poly(N-isopropylacrylamide) (OPV₅-b-PNIPAM₁₈, OPV₅-b-PNIPAM₄₉, and OPV₅-b-PNIPAM₇₅; the subscripts represent the number of repeat units of each block) with the same crystallizable core-forming OPV segment but different corona-forming PNIPAM blocks of various chain lengths were synthesized. Their solution self-assembly behavior was examined in methanol, ethanol, and isopropanol. Both the solvent and the length of the PNIPAM block were found to affect the self-assembly of the block copolymers. In methanol, OPV₅-b-PNIPAM₁₈ formed a mixture of fiber-like micelles of uniform width and two-dimensional platelet-like structures with fiber-like micelles protruding from the ends. In ethanol and in isopropanol, this polymer only formed long fiber-like micelles of uniform width. OPV₅-b-PNIPAM₄₉ formed long fiber-like micelles (several micrometers) of uniform width in all three solvents, but under the same self-assembly conditions, OPV₅-b-PNIPAM₇₅ only formed short fiber-like micelles with lengths of several hundred nanometers. We systematically examined the temperature-induced self-seeding behavior of all three block copolymers, exploring the influence of PNIPAM chain length, solvent, annealing time, and concentration of copolymer. The most remarkable result of these experiments is our finding that fiber-like micelles of uniform length with controlled lengths up to 1 μm can be easily prepared from all three block copolymers, even from OPV₅-b-PNIPAM₇₅ that formed only much shorter micelles under self-nucleated self-assembly. We also showed that the self-seeding strategy can be extended to other OPV-containing diblock copolymer such as OPV₅-b-poly(2-(diethylamino)ethyl methacrylate) for preparing monodisperse fiber-like micelles of controllable length. These results show that the self-seeding approach to crystallization-driven self-assembly can be a versatile route to prepare uniform fiber-like micelles with controllable lengths for OPV-containing block copolymers.