Uniform Continuous and Segmented Nanofibers Containing a π-Conjugated Oligo(<i>p</i>-phenylene ethynylene) Core via “Living” Crystallization-Driven Self-Assembly: Importance of Oligo(<i>p</i>-phenylene ethynylene) Chain Length

Abstract

π-Conjugated nanofibers of controlled length and composition show promising potential applications from biomedicine to optoelectronics. However, efficient preparation of uniform nanofibers from π-conjugated polymers with precise control over length and composition poses an outstanding challenge. Herein, we report the synthesis of a suite of block copolymers (BCPs) containing π-conjugated crystalline oligo(p-phenylene ethynylene) (OPE) segments of different chain lengths and a poly(N-isopropylacrylamide) (PNIPAM) or a poly(2-vinylpyridine) (P2VP) block (OPE5-b-PNIPAM47, OPE7-b-PNIPAM47, OPE9-b-PNIPAM47, and OPE9-b-P2VP56; subscripts indicate the number of repeat units). The length of OPE segment significantly affected the self-assembly OPE-based BCPs. OPE5-b-PNIPAM47 chains were molecularly dissolved in ethanol. Although OPE7-b-PNIPAM47 formed fiber-like micelles of uniform width initially, these micelles were not frozen at room temperature (23 °C), leading to the transformation from regular fiber-like micelles to irregular spherical aggregates upon aging for 7 days. Polydisperse fiber-like micelles of uniform width with kinetically frozen morphology at 23 °C were formed for OPE9-b-PNIPAM47 in ethanol by a direct heating–cooling cycle. The results were supported by the observations in dynamic light scattering, UV–vis, and fluorescence measurements, which indicated the resistance of OPE-based micelles toward micelle dissolution increased with the rising of OPE chain length. By the self-seeding approach of living crystallization-driven self-assembly (CDSA), uniform continuous micelles of controlled length (∼40 nm–1.2 μm) consisting of an OPE core and PNIPAM or P2VP shell can be obtained although micelles of OPE9-b-PNIPAM47 and OPE9-b-P2VP56 exhibited different resistance toward micelle dissolution. Significantly, a series of uniform segmented OPE-based fiber-like comicelles and their hybrid nanostructures with excellent length and composition tunability can be achieved by the seeded growth approach of living CDSA. Overall, we provided a facile access to the fabrication of OPE-based nanofibers with precise control over their length and composition along with instructive information about the influence of structure of π-conjugated block on the CDSA of BCPs containing a crystalline π-conjugated segment.