Uniform Continuous and Segmented Nanofibers Containing a π-Conjugated Oligo(p-phenylene ethynylene) Core via “Living” Crystallization-Driven Self-Assembly: Importance of Oligo(p-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 πconjugated crystalline oligo(p-phenylene ethynylene) (OPE) segments of different chain lengths and a poly(N-isopropylacrylamide) (PNIPAM) or a poly(2-vinylpyridine) (P2VP) block (OPE 5 -b-PNIPAM 47 , OPE 7 -b-PNIPAM 47 , OPE 9 -b-PNIPAM 47 , and OPE 9 -b-P2VP 56 ; subscripts indicate the number of repeat units). The length of OPE segment significantly affected the self-assembly OPE-based BCPs. OPE 5 -b-PNIPAM 47 chains were molecularly dissolved in ethanol. Although OPE 7 -b-PNIPAM 47 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 OPE 9 -b-PNIPAM 47 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 OPE 9 -b-PNIPAM 47 and OPE 9 -b-P2VP 56 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.

Uniform Nanowires Containing a Heterogeneousπ‐Conjugated Core of Controlled Length, Composition and Morphology

“…One can note that the length of comicelles increased from 49 nm to 170 nm as the temperature increased from 60 °C to 70 °C (Figures 2B–C, F, S3C−E and Table S1), and then sharply increased from 347 nm to 1216 nm with the increase of temperature from 75 °C to 82 °C (Figures 2D–F, S3F–H and Table S1). The formation of comicelles by co‐self‐seeding tracked the behavior of OPE 9 ‐ b ‐P2VP 56 with a higher resistance toward the heating, [19] which is consistent with the co‐self‐seeding of OPV 5 ‐ and OPV 6 ‐based BCPs [25] …”

“…Living crystallization‐driven self‐assembly (CDSA), including seeded growth and self‐seeding approaches, has emerged as a powerful strategy to generate uniform nanowires of controlled composition and dimension from block copolymers (BCPs) containing a crystalline π‐conjugated segment. However, this strategy is limited to the preparation of nanowires containing a homogeneous π‐conjugated core [14–19] . Recently, Manners and co‐workers [20] reported the preparation of coaxial heterojunction nanowires via a two‐step crystallization strategy.…”

“…To test this idea, we firstly used OPE 9 ‐ b ‐P2VP 56 and OPV 5 ‐ b ‐PNIPAM 47 (OPE=oligo( p ‐phenylene ethynylene), OPV=oligo( p ‐phenylene vinylene), P2VP=poly(2‐vinylpyridine) and PNIPAM=poly( N ‐isopropylacrylamide), subscripts represent exact and mean number of repeat units for OPV 5 /OPE 9 and P2VP 56 /PNIPAM 47 , respectively. Both samples are synthesized according to our previous reports [17,19] …”

“…The percentage of surviving OPV 5 ‐ b ‐PNIPAM 47 seeds was below 4 % upon annealing at 60 °C, [18] which indicated that almost all of micellar fragments of OPV 5 ‐ b ‐PNIPAM 49 would dissolve above 60 °C, whereas fragments of OPE 9 ‐ b ‐P2VP 56 had a much stronger resistance toward heating‐induced dissolution. Almost no fragments of OPE 9 ‐ b ‐P2VP 56 would dissolve at 60 °C and the percentage of dissolved seeds increased to 85 % as the temperature increased to 82 °C (Table S2) [19] . On the basis of width and contrast (P2VP domains have a higher affinity to phosphotungstic acid) difference between OPV 5 ‐ b ‐PNIPAM 47 (13.7 nm) and OPE 9 ‐ b ‐P2VP 56 (23.3 nm), the regions of micelles with width of about 13.7 and 23.3 nm were supposed to be rich with OPV 5 ‐ b ‐PNIPAM 47 and OPE 9 ‐ b ‐P2VP 56 , respectively.…”

Emerging applications for living crystallization-driven self-assembly

“…Studies reported since 2011 have involved PLLA, [165][166][167] PE, 168 PFDES, 38 and also hyperbranched poly(ether amine) capped with polyhedral oligomeric silsesquioxane, 143 PCL, 169,170 polycarbonate, 62,171 and π-conjugated polymers such polythiophene, [172][173][174][175] poly(3heptylselenophene), 176 PDHF, 177 oligo(p-phenylenevinylene) (OPV), 178,179 poly(cyclopentenylenevinylene) 180 and oligo(p-phenylene-ethynylene). 181 The use of the living CDSA seeded-growth approach to fabricate well-defined biopolymerbased assemblies has also been reported. Attachment of cyanine dyes to the termini of the DNA segment in DNA-polymer hybrids has been shown to direct self-assembly from spheres to 1D…”