No abstract

Fiber-like block copolymer (BCP) micelles offer considerable potential for a variety of applications, however, uniform samples of controlled length and with spatially tailored chemistry have not been accessible. Recently, a seeded growth method, termed 'living' crystallization-driven self-assembly (CDSA), has been developed to allow the formation of 1D micelles and block comicelles of precisely controlled dimensions from BCPs with a crystallizable segment. An expansion of the range of core forming blocks that participate in living CDSA is necessary for this technique to be compatible with a broad range of applications. Few examples currently exist of well-defined, water-dispersible BCP micelles prepared using this approach, especially from biocompatible and biodegradable polymers. Herein, we demonstrate that BCPs containing a crystallizable polycarbonate, poly(spiro[fluorene-9,5'-[1,3]dioxin]-2'-one) (PFTMC), can readily undergo living CDSA processes. PFTMC-b-poly(ethylene glycol) (PEG) BCPs with PFTMC:PEG block ratios of 1:11 and 1:25 were shown to undergo living CDSA to form near monodisperse fiber like micelles of precisely controlled lengths of up to ~1.6 m. Detailed structural characterization of these micelles by TEM, AFM, SAXS and WAXS, revealed that they comprise a crystalline, chain folded PFTMC core with a rectangular cross-section that is surrounded by a solvent swollen PEG corona. PFTMC-b-PEG fiber-like micelles were shown to be dispersible in water to give colloidally stable solutions. This allowed an assessment of the toxicity of these structures towards WI-38 and HeLa cells. From these experiments, we observed no discernable cytotoxicity from a sample of 119 nm fiber-like micelles to either the healthy (WI-38) or cancerous (HeLa) cell types. The living CDSA process was extended to PFTMC-b-poly(2-vinylpyridine) (P2VP), and addition of this BCP to PFTMC-b-PEG seed micelles led to the formation of well-defined segmented fibers with spatially localized coronal chemistries.

Efficient energy transport is desirable in organic semiconductor (OSC) devices. However, photogenerated excitons in OSC films mostly occupy highly localized states, limiting exciton diffusion coefficients to below ~10−2 cm2/s and diffusion lengths below ~50 nm. We use ultrafast optical microscopy and nonadiabatic molecular dynamics simulations to study well-ordered poly(3-hexylthiophene) nanofiber films prepared using living crystallization-driven self-assembly, and reveal a highly efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. We show that this enables exciton diffusion constants up to 1.1 ± 0.1 cm2/s and diffusion lengths of 300 ± 50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of exciton dynamics and suggesting design rules to engineer efficient energy transport in OSC device architectures not based on restrictive bulk heterojunctions.

The creation of 1D π-conjugated nanofibers with precise control and optimized optoelectronic properties is of widespread interest for applications as nanowires. “Living” crystallization-driven self-assembly (CDSA) is a seeded growth method of growing importance for the preparation of uniform 1D fiber-like micelles from a range of crystallizable polymeric amphiphiles. However, in the case of polythiophenes, one of the most important classes of conjugated polymer, only limited success has been achieved to date using block copolymers as precursors. Herein, we describe studies of the living CDSA of phosphonium-terminated amphiphilic poly­(3-hexylthiophene)­s to prepare colloidally stable nanofibers. In depth studies of the relationship between the degree of polymerization and the self-assembly behavior permitted the unveiling of the energy landscape of the living CDSA process. On the basis of the kinetic and thermodynamic insight provided, we have been able to achieve an unprecedented level of control over the length of low dispersity fiber-like micelles from 40 nm to 2.8 μm.