Scalable and Uniform Length-Tunable Biodegradable Block Copolymer Nanofibers with a Polycarbonate Core via Living Polymerization-Induced Crystallization-Driven Self-assembly.

Uniform 1D block copolymer (BCP) nanofibers prepared by the seeded-growth approach termed living crystallization-driven self-assembly (CDSA) offer promising potential for various applications due to their anisotropy, length tunability, and variable core and coronal chemistries. However, this procedure consists of a multi-step process involving independent BCP synthesis and self-assembly steps, where the latter is performed at low solution concentrations (<1 wt %), hindering scale-up. Here, we demonstrate the use of a one-pot BCP synthesis and self-assembly process, polymerization-induced CDSA (PI-CDSA), to access length-disperse nanofibers with a biodegradable crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core and a hydrophilic poly(ethylene glycol) (PEG) corona derived from PEG-b-PFTMC at concentrations up to 20 wt %, 400 times higher than those previously reported. Furthermore, living PI-CDSA could be used to access scalable, low dispersity, and length-tunable 1D PEG-b-PFTMC nanofibers at concentrations of up to 10 wt %. This provides the first example of living PI-CDSA involving an all-organic and biodegradable BCP that utilizes a conveniently implemented BCP synthesis protocol and does not involve living anionic polymerization. Significantly, samples of low-dispersity nanofibers of controlled lengths from 100 to 660 nm (Lw/Ln = 1.08-1.20) were prepared, allowing for upscaled access to well-defined biodegradable nanofibers at useful length-scales for applications in nanomedicine. Interestingly, detailed studies revealed a key role for PFTMC homopolymer impurities in the BCP prepared in situ in the formation of nanofibers under the reaction conditions used.

Seeded Self-Assembly of Charge-Terminated Poly(3-hexylthiophene) Amphiphiles Based on the Energy Landscape.

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.

Extending the Scope of "Living" Crystallization-Driven Self-Assembly: Well-Defined 1D Micelles and Block Comicelles from Crystallizable Polycarbonate Block Copolymers.

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]-dioxan]-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 µm. 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 toward WI-38 and HeLa cells. From these experiments, we observed no discernible cytotoxicity from a sample of 119 nm fiber-like micelles to either 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 in an organic semiconductor mediated by transient exciton delocalization.

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.