No abstract

All-conjugated block copolymers have significant potential for solution-processed optoelectronic applications, in particular those relying on a p/n junction. Herein, we report the synthesis and structure of all-conjugated diblock copolymers poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) and poly (3-hexylthiophene)-block-poly(9,9-dioctylfluorene-co-benzothiadiazole) in thin films and in the bulk. The diblock copolymers are prepared using a combination of Grignard metathesis polymerization and Suzuki polycondensation and characterized with NMR spectroscopy, size-exclusion chromatography, multiangle laser light scattering, and UV/vis spectroscopy. Structure in thin films and in the bulk is characterized using differential scanning calorimetry, X-ray diffraction, small-angle X-ray scattering, and atomic force microscopy. Diblock copolymer thin films self-assemble into a crystalline nanostructure with some long-range order after extended solvent annealing, and X-ray scattering measurements show that powder samples exhibit crystallinity throughout the bulk. By temperature dependent X-ray scattering measurements, we find that diblock copolymers self-assemble into crystalline nanowires with phase segregated block copolymer domains. These measurements show all-conjugated diblock copolymers may be useful for achieving solution-processed active layers in organic photovoltaics and light-emitting diodes with optimized structural and photophysical characteristics.

We investigate the effect of the ordering temperature (T) and film thickness (h(f)) on the surface morphology of flow-coated block copolymer (BCP) films of asymmetric poly(styrene-block-methyl methacrylate). Morphology transitions observed on the ordered film surface by atomic force microscopy (AFM) are associated with a perpendicular to a parallel cylinder BCP microphase orientation transition with respect to the substrate with increasing h(f). "Hybrid" surface patterns for intermediate h(f) between these limiting morphologies are correspondingly interpreted by a coexistence of these two BCP microphase orientations so that two "transitional" h(f) exist for each T. This explanation of our surface patterns is supported by both neutron reflectivity and rotational SANS measurements. The transitional h(f) values as a function of T define upper and lower surface morphology transition lines, h(fu) (T) and h(fl) (T), respectively, and a surface morphology diagram that should be useful in materials fabrication. Surprisingly, the BCP film surface morphology depends on the method of film formation (flow-coated versus spun-cast films) so that nonequilibrium effects are evidently operative. This morphological variability is attributed primarily to the trapping of residual solvent (toluene) within the film (quantified by neutron reflectivity) due to film vitrification while drying. This effect has significant implications for controlling film structure in nanomanufacturing applications based on BCP templates.

Nanostructures that have dimensions commensurate with the wavelength of the electromagnetic radiation exhibit near-field effects and, as optical antennas, can couple laser radiation to the local environment. Laser-induced silicon microcolumn arrays behave as nanophotonic ion sources that can be modulated by rotating the plane of light polarization. However, the limited range of surface morphologies available for these substrates makes it difficult to study the underlying mechanism that governs ion production. Here we demonstrate that nanopost arrays (NAPAs) can be tailored to exhibit resonant ion production. Ion yields from posts with subwavelength diameter show sharp resonances at high aspect ratios. The resonant enhancement in ion intensities can be modulated by adjusting the periodicity. In addition to strong molecular ion formation, the presence of high-energy fragmentation channels is observed. Ion yields from NAPAs exhibit dramatic differences for p- and s-polarized laser beams, indicating that energy coupling is similar to antenna arrays. These nanophotonic ion sources can control the degree of ion fragmentation and could eventually be integrated with micromachined mass spectrometers and microfluidic devices.

High-vacuum polymerization of R-(amino acid)-N-carboxyanhydrides (NCAs) affords polymers with controlled molecular weights and narrow polydispersities; however, a comprehensive study of the end-group composition of the resulting polypeptides has not yet been performed. This reveals crucial information, as the end-groups are indicative of both the polymerization mechanism (i.e., initiation event) and the termination pathways. To this end, poly(O-benzyl-L-tyrosine) initiated by 1,6-diaminohexane was synthesized and subsequently characterized by MALDI-TOF MS, NALDI-TOF MS, and 13 C NMR spectroscopy to ascertain the end-group structure. Polymers were prepared by both high-vacuum and glovebox techniques in DMF/THF. Preparation of poly(O-benzyl-L-tyrosine) by high-vacuum techniques yielded a polymer initiated exclusively by the normal amine mechanism, and termination by reaction with DMF was observed. In contrast, polymers prepared in the glovebox were initiated by the normal amine and activated monomer mechanisms, and several termination products are evident. To our knowledge, this is the first rigorous and comparative analysis of the end-group structure, and it demonstrates the advantage of highvacuum techniques for polymerization of NCAs for the preparation of well-defined polypeptides with endgroup fidelity.

Nanophase separation plays a critical role in the performance of donor-acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-bpoly(ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI ¼ 1.3) is added to a binary blend of P3HT and 6,6-phenyl C 61 -butyric acid methyl ester (PCBM) at different concentrations (0-20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. If coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

The synthesis of well-defined, end-functional poly(3-hexylthiophene)s (P3HTs) by in situ quenching of the Grignard metathesis (GRIM) polymerization is complicated by the extreme tendency to favor difunctional products in all but a few cases. A facile one-pot method for preparing 2-pyridyl and 3pyridyl P3HTs with high abundance of monofunctional products is established via an examination of the kinetics of the endfunctionalization quenching reaction with lithium chloride complexes of 2-and 3-pyridyl Grignard reagents. Density functional theory calculations guide the selection of pyridine as the end group, which provides the capacity to ligate cadmium selenide (CdSe) nanocrystals and arrests aggregation upon thermal annealing when dispersed in a P3HT matrix. The relative abundances of various end-functional products, as ascertained by high-resolution matrix assisted laser desorption ionization timeof-flight mass spectrometry, can be altered through the use of 1-pentene as an additive: GRIM polymerizations quenched with 3pyridyl and 2-pyridyl Grignard reagents show 5% and 18% abundances of difunctional, pyridyl-capped P3HTs, respectively, when 1-pentene is present at 1000:1 relative to the nickel catalyst. This represents a significant improvement compared to quenching with aryl Grignard reagents, where difunctional products predominate. The ability to manipulate end group compositions coupled with the propensity of pyridyl-functionalized P3HTs to ligate semiconductor quantum dots (SQDs) opens new possibilities for tuning the morphology of conjugated polymer/SQD blends.

Conjugated block copolymers have the potential to improve solution processed optoelectronic devices such as organic photovoltaics (OPVs), but significant synthetic challenges exist and systematic studies investigating structure− property relationships are lacking. We demonstrate a new route to conjugated block copolymers via copper-catalyzed click coupling and apply this method to synthesize a series of poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) (P3HT-b-PF) conjugated block copolymers with varying block weight fractions. The resulting block copolymers are comprised of two conjugated polymers joined by a flexible, nonconjugated linker. The series of conjugated block copolymers prepared enables an investigation into the role of polymer block lengths and composition on crystallization and self-assembly behavior. Grazing incidence wide-angle X-ray scattering measurements indicate the formation of highly oriented P3HT and/or PF crystallites in thermally annealed block copolymer films. Crystallization of either P3HT or PF blocks is predominant in all block copolymers studied, but at intermediate ratios crystallization of both blocks is observed.