Self-assembly Of Rod Research Articles

Correlations between Morphologies and Photovoltaic Properties of Rod−Coil Block Copolymers

We present results obtained using a drift-diffusion model for the structure−property correlations in photovoltaic devices based on self-assembly of rod−coil block copolymers. We use a self-consistent field theory model to generate the self-assembly morphologies of rod−coil block copolymers in confined situations. The density and orientational order parameter profiles so-obtained are then used as input to a recently proposed drift-diffusion model which predicts the photovoltaic device characteristics. The latter model allows for prescription of arbitrary morphologies of donor and acceptor phases while simultaneously incorporating the role of anisotropic charge transport of holes and excitons that arise in the ordered phases of rod−coil block copolymers. We present results elucidating the role of morphology of self-assembly, orientation of lamellar phases, domain widths, and the degree of phase separation and orientational ordering, upon the photovoltaic device characteristics.

Self-Assembly of Rod−Coil−Rod Triblock Copolymer and Homopolymer Blends

The self-assembly structures and their transformation of blends of rod−coil−rod triblock copolymers and homopolymers were first detected. Adding coil homopolymers into block copolymers with a lamellar structure resulted in an increase of d-spacing. When coil homopolymers were added into triblock copolymer with a rod continuous hexagonally packed-cylinder structure, lamellar structures were obtained, and the lamellar d-spacing were smaller than that of the original structure. The addition of rod-like homopolymer into the block copolymers with a lamellar structure also resulted in the increase of the d-spacing. As a result of the interdigitation, the rod blocks must rearrange to a larger angle with respect to the interface.

Self-Assembly of Rod−Coil Block Copolymers and Their Application in Electroluminescent Devices

The formation of alternating electron transporting and hole transporting 15 nm lamellae within the active layer of an organic light-emitting diode (OLED) is demonstrated to improve device performance. A new multifunctional bipolar rod−coil block copolymer containing a poly(alkoxy phenylenevinylene) (PPV) rod-shaped block as the hole transporting and emitting material and a poly(vinyloxadiazole) coil-shaped electron transporting block is synthesized. This new block copolymer is the active material of a self-assembling multicomponent electroluminescent device that can be deposited in a single step. In the thin film, grazing incidence X-ray scattering and transmission electron microscopy demonstrate that the layers form grains which are oriented bimodally: parallel and perpendicular from the anode. In this mixed orientation, the device demonstrates better performance than those with either pure PPV or a blend of the two analogous homopolymers as the active materials, i.e., higher external quantum efficiency (EQE) and brightness. This improved device performance is mainly attributed to the bipolar functionality and microphase separation of the block copolymer, which provide highly efficient hole and electron recombination at the nanodomain interfaces.

Self-Assembly of Poly(diethylhexyloxy-p-phenylenevinylene)-b- poly(4-vinylpyridine) Rod−Coil Block Copolymer Systems

We describe the synthesis, the morphology, and self-assembly behavior of semiconducting poly(4-vinylpyridine-b-diethylhexyloxy-p-phenylenevinylene) (P4VP-b-PPV) rod−coil block copolymer systems. Three different block copolymers with 55%, 80%, and 88% coil volume fraction were synthesized by convergent anionic polymerization in THF using lithium α-methylstyrene as initiator. The morphology of the block copolymers was studied by transmission electron microscopy, small-angle X-ray scattering, and small-angle neutron scattering as a function of the volume fraction of the rod block as well as different annealing conditions. The microphase-separated morphologies in these block copolymers vary from lamellar, to hexagonal, and spherical, when the volume fraction of the rod is progressively reduced. By combining the lattice parameter measured by scattering techniques with the volume fraction of rod domains obtained by nuclear magnetic resonance, it was shown that the block copolymers in the lamellar structure are organized in a smectic C double layer, while in the hexagonal phase they self-organize in a homeotropic arrangement, with the rod blocks forming the dispersed phase. Furthermore, while self-assembly of rod−coil block copolymers in columnar hexagonal phase prevents close rod packing, for the lamellar phase evidence of this configuration among rods is shown by wide-angle X-ray scattering. As a consequence, the morphology and long-range order in the lamellar phase are the result of simultaneous inter-rods liquid crystalline interactions and the tendency to microphase segregation of rod and coil. As a result, depending on temperature, the lamellar phase can exist both with rods oriented in a smectic configuration and with randomly packed rods. We show that annealing the lamellar phase below its order−disorder transition temperature, TODT, but above the maximum affordable temperature for inter-rods liquid crystalline interactions, called smectic-in-lamellar to lamellar order−disorder transition temperature, TSL (with TSL< TODT), leads to highly improved long-range lamellar order, which is then preserved when the system is cooled below TSL, at which temperature rod close packing is fully recovered.

Supramolecular reactor from self-assembly of rod-coil molecule in aqueous environment.

We synthesized an amphiphilic coil-rod-coil triblock molecule consisting of hexa-p-phenylene as a rod block and poly(ethylene oxide) with the number of repeating units of 17 as coil blocks and investigated aggregation behavior in aqueous environment. The rod-coil molecule was observed to aggregate into discrete micelles consisting of hydrophobic disklike rod bundles encapsulated by hydrophilic poly(ethylene oxide) coils. The aromatic bundles of the micelles were demonstrated to be used as an efficient supramolecular reactor for the room temperature Suzuki cross-coupling reaction of a wide range of aryl halides, including even aryl chlorides with phenylboronic acids in aqueous environment. These results demonstrate that self-assembly of amphiphilic rod-coil molecules can provide a useful strategy to construct an efficient supramolecular reactor for aromatic coupling reaction.

Self-assembly of rod-coil molecules into molecular length-dependent organization.

A series of rod-coil molecules (n-x, where n represents the number of repeating units in a PPO coil and x the number of phenyl groups in a rod segment) with variation in the molecular length, but an identical rod to coil volume ratio was synthesized, and their self-assembling behavior was investigated by using DSC and X-ray scatterings. The molecule with a short rod-coil molecule (16-4) shows a 3-D tetragonal structure based on a body-centered symmetry of the discrete bundles in addition to a lamellar structure. This 3-D lattice, on heating, collapses to generate a disordered micellar structure. Remarkably, the molecules based on longer molecular length (21-5 and 24-6) were observed to self-organize into, on heating, lamellar, tetagonally perforated lamellar, 2-D hexagonal columnar and finally disordered micellar structures. Further increase in the molecular length as in the case of 29-7 and 32-8 induces a 3-D hexagonally perforated lamellar structure as an intermediate structure between the lamellar and tetragonally perforated lamellar structures. Consequently, these systems demonstrate the ability to regulate the domain nanostructure, from 2-dimensionally continuous layers, long strips to discrete bundles via periodic perforated layers by small changes in the molecular length, at an identical rod-to-coil volume fraction.

Self-Assembly of Rod−Coil Diblock Oligomers Based on α-Helical Peptides

A detailed study of the supramolecular organization of two series of peptide-based diblock oligomers is reported. The diblock oligomers are comprised of an α-helical oligopeptide rod, which is either based on γ-benzyl-l-glutamate or ε-benzyloxycarbonyl-l-lysine, and a short oligo(styrene) coil. The unique feature of these rod−coil molecules is that the conformation of the α-helical peptide rod is sensitive to temperature. As a result, the supramolecular organization of these rod−coil oligomers can be manipulated not only via changes in the block length ratio or chemical composition but also via temperature-induced conformational changes in the rod segments. This makes these molecules attractive building blocks for the development of stimuli-sensitive self-assembled materials.

Self-Assembly of Rod−Coil Diblock Copolymers

A two-dimensional self-consistent-field lattice model incorporating the rotational isomeric state scheme to account for chain rigidity was developed to study the phase diagram of rod−coil diblock copolymer melts. Several morphologies such as lamella, zigzag lamella, elliptical cross-sectional cylinders, and hexagonally packed cylinders were observed for rod−coil diblock copolymers. A free energy analysis suggests that the zigzag and elliptical cross-sectional cylindrical structures are metastable. The effect of decreasing the flexibility of the coil segment of a rod−coil diblock copolymer was determined. Our calculation showed that as the flexible block of the copolymer becomes more rigid, part of the cylindrical domain in the phase diagram is replaced by lamella structures.