Magnetic Field Alignment of a Diblock Copolymer Using a Supramolecular Route

Large-area uniform magnetic alignment of a self-assembled diblock copolymer has been achieved by the selective sequestration of rigid moieties with anisotropic diamagnetic susceptibility within one block of the system. The species is based on a biphenyl core and is confined in the acrylic acid domains of a poly­(styrene-<i>b</i>-acrylic acid) block copolymer by hydrogen bonding between an imidazole headgroup and the acrylic acid units. Microphase separation produces hierarchically ordered systems of smectic layers within lamellae and smectic layers in the matrix surrounding hexagonally packed poly­(styrene) cylinders, as a function of imidazole/acrylic acid stoichiometry. The magnetic field aligns the smectic layers as well as the block copolymer superstructure in a manner dependent on the anchoring condition of the biphenyl species at the block copolymer interface. Surprisingly, this is found to depend on the composition of the system. This approach is synergistic with recent efforts to engineer functional supramolecular block copolymer assemblies based on rigid chromophores. It offers a facile route to large area control of microstructure as required for full exploitation of functional properties in these systems

Hierarchically Self-Assembled Photonic Materials from Liquid Crystalline Random Brush Copolymers

Here we report a general methodology to attain novel hierarchical nanostructures using new polymer scaffolds that self-assemble to form cholesteric 1D photonic mesophases existing in conjunction with microphase segregated domains. To achieve this, a series of liquid-crystalline random brush copolymers (LCRBC) consisting of cholesteryl liquid crystalline (LC) mesogen and brushlike PEG as side chain functionality are synthesized. At room temperature, all LCRBCs exhibits microphase segregation of PEG side chains on length scale of 10–15 nm, whereas LC domain forms smectic mesophase (3–7 nm LC layers). Interestingly, upon heating a cholesteric mesophase is exclusively observed for copolymer containing 78 and 85 wt % of LC content (LCRBC78 and LCRBC85, respectively) existing along with microphase segregated PEG domains. Moreover, the phase behavior of these copolymers studied by temperature-controlled small-angle X-ray scattering (SAXS) suggests the order–disorder transition for the microphase segregated structure coincides with the cholesteric–isotropic transition. Remarkably, LCRBC78 and LCRBC85 quenched from cholesteric mesophase exhibits nanoscale hierarchical order consisting of (1) smectic LC ordering with 3–7 nm periodicity, (2) microphase segregation of PEG side chain on 10–12 nm length scale, and (3) periodicities from helical mesophase (cholesteric phase) on optical length scales of 150–200 nm. Thus, by exploiting LCRBC molecular architecture and composition, hierarchical nanostructure can be obtained and preserved which allows for the creation of unique 1D-photonic materials

Nanoimprinting Sub-100 nm Features in a Photovoltaic Nanocomposite using Durable Bulk Metallic Glass Molds

The use of bulk metallic glass (BMG) for the nanoimprint of high-aspect-ratio (>3) features into functional polymers is investigated. To accomplish this, the most critical aspect is the successful demolding of the imprinted polymer. By fluorosilane functionalization of the BMG surface and optimization of processing temperature, high aspect pore features down to 45 nm diameters are introduced into the surface of two organic photovoltaic systems: poly­(3-hexylthiophene-2,5-diyl) (P3HT) and 1:1 mixtures of P3HT with Phenyl-C61-butyric acid methyl ester (PCBM). The crystallinity of P3HT demands higher forming temperatures and pressures that are difficult to obtain with conventional soft nanoimprint lithography molds. The ability to accommodate a wide range of processing conditions and the low cost of fabricating molds with nanometer-scale features point to the large potential of nanotextured BMGs as an economical and scalable imprint material for high-resolution applications

Monoliths of Semiconducting Block Copolymers by Magnetic Alignment

Achieving highly ordered and aligned assemblies of organic semiconductors is a persistent challenge for improving the performance of organic electronics. This is an acute problem in macromolecular systems where slow kinetics and long-range disorder prevail, thus making the fabrication of high-performance large-area semiconducting polymer films a nontrivial venture. Here, we demonstrate that the anisotropic nature of semiconducting chromophores can be effectively leveraged to yield hierarchically ordered materials that can be readily macroscopically aligned. An n-type mesogen was synthesized based on a perylene diimide (PDI) rigid core coupled to an imidazole headgroup <i>via</i> an alkyl spacer. Supramolecular assembly between the imidazole and acrylic acid units on a poly(styrene-<i>b</i>-acrylic acid) block copolymer yielded self-assembled hexagonally ordered polystyrene cylinders within a smectic A mesophase of the PDI mesogen and poly(acrylic acid). We show that magnetic fields can be used to control the alignment of the PDI species and the block copolymer superstructure concurrently in a facile manner during cooling from a high-temperature disordered state. The resulting materials are monoliths, with a single well-defined orientation of the semiconducting chromophore and block copolymer microdomains throughout the sample. This synergistic introduction of both functional properties and the means of controlling alignment by supramolecular attachment of mesogenic species to polymer backbones offer new possibilities for the modular design of functional nanostructured materials

Optically Active Elastomers from Liquid Crystalline Comb Copolymers with Dual Physical and Chemical Cross-Links

We report on the synthesis and properties of cholesteric liquid crystalline random terpolymers with comblike architecture as a modular platform for preparation of stimuli-responsive photonic elastomers. Ring-opening metathesis of norbornene monomers bearing <i>n</i>-alkyloxy cholesteryl (Ch₉), <i>n</i>-alkoxy cyanobiphenyl (CB₆ or CB₁₂), and poly­(ethylene glycol) (PEG) side chains is efficient and quantitatively yields low polydispersity random terpolymers. This terpolymer scaffold self-assembles to form cholesteric mesophases (N*) in which microphase-segregated domains of PEG side chains are randomly embedded. The cholesteric mesophase provides a 1D photonic band gap structure at optical wavelengths, which is maintained during chemical cross-linking of the norbornene backbone to form elastomers. The presence of cyanobiphenyl mesogens leads to an increase in the helical pitch of the cholesteric mesophase, resulting in a red-shift of the reflectivity relative to the pure cholesteric mesophase. By contrast, the presence of PEG blue-shifts the reflectivity, such that the overall optical properties can be readily tailored by the composition of the terpolymer. Furthermore, the mechanical properties of the materials are enhanced by the presence of the microphase-separated PEG domains which act as physical cross-links and also provide plasticization of the system. The terpolymers described here provide a modular and versatile platform for the realization of photopatternable materials that exhibit shape memory and thermochromic properties

Poly(ethylenimine)-Based Polymer Blends as Single-Ion Lithium Conductors

Highly conductive solid polymer electrolytes were generated by blending linear poly­(ethyleneimine)-<i>graft</i>-poly­(ethylene glycol) with linear poly­(ethyleneimine) bearing lithium <i>N</i>-propylsulfonate groups as the lithium source. The effect of polymer backbone structure on Li⁺ conductivity was determined by comparing a series of blends made from the PEI-based materials with those from polymethacrylate backbone analogues. The use of PEI backbones promoted ion-pair dissociation, stabilized the macromolecular mix and generated blends with ionic conductivities up to 2 orders of magnitude higher than those of the polymethacrylate-based systems. Blends containing the PEI-bound lithium sulfonates exhibited lithium conductivities higher than those measured for PEG doped with lithium bis­(trifluoromethyl)­sulfonimide. Shifts in the ν_<i>s</i>(SO₃) IR absorption band suggest that the solvation environment for the lithium sulfonates changes with polymer structure. The PEI-based blends are thermally stable up to 200 °C, electrochemically stable in the ±5 V range, and showed unparalleled ionic conductivities (0.4 mS/cm at room temperature and 5 mS/cm at 80 °C) for solvent-free systems with polymer-bound anions

Flat Drops, Elastic Sheets, and Microcapsules by Interfacial Assembly of a Bacterial Biofilm Protein, BslA

Protein adsorption and assembly at interfaces provide a potentially versatile route to create useful constructs for fluid compartmentalization. In this context, we consider the interfacial assembly of a bacterial biofilm protein, BslA, at air–water and oil–water interfaces. Densely packed, high modulus monolayers form at air–water interfaces, leading to the formation of flattened sessile water drops. BslA forms elastic sheets at oil–water interfaces, leading to the production of stable monodisperse oil-in-water microcapsules. By contrast, water-in-oil microcapsules are unstable but display arrested rather than full coalescence on contact. The disparity in stability likely originates from a low areal density of BslA hydrophobic caps on the exterior surface of water-in-oil microcapsules, relative to the inverse case. In direct analogy with small molecule surfactants, the lack of stability of individual water-in-oil microcapsules is consistent with the large value of the hydrophilic–lipophilic balance (HLB number) calculated based on the BslA crystal structure. The occurrence of arrested coalescence indicates that the surface activity of BslA is similar to that of colloidal particles that produce Pickering emulsions, with the stability of partially coalesced structures ensured by interfacial jamming. Micropipette aspiration and flow in tapered capillaries experiments reveal intriguing reversible and nonreversible modes of mechanical deformation, respectively. The mechanical robustness of the microcapsules and the ability to engineer their shape and to design highly specific binding responses through protein engineering suggest that these microcapsules may be useful for biomedical applications

Flat Drops, Elastic Sheets, and Microcapsules by Interfacial Assembly of a Bacterial Biofilm Protein, BslA

Protein adsorption and assembly at interfaces provide a potentially versatile route to create useful constructs for fluid compartmentalization. In this context, we consider the interfacial assembly of a bacterial biofilm protein, BslA, at air–water and oil–water interfaces. Densely packed, high modulus monolayers form at air–water interfaces, leading to the formation of flattened sessile water drops. BslA forms elastic sheets at oil–water interfaces, leading to the production of stable monodisperse oil-in-water microcapsules. By contrast, water-in-oil microcapsules are unstable but display arrested rather than full coalescence on contact. The disparity in stability likely originates from a low areal density of BslA hydrophobic caps on the exterior surface of water-in-oil microcapsules, relative to the inverse case. In direct analogy with small molecule surfactants, the lack of stability of individual water-in-oil microcapsules is consistent with the large value of the hydrophilic–lipophilic balance (HLB number) calculated based on the BslA crystal structure. The occurrence of arrested coalescence indicates that the surface activity of BslA is similar to that of colloidal particles that produce Pickering emulsions, with the stability of partially coalesced structures ensured by interfacial jamming. Micropipette aspiration and flow in tapered capillaries experiments reveal intriguing reversible and nonreversible modes of mechanical deformation, respectively. The mechanical robustness of the microcapsules and the ability to engineer their shape and to design highly specific binding responses through protein engineering suggest that these microcapsules may be useful for biomedical applications

Flat Drops, Elastic Sheets, and Microcapsules by Interfacial Assembly of a Bacterial Biofilm Protein, BslA

Protein adsorption and assembly at interfaces provide a potentially versatile route to create useful constructs for fluid compartmentalization. In this context, we consider the interfacial assembly of a bacterial biofilm protein, BslA, at air–water and oil–water interfaces. Densely packed, high modulus monolayers form at air–water interfaces, leading to the formation of flattened sessile water drops. BslA forms elastic sheets at oil–water interfaces, leading to the production of stable monodisperse oil-in-water microcapsules. By contrast, water-in-oil microcapsules are unstable but display arrested rather than full coalescence on contact. The disparity in stability likely originates from a low areal density of BslA hydrophobic caps on the exterior surface of water-in-oil microcapsules, relative to the inverse case. In direct analogy with small molecule surfactants, the lack of stability of individual water-in-oil microcapsules is consistent with the large value of the hydrophilic–lipophilic balance (HLB number) calculated based on the BslA crystal structure. The occurrence of arrested coalescence indicates that the surface activity of BslA is similar to that of colloidal particles that produce Pickering emulsions, with the stability of partially coalesced structures ensured by interfacial jamming. Micropipette aspiration and flow in tapered capillaries experiments reveal intriguing reversible and nonreversible modes of mechanical deformation, respectively. The mechanical robustness of the microcapsules and the ability to engineer their shape and to design highly specific binding responses through protein engineering suggest that these microcapsules may be useful for biomedical applications

Molecular Design of Liquid Crystalline Brush-Like Block Copolymers for Magnetic Field Directed Self-Assembly: A Platform for Functional Materials

We report on the development of a liquid crystalline block copolymer with brush-type architecture as a platform for creating functional materials by magnetic-field-directed self-assembly. Ring-opening metathesis of <i>n</i>-alkyloxy cyanobiphenyl and polylactide (PLA) functionalized norbornene monomers provides efficient polymerization yielding low polydispersity block copolymers. The mesogenic species, spacer length, monomer functionality, brush-chain length, and overall molecular weight were chosen and optimized to produce hexagonally packed cylindrical PLA domains which self-assemble and align parallel to an applied magnetic field. The PLA domains can be selectively removed by hydrolytic degradation resulting in the production of nanoporous films. The polymers described here provide a versatile platform for scalable fabrication of aligned nanoporous materials and other functional materials based on such templates