Pawel W. Majewski

Anisotropic ionic conductivity in block copolymer membranes by magnetic field alignment.

The self-assembly of diblock copolymers provides a convenient route to the formation of mechanically robust films with precise and tunable periodic arrangements of two physically demixed but chemically linked polymeric materials. Chemoselective transport membranes may be realized from such films by selective partitioning of an active species into one of the polymer domains. Here, lithium ions were selectively sequestered within the poly(ethylene oxide) block of a liquid crystalline diblock copolymer to form polymer electrolyte membranes. Optimization of the membrane conductivity mandates alignment of self-assembled structures such that conduction occurs via direct as opposed to tortuous transport between exterior surfaces. We show here that magnetic fields can be used in a very simple and scalable manner to produce highly aligned hexagonally packed cylindrical microdomains in such membranes over macroscopic areas. We systematically explore the dependence of the ionic conductivity of the membrane on both temperature and magnetic field strength. A surprising order of magnitude increase in conductivity relative to the nonaligned case is found in films aligned at the highest magnetic field strengths, 6 T. The conductivity of field aligned samples shows a nonmonotonic dependence on temperature, with a marked decrease on heating in the proximity of the order-disorder transition of the system before increasing again at elevated temperatures. The data suggest that domain-confined transport in hexagonally packed cylindrical systems differs markedly in anisotropy by comparison with lamellar systems.

Millisecond Ordering of Block Copolymer Films via Photothermal Gradients

For the promise of self-assembly to be realized, processing techniques must be developed that simultaneously enable control of the nanoscale morphology, rapid assembly, and, ideally, the ability to pattern the nanostructure. Here, we demonstrate how photothermal gradients can be used to control the ordering of block copolymer thin films. Highly localized laser heating leads to intense thermal gradients, which induce a thermophoretic force on morphological defects. This increases the ordering kinetics by at least 3 orders of magnitude compared to conventional oven annealing. By simultaneously exploiting the thermal gradients to induce shear fields, we demonstrate uniaxial alignment of a block copolymer film in less than a second. Finally, we provide examples of how control of the incident light field can be used to generate prescribed configurations of block copolymer nanoscale patterns.

Order-disorder transition and alignment dynamics of a block copolymer under high magnetic fields by in situ x-ray scattering.

We examine the influence of magnetic fields on the order-disorder transition (ODT) in a liquid crystalline block copolymer. This is motivated by a desire to understand the dynamics of microstructure alignment during field annealing as potentially dictated by selective destabilization of nonaligned material. Temperature resolved scattering across the ODT and time-resolved measurements during isothermal field annealing at sub-ODT temperatures were performed in situ. Strongly textured mesophases resulted in each case but no measurable field-induced shift in T(ODT) was observed. This suggests that selective melting does not play a discernable role in the systems field response. Our data indicate instead that alignment occurs by slow grain rotation within the mesophase. We identify an optimum subcooling that maximizes alignment during isothermal field annealing. This is corroborated by a simple model incorporating the competing effects of an exponentially decreasing mobility and divergent, increasing magnetic anisotropy on cooling below T(ODT). The absence of measurable field effects on T(ODT) is consistent with estimates based on the relative magnitudes of the field interaction energy and the enthalpy associated with the ODT.

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 via an alkyl spacer. Supramolecular assembly between the imidazole and acrylic acid units on a poly(styrene-b-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.

Thermally Switchable Aligned Nanopores by Magnetic‐Field Directed Self‐Assembly of Block Copolymers

A scalable approach for developing large area polymer films, with stimuli responsive vertically aligned nanopores is reported. Magnetic fields are used to create highly aligned hexagonally packed block copolymer cylindrical microdomains with order parameters exceeding 0.95. Selective etch removal of material yields nanoporous films which demonstrate reversible pore closure on heating.

Arbitrary lattice symmetries via block copolymer nanomeshes

Self-assembly of block copolymers is a powerful motif for spontaneously forming well-defined nanostructures over macroscopic areas. Yet, the inherent energy minimization criteria of self-assembly give rise to a limited library of structures; diblock copolymers naturally form spheres on a cubic lattice, hexagonally packed cylinders and alternating lamellae. Here, we demonstrate multicomponent nanomeshes with any desired lattice symmetry. We exploit photothermal annealing to rapidly order and align block copolymer phases over macroscopic areas, combined with conversion of the self-assembled organic phase into inorganic replicas. Repeated photothermal processing independently aligns successive layers, providing full control of the size, symmetry and composition of the nanoscale unit cell. We construct a variety of symmetries, most of which are not natively formed by block copolymers, including squares, rhombuses, rectangles and triangles. In fact, we demonstrate all possible two-dimensional Bravais lattices. Finally, we elucidate the influence of nanostructure on the electrical and optical properties of nanomeshes.

Directed self-assembly of hybrid oxide/polymer core/shell nanowires with transport optimized morphology for photovoltaics

Hybrid organic-inorganic solar cells are promising for the development of next generation low-cost, high efficiency photovoltaics (PVs). They combine the facile solution processability, large optical extinction coefficients, and good hole mobility of conjugated polymers with the high electron affinity and electron mobility of inorganic nanoparticles.[1] The most common configuration for effectively generating photocurrent with these materials is the bulk heterojunction (BHJ) device where intimate mixing of the polymer with inorganic nanoparticles generates a random bicontinuous morphology with nanometerscale dimensions. The morphology and ultimately the performance of this photoactive layer depend on a complex interplay of materials parameters and processing conditions such as temperature, polymer-solvent and particle-solvent interactions, solvent evaporation rate, solution composition (polymer volume fraction), and post-deposition treatments.[2] In particular, the role of polymer–particle miscibility has been well highlighted by recent reports.[3] Although there has been steady progress in improving device efficiency, the inherently disordered and kinetically-dictated structure of BHJ devices is suboptimal, particularly in terms of charge transport, but also in exciton utilization.[1a,4] An ideal hybrid device is one in which donor and acceptor materials are arranged in a densely packed vertical array. Nanometer-scale periodicity would minimize radiative decay of excitons, and the vertical alignment of the materials ensures a non-tortuous path for charge transport.[1a,4a,5] Such an ordered BHJ motif (OBHJ) has thus been the focus of recent interest,[6] but its realization, particularly in a manner compatible with low-cost fabrication of devices, remains elusive. A further refinement of the OBHJ entails atomic and molecularscale control of the donor and acceptor materials such that their

Liquid Crystalline Order and Magnetocrystalline Anisotropy in Magnetically Doped Semiconducting ZnO Nanowires

Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.

Photopolymerized polypyrrole microvessels.

We report on the preparation of water-filled polymer microvessels through the photopolymerization of pyrrole in a water/chloroform emulsion. The resulting structures were characterized by complementary spectroscopic and microscopic techniques, including Raman spectroscopy, XPS, SEM, and TEM. The encapsulation of fluorescent, magnetic, and ionic species within the microvessels has been demonstrated. Confocal microscopy and fluorescence anisotropy measurements revealed that the encapsulated chromophore (Rhodamine 6G) resides within voids in the capsules; however, strong interaction of the dye with polypyrrole results in a measurable decrease in its rotational dynamics. Microvessels loaded with ferrofluid exhibit magnetic properties, and their structures can be directed with an external magnetic field. TEM measurements allowed imaging of individual nanoparticles entrapped within the vessels. The application of Cu(2+)-loaded microvessels as a transducer layer in all-solid-state ion-selective electrodes was also demonstrated.

Rapid ordering of block copolymer thin films

Block-copolymers self-assemble into diverse morphologies, where nanoscale order can be finely tuned via block architecture and processing conditions. However, the ultimate usage of these materials in real-world applications may be hampered by the extremely long thermal annealing times-hours or days-required to achieve good order. Here, we provide an overview of the fundamentals of block-copolymer self-assembly kinetics, and review the techniques that have been demonstrated to influence, and enhance, these ordering kinetics. We discuss the inherent tradeoffs between oven annealing, solvent annealing, microwave annealing, zone annealing, and other directed self-assembly methods; including an assessment of spatial and temporal characteristics. We also review both real-space and reciprocal-space analysis techniques for quantifying order in these systems.