Christian W. Pester

Electric Field Induced Selective Disordering in Lamellar Block Copolymers

External electric fields align nanostructured block copolymers by either rotation of grains or nucleation and growth depending on how strongly the chemically distinct block copolymer components are segregated. In close vicinity to the order-disorder transition, theory and simulations suggest a third mechanism: selective disordering. We present a time-resolved small-angle X-ray scattering study that demonstrates how an electric field can indeed selectively disintegrate ill-aligned lamellae in a lyotropic block copolymer solution, while lamellae with interfaces oriented parallel to the applied field prevail. The present study adds an additional mechanism to the experimentally corroborated suite of mechanistic pathways, by which nanostructured block copolymers can align with an electric field. Our results further unveil the benefit of electric field assisted annealing for mitigating orientational disorder and topological defects in block copolymer mesophases, both in close vicinity to the order-disorder transition and well below it.

Engineering Surfaces through Sequential Stop-Flow Photopatterning.

Solution-exchange lithography is a new modular approach to engineer surfaces via sequential photopatterning. An array of lenses reduces features on an inkjet-printed photomask and reproduces arbitrarily complex patterns onto surfaces. In situ exchange of solutions allows successive photochemical reactions without moving the substrate and affords access to hierarchically patterned substrates.

Cross-linking density and temperature effects on the self-assembly of SiO2-PNIPAAm core-shell particles at interfaces.

SiO2-PNIPAAm core-shell microgels (PNIPAAm=poly(N-isopropylacrylamide)) with various internal cross-linking densities and different degrees of polymerization were prepared in order to investigate the effects of stability, packing, and temperature responsiveness at polar-apolar interfaces. The effects were investigated using interfacial tensiometry, and the particles were visualized by cryo-scanning electron microscopy (SEM) and scanning force microscopy (SFM). The core-shell particles display different interfacial behaviors depending on the polymer shell thickness and degree of internal cross-linking. A thicker polymer shell and reduced internal cross-linking density are more favorable for the stabilization and packing of the particles at oil-water (o/w) interfaces. This was shown qualitatively by SFM of deposited, stabilized emulsion droplets and quantitatively by SFM of particles adsorbed onto a hydrophobic planar silicon dioxide surface, which acted as a model interface system. The temperature responsiveness, which also influences particle-interface interactions, was investigated by dynamic temperature protocols with varied heating rates. These measurements not only showed that the particles had an unusual but very regular and reversible interface stabilization behavior, but also made it possible to assess the nonlinear response of PNIPAAm microgels to external thermal stimuli.

Electric-Field-Induced Alignment of Block Copolymer/Nanoparticle Blends

External electric fields readily align birefringent block-copolymer mesophases. In this study the effect of gold nanoparticles on the electric-field-induced alignment of a lamellae-forming polystyrene-block-poly(2-vinylpyridine) copolymer is assessed. Nanoparticles are homogeneously dispersed in the styrenic phase and promote the quantitative alignment of lamellar domains by substantially lowering the critical field strength above which alignment proceeds. The results suggest that the electric-field-assisted alignment of nanostructured block copolymer/nanoparticle composites may offer a simple way to greatly mitigate structural and orientational defects of such films under benign experimental conditions.

Piezoelectric Properties of Non-Polar Block Copolymers

Most polymers are typical dielectric materials, but recent research in our group has shown that nanostructured block copolymer morphologies exhibit new and unexpected electroactive behavior. We present herein the fi rst study of converse piezoelectric properties in non-crystalline polymer systems, consisting of non-polar monomers, and evaluate its evolution with temperature to yield detailed information on electric fi eld‐ polymer interaction on a molecular level. The observed properties should hold generally and suggest that block copolymers may provide a valuable new route to piezoelectric materials. So far, mostly inorganic, perovskite structured ceramics, many of them titanate derivatives such as lead zirconate titanate (PZT), dominate the fi eld of commercial piezoelectric applications. Nanogenerators, fi eld-effect transistors, strain, and optoelectronic sensors are only few of many applications for this interesting class of materials. [ 1 , 2 ] Whilst the piezoelectric properties of many of these crystals and ceramics are well

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red–green–blue arrays to yield white emission.

Simultaneous Preparation of Multiple Polymer Brushes under Ambient Conditions using Microliter Volumes

The fabrication of well-defined, multifunctional polymer brushes under ambient conditions is described. This facile method uses light-mediated, metal-free atom-transfer radical polymerization (ATRP) to grow polymer brushes with only microliter volumes required. Key to the success of this strategy is the dual action of N-phenylphenothiazine (PTH) as both an oxygen scavenger and polymerization catalyst. Use of simple glass cover slips results in a high degree of spatial and temporal control and allows for multiple polymer brushes to be grown simultaneously. The preparation of arbitrary 3D patterns and functional/emissive polymer brushes demonstrates the practicality and versatility of this novel strategy.

Reorientation mechanisms of block copolymer/CdSe quantum dot composites under application of an electric field

Time- and temperature-resolved in situ birefringence measurements were applied to analyze the effect of nanoparticles on the electric field-induced alignment of a microphase separated solution of poly(styrene)-block-poly(isoprene) in toluene. Through the incorporation of isoprene-confined CdSe quantum dots the reorientation behavior is altered. Particle loading lowers the order-disorder transition temperature, and increases the defect density, favoring nucleation and growth as an alignment mechanism over rotation of grains. The temperature dependent alteration in the reorientation mechanism is analyzed via a combination of birefringence and synchrotron SAXS. The detailed understanding of the effect of nanoparticles on the reorientation mechanism is an important prerequisite for optimization of electric-field-induced alignment of block copolymer/nanoparticle composites where the block copolymer guides the nanoparticle self-assembly into anisotropic structures.