Heiko G. Schoberth

Nanostructured wrinkled surfaces for templating bionanoparticles—controlling and quantifying the degree of order

We present a novel method to align the tobacco mosaic virus (TMV) on topographically structured surfaces. In order to gain defined patterns we use wrinkled polydimethlysiloxane (PDMS) sheets as templates. We aligned the virus with a simple spin-coating procedure on the PDMS sheet. The concentration of the virus solution and the spin speed are varied in order to identify ideal conditions for the arrangement of the viruses on the wrinkled templates. Here, we establish a simple analytical approach which allows quantifying the degree of order of the patterns, which is the basis for a quantitative discussion of templating efficiency. Furthermore, we discuss the role of dewetting processes for the particle assembly. TMVs can be used as reactive nanoparticles due to their well-defined surface chemistry. They can as well serve as a model system for alignment of anisotropic particles via spin coating from solution.

Scaling behavior of the reorientation kinetics of block copolymers exposed to electric fields

We have followed the reorientation kinetics of various block copolymer solutions exposed to an external electric DC field. The characteristic time constants follow a power law indicating that the reorientation is driven by a decrease in electrostatic energy. Moreover, the observed exponent suggests an activated process in line with the expectations for a nucleation and growth process. When properly scaled, the data collapse onto a single master curve spanning several orders of magnitude both in reduced time and in reduced energy. The power law dependence of the rate of reorientation derived from computer simulations based on dynamic density functional theory agrees well with the experimental observations. First experiments in AC electric fields at sufficiently high frequencies confirm the notion that the reorientation process is dominated by differences in the dielectric constants rather than by mobile ions.

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.

Lamellar microstructure and dynamic behavior of diblock copolymer/nanoparticle composites under electric fields

We investigate the microstructure and dynamic behavior of diblock copolymer/nanoparticle composites under electric fields by a series of cell dynamical system simulations. The study focuses on the effects of two factors, i.e., the electric field strength and the polymer/nanoparticle coupling interaction strength. Our simulations demonstrate that the presence of nanoparticles can evidently influence the morphology and orientation dynamics of the lamellar microstructure in diblock copolymer nanocomposites under electric fields. The nanoparticles can be still confined within their preferential phase during the electric field-induced orientation process and exhibit a spatial distribution with both short and long range order. The diblock/nanoparticle coupling strength and the electric field strength permit effective accesses to control the distribution of nanoparticles in the systems with hierarchical nanostructures. Our simulations shed light on the possible use of nanoparticles in designing and processing of various functional nanostructured systems.

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

“Micro-structure–macro-response” relationship in swollen block copolymer films

We demonstrate the effect of confinement on the thickness-dependent swelling behavior of poly(styrene)-b-poly(2-vinylpyridine) diblock copolymer films as revealed by in situspectroscopic ellipsometry. “Microscopic” molecular confinement to the dimension smaller than the characteristic lamella spacing, as well as to the non-equilibrium microstructures limits the solvent up-take by the polymer film. The maximum degree of swelling (“macroscopic response”) is achieved for films with a starting thickness in a dry state hdr in the range of ∼one–two characteristic lamella dimension in bulk L0 and drops down according to ∼hdr−0.1 as the film thickness increases up to ∼μm scale (∼tens of L0). These findings bring novel fundamental insights into the stimuli-responsive behavior of confined soft matter.

Effects of Electric Fields on Block Copolymer Nanostructures

In this chapter we overview electric-field-induced effects on block copolymer microdomains. First, we will consider the thin film behavior and elucidate the parameters governing electric-field-induced alignment. We describe the structural evolution of the alignment in an electric field via quasi in situ scanning force microscopy (SFM) using a newly developed SFM setup that allows solvent vapor treatment in the presence of high electric fields. Second, we will turn to bulk structures and show novel effects of high field strengths on the block copolymer phase behavior. We will describe a procedure that allows tuning the morphology and size of the nanoscopic patterns by application of high electric fields and present experimental evidence for the electric-field-induced decrease of the order–disorder transition temperature in a block copolymer.

Going beyond the Surface: Revealing Complex Block Copolymer Morphologies with 3D Scanning Force Microscopy

We report on the quasi in situ scanning force microscopy nanotomography which proved to be a key method to effectively obtain a three-dimensional (3D) microdomain structure of a complex ABC triblock morphology. As an example, we studied polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (BVT) thin triblock terpolymer films. We realized a controlled erosion of the material by using low-pressure plasma etching coupled to the scanning force microscope. The 3D reconstruction provides insights into the structural behavior in very thin volume elements revealing morphological details not accessible with other methods.

Miscibility Gap in the Phase Diagrams of Thermoelectric Half-Heusler Materials CoTi_{1-x}Y_xSb (Y = Sc, V, Mn, Fe)

The half-Heusler system CoTi1-xYx\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}