Gradient Photonic Materials Based on One-Dimensional Polymer Photonic Crystals.

In nature, animals such as chameleons are well-known for the complex color patterns of their skin and the ability to adapt and change the color by manipulating sophisticated photonic crystal systems. Artificial gradient photonic materials are inspired by these color patterns. A concept for the preparation of such materials and their function as tunable mechanochromic materials is presented in this work. The system consists of a 1D polymer photonic crystal on a centimeter scale on top of an elastic poly(dimethylsiloxane) substrate with a gradient in stiffness. In the unstrained state, this system reveals a uniform red reflectance over the entire sample. Upon deformation, a gradient in local strain of the substrate is formed and transferred to the photonic crystal. Depending on the magnitude of this local strain, the thickness of the photonic crystal decreases continuously, resulting in a position-dependent blue shift of the reflectance peak and hence the color in a rainbow-like fashion. Using more sophisticated hard-soft-hard-soft-hard gradient elastomers enables the realization of stripe-like reflectance patterns. Thus, this approach allows for the tunable formation of reflectance gradients and complex reflectance patterns. Envisioned applications are in the field of mechanochromic sensors, telemedicine, smart materials, and metamaterials.

Confinement templates for hierarchical nanoparticle alignment prepared by azobenzene-based surface relief gratings.

Alignment of nanoparticles to hierarchical periodic structures is an emerging field in the development of patterned surfaces. Common alignment methods are based on templates that guide particle self-assembly. These can be formed using lithographic methods offering an almost free choice of the motif, while being expensive and time-consuming for large-scale production. Alternatively, template formation by controlled wrinkling offers a low-cost formation, but often suffers from the formation of defect structures like line-defects and cracks. Here, we show a preparation technique for nanoparticle alignment substrates that is based on the inscription of holographic surface relief gratings with a periodic sinusoidal wave pattern on the surface of azobenzene films. As interference patterns are employed for structure formation, very uniform and defect-free gratings with tunable grating height and grating period can be prepared. These substrates were successfully replicated to poly(dimethyl siloxane) and the replicas used for the alignment of polystyrene latex particles. Accordingly produced substrates exhibiting gratings with a variation in grating height allow for efficient screening of nanoparticle alignment in a geometrical confinement in one single experiment. We anticipate our studies as a promising tool for the development of sensors, tunable gratings and metamaterials.

Controlled Wrinkling of Gradient Metal Films.

Controlled wrinkling is a rather simple method of fabricating surface topographies. The production process is based on the spontaneous formation of wrinkles upon compression of a hard film attached to a soft elastic substrate. Controlled wrinkling typically features large-scale wrinkled samples with a discrete wavelength and amplitude. In this report, we employ an approach utilizing linear metal layer thickness gradients for the controlled formation of gradient wrinkle patterns. The observed wavelength modulation was experimentally achieved by preparing layer thickness gradients of gold, chromium, and indium by physical vapor deposition in combination with a poly(dimethyl siloxane) elastomer substrate. In case of chromium and indium, a thin SiO x surface layer was sufficient to ensure adhesion. However, in case of gold, an additional thin chromium adhesion layer was required. For the wrinkled gradient gold film, it was possible to tune the wavelength from 3.4 to 12.2 µm on a single substrate. The experimental data correspond well to the theoretical bilayer model from Stafford et al. Chromium has a significant higher Young's modulus and melting temperature than gold. However, chromium was successfully evaporated and gradient wrinkle patterns with wavelengths from 1.0 to 3.5 µm were realized. In contrast, indium has a considerable lower Young's modulus than gold and chromium, respectively. Consequently, lower wavelengths (0.6-1.0 µm) of the wrinkled gradient indium film were observed. These tunable wrinkled gradient metal films can be envisioned as components in sensors and optical and electro-optical devices.

Hierarchical line-defect patterns in wrinkled surfaces.

We demonstrate a novel approach for controlling the formation of line-defects in wrinkling patterns by introducing step-like changes in the Young's modulus of elastomeric substrates supporting thin, stiff layers. Wrinkles are formed upon treating the poly(dimethylsiloxane) (PDMS) substrates by UV/Ozone (UVO) exposure in a uniaxially stretched state and subsequent relaxation. Line defects such as minutiae known from fingerprints are a typical feature in wrinkling patterns. The position where these defects occur is random for homogenous substrate elasticity and film thickness. However, we show that they can be predetermined by using PDMS substrates consisting of areas with different cross-linking densities. While changing the cross-linking density is well known to influence the wrinkling wavelength, we use this parameter in this study to force defect formation. The defect formation is monitored in situ using light microscopy and the mechanical parameters/film thicknesses are determined using imaging AFM indentation measurements. Thus the observed wrinkle-wavelengths can be compared to theoretical predictions. We study the density and morphology of defects for different changes in elasticity and compare our findings with theoretical considerations based on a generalized Swift-Hohenberg-equation to simply emulate the observed pattern-formation process, finding good agreement. The fact that for suitable changes in elasticity, well-ordered defect patterns are observed is discussed with respect to formation of hierarchical structures for applications in optics and nanotechnology.

Length control and block-type architectures in worm-like micelles with polyethylene cores.

We present evidence for "living"-like behavior in the crystallization-driven self-assembly of triblock copolymers with crystallizable polyethylene middle blocks into worm-like crystalline-core micelles (CCMs). A new method of seed production is introduced utilizing the selective self-assembly of the triblock copolymers into spherical CCMs in appropriate solvents. Seeded growth of triblock copolymer unimers from these spherical CCMs results in worm-like CCMs with narrow length distributions and mean lengths that depend linearly on the applied unimer-to-seed ratio. Depending on the applied triblock copolymer, polystyrene-block-polyethylene-block-polystyrene (SES) or polystyrene-block-polyethylene-block-poly(methyl methacrylate) (SEM), well-defined worm-like CCMs with a homogeneous or patch-like corona, respectively, can be produced. In a subsequent step, these worm-like CCMs can be used as seeds for the epitaxial growth of a different polyethylene containing triblock copolymer. In this manner, ABA-type triblock co-micelles containing blocks with a homogeneous polystyrene corona and those with a patch-like polystyrene/poly(methyl methacrylate) corona were prepared. While the epitaxial growth of SEM unimers from worm-like SES CCMs with a homogeneous corona yields triblock co-micelles almost quantitatively, the addition of SES unimers to patchy SEM wCCMs results in a mixture of ABA- and AB-type block co-micelles together with residual patchy wCCMs. Following reports on self-assembled block-type architectures from polymers containing core-forming polyferrocenylsilane blocks, these structures represent the first extension of the concept to block co-micelles from purely organic block copolymers.