X-ray scattering from freestanding polymer films with geometrically curved surfaces

Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (T_g) are ideal because phase separation occurs because of π-π stacking during the annealing process, if T_g is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than T_g. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

Structural instabilities in lamellar diblock copolymer thin films during solvent vapor uptake

We have studied the structural rearrangements in a lamellae-forming poly(styrene-b-butadiene) film during exposure to toluene vapor. Real-time, in situ grazing-incidence small-angle X-ray scattering allowed us to identify distinct kinetic regimes: First, the lamellae swell linearly, and the blocks stretch uniaxially. Then, the blocks relax to a more globular molecular conformation, and the lamellar thickness abruptly shrinks again. The increased interfacial area culminates in a dramatic instability causing a major rearrangement of the lamellar stack and the formation of new lamellae. Finally, the lamellar thickness levels off, and the interfaces flatten again.

Freely suspended nanocomposite membranes as highly sensitive sensors

Highly sensitive sensor arrays are in high demand for prospective applications in remote sensing and imaging. Measuring microscopic deflections of compliant micromembranes and cantilevers is developing into one of the most versatile approaches for thermal, acoustic and chemical sensing. Here, we report on an innovative fabrication of compliant nanocomposite membranes with nanoscale thickness showing extraordinary sensitivity and dynamic range, which makes them candidates for a new generation of membrane-based sensor arrays. These nanomembranes with a thickness of 25-70 nm, which can be freely suspended over large (hundred micrometres) openings are fabricated with molecular precision by time-efficient, spin-assisted layer-by-layer assembly. They are designed as multilayered molecular composites made of a combination of polymeric monolayers and a metal nanoparticle intralayer. We demonstrate that these nanocomposite membranes possess unparalleled sensitivity and a unique autorecovering ability. The membrane nanostructure that is responsible for these outstanding properties combines multilayered polymer/nanoparticle organization, high polymer-chain orientation, and a pre-stretched state.