Aa Alexander Alexeev

Accurately evaluating Young’s modulus of polymers through nanoindentations: A phenomenological correction factor to the Oliver and Pharr procedure

The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.

Metal ion assisted folding and supramolecular organization of a De Novo designed metalloprotein

This paper describes the supramolecular organization of a novel de novo designed metalloprotein, which consists of two N-terminal terpyridine modified coiled-coil protein folding motif sequences held together by an iron(II) ion. The self-assembly of the metalloprotein is the result of the interplay of metal ion complexation and protein folding, and can be manipulated by changes in concentration, temperature, and solvent. At low concentrations, folding and organization of the metalloprotein resembles that of the native coiled-coil peptide. Besides unimeric species, also dimeric and tetrameric metalloprotein assemblies were found. Several indications suggest that at least part of these unimeric species may exist as intramolecularly folded coiled-coils, however, unambiguous proof is lacking at the moment. At higher concentrations, folding and organization is dominated by the large octahedral [FeII(terpy)2] complexes (terpy = 2,2′:6′,2″-terpyridine) and considerable amounts of large, ill-defined aggregates are formed.

Automated scanning probe microscopy for combinatorial polymer research

With the development of combinatorial materials research (CMR) methods and high throughput experimentation (HTE) workflows for polymer research applications, the demand for automated, high throughput characterization methods is evident. Solution-based characterization methods like NMR, GPC, viscosimetry, for example and UV-Vis as well as fluorescence plate readers are available. Here we report on the incorporation of automated scanning probe microscopy in the HTE-workflow by demonstrating the evaluation of surface properties and topographies for thin polymer film libraries.

Scanning probe microscopy on polymer solar cells

Polymer solar cells have the potential to become a major electrical power generating tool in the twenty-first century. Research and development endeavors are focusing on continuous roll-to-roll printing of polymeric or organic compounds from solution—like newspapers—to produce flexible and lightweight devices at low cost. It is recognized, though, that besides the functional properties of the compounds, the organization of structures on the nanometre level— forced and controlledmainly by the processing conditions applied—determines the performance of state-of-the-art polymer solar cells. In such devices the photoactive layer is composed of at least two functional materials that form nanoscale interpenetrating phases with specific functionalities, a so-called bulk heterojunction. In this study, we discuss our current knowledge of the main factors determining the morphology formation and evolution—based on systematic scanning probe microscopy studies—and gaps in our understanding of nanoscale structure–property relations in the field of high-performance polymer solar cells are addressed.

Influence of the polymer architecture on morphology and device properties of polymer bulk heterojunction photovoltaic cells

Polymer bulk hetero junction solar cells were made from poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) as donor and poly(cyanoetherphenylenevinylene) (PCNEPV) derivatives as acceptor material. In this paper we start out with discussing the synthesis of the materials. Subsequently, the main issues concerning the devices are treated. Annealing the devices yielded devices with encouraging efficiencies of 0.5% (1 sun, 100mW/cm2), as calculated from the maximum power points (MPP). AFM studies revealed that this anneal step improves especially the interface of the active layer with the under laying PEDOT:PSS, although mobility and morphology changes can not be ruled out. Lowering the molecular weight (Mw) of the MDMO-PPV gave a slight improvement of the device performance. Decreasing the Mw of the acceptor material, MDMO-PCNEPV (PCNEPV derivative with the same side chains as MDMO-PPV) and optimizing the layer thickness led to a device with an efficiency of 0.65%. Finally we looked into the influence of the nature of the side chains on the acceptor polymer. The results suggest that the closer the resemblance between donor and acceptor is the better the device performance.

Polymerization-induced diffusion as a tool to generate periodic relief structures: a combinatorial study

Polymeric relief structures are extensively used in display technology due to their ability to redirect light in a controlled way. Photo-embossing is a new photo-lithographic technique to generate surface relief structures using photopolymers. In the present paper we show a combinatorial methodology to explore this technique. We have prepared and evaluated (using automated atomic force microscopy) 2-dimensional libraries of photo-embossed gratings, each library with a gradient in period and a gradient in either exposure energy or development temperature or film thickness or photoinitiator concentration or monomer to binder ratio. We show how this combinatorial approach helps us to better understand the photo-embossing process. In addition, we show that this methodology is an effective tool to identify processing conditions resulting in optimum shape and height of the polymeric relief micro-structures to be used in specific applications.