Surface Characteristics of Polyfluorene Films Studied by Polarization-Dependent NEXAFS Spectroscopy

Influence of side chains on the self-alignment capability of electroluminescent polyfluorenes

We report a significant role of side chains in the propagation of molecular orientation upon annealing the liquid crystal phase of polyfluorenes. Direct rubbing of poly(9,9-di(octyl)fluorene) led to the orientation of polymer segments in the top-most region of the film and enhanced propagation of this orientation along the rubbing direction was observed upon annealing. In contrast, the rubbing-induced molecular orientation of poly(9,9-di(ethylhexyl)fluorene) segments completely disappeared upon annealing in the nematic melt state. The higher order of the side chain structures in poly(9,9-di(octyl)fluorene) were found to allow the propagation of the three-dimensional molecular alignment. From integrated experimental and density functional theory studies, we propose that side chain interdigitation generates a unique alignment behavior of poly(9,9-di(octyl)fluorene).

Block Copolymer Nanocomposites in Electric Fields: Kinetics of Alignment

We investigate the kinetics of block copolymer/nanoparticle composite alignment in an electric field using in situ transmission small-angle X-ray scattering. As a model system, we employ a lamellae forming polystyrene-block-poly(2-vinyl pyridine) block copolymer with different contents of gold nanoparticles in thick films under solvent vapor annealing. While the alignment improves with increasing nanoparticle fraction, the kinetics slows down. This is explained by changes in the degree of phase separation and viscosity. Our findings provide extended insights into the basics of nanocomposite alignment.

Calibrated NEXAFS spectra of common conjugated polymers

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy has evolved into a powerful characterization tool for polymeric materials and is increasingly being used to elucidate composition and orientation in thin films of relevance to organic electronic devices. For accurate quantitative compositional analysis, insight into the electronic structure and the ability to assess molecular orientation, reliable reference spectra with known energy resolution and calibrated energy scale are required. We report a set of such NEXAFS spectra from 23 semiconducting polymers and some related materials that are frequently used in organic device research.

Molecular characterization of organic electronic films

Organic electronics have emerged as a viable competitor to amorphous silicon for the active layer in low-cost electronics. The critical performance of organic electronic materials is closely related to their morphology and molecular packing. Unlike their inorganic counterparts, polymers combine complex repeat unit structure and crystalline disorder. This combination prevents any single technique from being able to uniquely solve the packing arrangement of the molecules. Here, a general methodology for combining multiple, complementary techniques that provide accurate unit cell dimensions and molecular orientation is described. The combination of measurements results in a nearly complete picture of the organic film morphology.

Microstructural characterization and charge transport in thin films of conjugated polymers

The performance of semiconducting polymers has been steadily increasing in the last 20 years. Improved control over the microstructure of these materials and a deeper understanding of how the microstructure affects charge transport are partially responsible for such trend. The development and widespread use of techniques that allow to characterize the microstructure of semiconducting polymers is therefore instrumental for the advance of these materials. This article is a review of the characterization techniques that provide information used to enhance the understanding of structure/property relationships in semiconducting polymers. In particular, the applications of optical and X-ray spectroscopy, X-ray diffraction, and scanning probe techniques in this context are described.

Molecular ordering in a biaxial smectic-A phase studied by scanning transmission X-ray microscopy (STXM)

Results of STXM investigations of a binary mixture (-TNF = 2 : 1; SmA(b) 140 M 180 Iso) known to form a SmA(b) phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the -TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring pi* orbitals (284.5-286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the -TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive +/-(1/2) disclination theoretically predicted for the SmA(b) phase.