@phdthesis{Grenzer2007,
  author    = {Grenzer, Marina},
  title     = {Photoinduced material transport in amorphous azobenzene polymer films},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-15771},
  school      = {Universit{\"a}t Potsdam},
  year      = {2007},
  abstract  = {The role played by azobenzene polymers in the modern photonic, electronic and opto-mechanical applications cannot be underestimated. These polymers are successfully used to produce alignment layers for liquid crystalline fluorescent polymers in the display and semiconductor technology, to build waveguides and waveguide couplers, as data storage media and as labels in quality product protection. A very hot topic in modern research are light-driven artificial muscles based on azobenzene elastomers. The incorporation of azobenzene chromophores into polymer systems via covalent bonding or even by blending gives rise to a number of unusual effects under visible (VIS) and ultraviolet light irradiation. The most amazing effect is the inscription of surface relief gratings (SRGs) onto thin azobenzene polymer films. At least seven models have been proposed to explain the origin of the inscribing force but none of them describes satisfactorily the light induced material transport on the molecular level. In most models, to explain the mass transport over micrometer distances during irradiation at room temperature, it is necessary to assume a considerable degree of photoinduced softening, at least comparable with that at the glass transition. Contrary to this assumption, we have gathered a convincing evidence that there is no considerable softening of the azobenzene layers under illumination. Presently we can surely say that light induced softening is a very weak accompanying effect rather than a necessary condition for the formation of SRGs. This means that the inscribing force should be above the yield point of the azobenzene polymer. Hence, an appropriate approach to describe the formation and relaxation of SRGs is a viscoplastic theory. It was used to reproduce pulse-like inscription of SRGs as measured by VIS light scattering. At longer inscription times the VIS scattering pattern exhibits some peculiarities which can be explained by the appearance of a density grating that will be shown to arise due to the final compressibility of the polymer film. As a logical consequence of the aforementioned research, a thermodynamic theory explaining the light-induced deformation of free standing films and the formation of SRGs is proposed. The basic idea of this theory is that under homogeneous illumination an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. Finally, some ideas about further development of this controversial topic will be discussed.},
  language  = {en}
}
@article{ZenSaphiannikovaNeheretal.2006,
  author    = {Zen, Achmad and Saphiannikova, Marina and Neher, Dieter and Grenzer, J{\"o}rg and Grigorian, Souren A. and Pietsch, Ullrich and Asawapirom, Udom and Janietz, Silvia and Scherf, Ullrich and Lieberwirth, Ingo and Wegner, Gerhard},
  title     = {Effect of molecular weight on the structure and crystallinity of poly(3-hexylthiophene)},
  doi       = {10.1021/Ma0521349},
  year      = {2006},
  abstract  = {Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites},
  language  = {en}
}