@article{HollaenderKossackKolloscheetal.2016,
  author    = {Holl{\"a}nder, Lars and Kossack, Wilhelm and Kollosche, Matthias and Wirges, Werner and Kremer, Friedrich and Gerhard, Reimund},
  title     = {Influence of the remanent polarisation on the liquid crystal alignment in composite films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and a cyanobiphenyl-based liquid crystal},
  series = {Liquid crystals : an international journal of science and technology},
  volume    = {43},
  journal   = {Liquid crystals : an international journal of science and technology},
  publisher = {Editions Rodopi BV},
  address   = {Abingdon},
  issn      = {0267-8292},
  doi       = {10.1080/02678292.2016.1185174},
  pages     = {1514 -- 1521},
  year      = {2016},
  abstract  = {Polymer-dispersed liquid crystals (PDLCs) of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and nematic 4-cyano-4\&\#697;-n-hexylbiphenyl (6CB) or 4-cyano-4\&\#697;-n-pentylbiphenyl (5CB) were prepared to study the effect of the remanent polarisation of the polymer on the liquid crystal alignment. We measured the macroscopic alignment of the liquid crystal molecules in the thickness direction by means of Infrared Transition-Moment Orientational Analysis. Electrical poling at 100 V/µm caused an increased order parameter up to 0.15. After subsequent annealing above the nematic-to-isotropic phase-transition temperature, the order parameter was reduced to 0.02. Nevertheless, the order parameter was still higher than for non-poled film indicating a slight orientation in thickness direction. Both values are lower than those expected from model calculations. In agreement with dielectric measurements, we attribute this result to the shielding effect of mobile charge carriers within the liquid crystal inclusions.},
  language  = {en}
}
@article{QiuHollaenderWirgesetal.2013,
  author    = {Qiu, Xunlin and Holl{\"a}nder, Lars and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury},
  title     = {Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit},
  series = {Journal of applied physics},
  volume    = {113},
  journal   = {Journal of applied physics},
  number    = {22},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.4809556},
  pages     = {8},
  year      = {2013},
  abstract  = {Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance-which is, however, still considerably larger than the sample capacitance-is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.},
  language  = {en}
}