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Articles by Roman Dabrowski
Total Records ( 4 ) for Roman Dabrowski
  Roberta Cassano , Roman Dabrowski , Jerzy Dziaduszek , Sonia Trombino , Francesca Iemma , Fiore Pasquale Nicoletta , Giovanni De Filpo and Nevio Picci
  The synthesis, characterisation, helical pitch and electro-optical response of new ferroelectric liquid crystals, 4’-(ω-acryloxyalkoxy)-, 4’-(ω-alkanoyloxyalkoxy)- or 4’-(ω-perfluoroalkanoyloxy)alkoxybiphenyl-4-yl (S)-4-(1-octyloxy)benzoates, bearing either polymerisable or non-polymerisable terminal groups, are described. The synthesised liquid crystals exhibit both a chiral smectic C phase and a chiral nematic phase. The helical pitch and electro-optical properties of the new compounds were determined as a function of temperature.
  Sebastian Gauza , Amanda Parish , Shin-Tson Wu , Anna Spadlo and Roman Dabrowski
  We have designed, synthesised and evaluated the physical properties of high-birefringence (Δn) isothiocyanato phenyl-tolane and terphenyl liquid crystals. These compounds exhibit a Δn∼0.36-0.52 at room temperature and wavelength λ = 633 nm. A different mesomorphic sequence was observed depending on the terminal alkyl chain length. Compounds with a longer chain show smectic and nematic phases, whereas short-chain homologues only exhibit a nematic phase. In the vicinity of the nematic-to-smectic transition, the viscoelastic coefficient decreases, instead of increases, as the temperature decreases. The observed change is as large as 75% for the 4`-(3,5-difluoro-4-isothiocyanatophenylethynyl)-4-pentylbiphenyl compound studied. This behaviour is useful for improving the response time of a liquid crystal device, if the operating temperature can be controlled accurately.
  Marzena Tykarska , Roman Dabrowski , Jan Przedmojski , Wiktor Piecek , Katarzyna Skrzypek , Bertrand Donnio and Daniel Guillon
  Two ferroelectric three-ring chiral esters, one with a partially fluorinated alkyl chain and another with a cyano terminal group, were mixed with a structurally similar compound having an alkyl terminal chain. In their mixtures an antiferroelectric phase was induced. The phase behaviour, spontaneous polarisation, tilt angle, smectic layer spacing and helical pitch of both systems were determined. The mechanism of the induction of an antiferroelectric phase is different in both cases, with highly tilted phases in former system and less tilted phases in the latter.
  Anoop Kumar Srivastava , Ravindra Dhar , Vinai Krishna Agrawal , Seung Hee Leeb and Roman Dabrowski
  Switching and dielectric relaxation phenomena were investigated for an antiferroelectric liquid crystal, 4,4-(1-methyloctyloxycarbonyl)phenyl]-4`-[3-(butanoyloxy)prop-1-oxy]biphenyl carboxylate, exhibiting chiral smectic A (SmA*), smectic C (SmC*) and antiferroelectric (SmCA*) phases. Spontaneous polarisations, rotational viscosities, relaxation frequencies, dielectric strengths and distribution parameters were determined as a function of temperature. The electric field required for saturation of the spontaneous polarisation increased with a decrease in temperature. In the SmA* phase, only one relaxation mechanism was observed that behaves as soft mode. Two relaxation processes were detected in the SmC* phase. A high-frequency relaxation process invariant at 2.2 kHz was due to a Goldstone mode, but the origin of low-frequency relaxation process (1-20 Hz) is unclear; however, it may belong to an X-mode. The dielectric spectrum of the SmCA* phase exhibits two absorption peaks separated by two decades of frequency. The low-frequency peak is related to the antiferroelectric Goldstone mode, whereas the high-frequency peak originates from the anti-phase fluctuation of the directors in the anti-tilt pairs of the SmCA* phase.
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