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1999 (1)
1998 (1)
1Author    M. Sandmann, A. WürflingerRequires cookie*
 Title    PVT Measurements on 4'-/i-Octyl-Biphenyl-4-Carbonitrile (8CB) up to 300 MPa  
 Abstract    P, V m , Tdata have been measured for the smectic, nematic and isotropic phases of 4'-n-octyl-biphen-yl-4-carbonitrile (8CB) in the temperature range 300-370 K and pressures up to 300 MPa. At atmos-pheric pressure all phase transitions appear to be of first order due to a discontinuity in the density. The volume change at the smectic A -nematic transition is only a tenth of the volume change at the clear-ing temperature. At moderate pressures below 80 MPa the S A -N transition could be detected as a dis-continuity in the period of oscillation in measurements with a high-pressure vibrating tube densimeter. At higher pressures the discontinuity seems to die away, possibly indicating a change from first order to second order transition. From the volume changes and the slopes of the transition lines we calculate the enthalpy changes at the phase transition. The p, V m , T data enable us to calculate the volume part of the entropy and the molecular field parameter y=9ln 7 N i/3ln V NI . 
  Reference    Z. Naturforsch. 53a, 787—792 (1998); received July 10 1998 
  Published    1998 
  Keywords    8CB, High Pressure, pVT, Phase Transitions, Thermodynamics 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0787.pdf 
 Identifier    ZNA-1998-53a-0787 
 Volume    53 
2Author    Phineus Markwick, Stanislaw Urban, Albert WürflingerRequires cookie*
 Title    High Pressure Dielectric Studies of 8CB in the Isotropic, Nematic, and Smectic A Phases  
 Abstract    The static and complex permittivity of 4-n-octyl-4'-cyanobiphenyl (8CB) has been measured for the isotropic, nematic and smectic A phases as functions of temperature and pressure. The ranges of 297 -361 K, 0.1 -220 MPa, and 0.1 -13 MHz, were covered. Only the parallel component of the complex permittivity, £*(/) = e'(f) — ie"(f), was measured. The relaxation times T||(p, T) as well as ris(p, T) were analysed at constant temperature, pressure and volumes yielding the activation volume, A ¥ V(T), activation enthalpy A^H(p), and activation energy lyU(V), respectively. All activation parameters calculated for the smectic A phase of 8CB are smaller than those obtained for the nematic phase. The activation energy constitutes approximately half of the activation enthalpy value in all three phases studied. The pressure study allowed to calculate the pressure dependence of the retardation factor g\\(p, T), from which the nematic potential q(p,T) can be derived. Using the relationships between g\\ and q/RT proposed by Kalmykov and Coffey, the order parameter (Piip, T)) was calculated as a function of pressure. 
  Reference    Z. Naturforsch. 54a, 275—280 (1999); received March 13 1999 
  Published    1999 
  Keywords    Liquid Crystals, 8CB, High Pressure, Dielectric, Thermodynamics, Order Parameter 
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 TEI-XML for    default:Reihe_A/54/ZNA-1999-54a-0275.pdf 
 Identifier    ZNA-1999-54a-0275 
 Volume    54