für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. pVT Measurements and Related Studies on the Binary System /iC16H34 tiC17H36 and on wC18H38 at High Pressures

The dielectric permittivity tensor components, epsilon(II) and epsilon(perpendicular to), in the nematic phase of 6CB (4-n-hexyl-4'- cyanobiphenyl) were measured in the pressure range 0.1 - 130MPa and the temperature range 12 - 58 degrees C. The dielectric anisotropy, Delta epsilon(p, V, T) = epsilon(II) - epsilon(perpendicular to), was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter c relating the interaction potential with the volume (density). Its value gamma = 4.1 agrees very well with other estimates.

Results of dielectric studies under elevated pressure of selected liquid crystalline materials are reviewed. The tensor components of the static permittivity of two nematics, 60CB and 8PCH, measured at p = 1 atm as a function of temperature, and at T = constant as a function of external pressure, are compared. They show striking similarities, if the ranges of the nematic phase at the isobaric and isothermal conditions are normalized. The Maier and Meier equations are applied for discussing the observed behaviors. The dielectric relaxation processes in many substances exhibiting the nematic, smectic A, smectic C and smectic (crystalline) E phases were studied in the past in our group. The low frequency relaxation times characterizing the molecular rotations around the short axes were determined at the isothermal, isobaric and isochoric conditions. This allowed us to calculate three activation parameters: activation volume, activation enthalpy and activation energy, which are interrelated. In particular, it was found that the isobaric activation enthalpy is twice as large as the isochoric activation energy, independently of the phase studied. This indicates the role of steric constraints against the molecular rotations around the short axes.

The low frequency relaxation times tau//, which characterize the flip-flop molecular motions in liquid crystalline phases, recently determined in high-pressure experiments for eight liquid crystalline substances, were reanalyzed considering a relation proposed for the glass-forming liquids [C. Dreyfus, Phys. Rev. E 68, 011204 (2003); R. Casalini and C. M. Roland, Phys. Rev. E 69, 062501 (2004)]. The data, measured at constant pressure, constant temperature, and constant molar volume, could be rescaled onto a master line in the ln tau// vs 1/(T V(gamma)m) plot, with gamma as an adjustable parameter (Vm=1/rho is the specific volume). The obtained gamma values are in good agreement with other estimations; here, the value of gamma parameter was determined for the crystal-like smectic- E phase.

The structure of the mesogenic compound 3-Cyanobenzyi-2,5-bis(4-n-octyloxybenzoyl-oxy)benzoate (3CNBOB) was investigated by single and melt crystal methods. Using a single crystal of 3CNBOB at 20degreesC, the unit cell structure was determined as triclinic with two molecules in a unit cell of dimensions: a = 8.792 Angstrom, b = 11.760 Angstrom, c = 19.121 Angstrom, alpha = 96.61degrees, beta = 90.56degrees, and gamma = 90.28degrees. Three benzene rings of the compound exhibit pi-pi interactions in the crystalline form. The 3CNBOB showed a liquid crystalline state in the region of 82.9 to 101.5degreesC. Melt crystallization was observed by in-situ x-ray diffraction using a temperature slope apparatus. A smectic phase appeared in the liquid crystalline region near the boundary to the crystalline phase. The smectic molecules showed spontaneous orientation aligned with their long axes parallel to the crystallizing plane. There were two crystalline states in the bulk material. The room-temperature crystalline form was the same as the one observed using the single crystal. The high-temperature crystalline state developed from the smectic state. During the smectic-crystalline transition, the packing became denser and the spontaneous orientation decreased.

The nematic order parameter S = (P-2(costheta)) was calculated from the dielectric relaxation time characterizing the molecular rotation around the short axes using formulae derived by Coffey et al. (W. T. Coffey, Yu. P. Kalmykov and J. P. Waldron, Liq. Cryst., 1995, 18, 677; W. T. Coffey, D. S. F. Crothers, Yu. P. Kalmykov and J. P. Waldron, Physica A, 1995, 213, 551; Yu. P. Kalmykov and W. T. Coffey, Liq. Cryst., 1998, 25, 329). In order to test the approximations on which the theory is based (rod-like shape of molecules, the diffusional model of reorientational motion in the presence of an ordering Maier-Saupe potential) several nematogens were chosen for which the S ( T) behaviour is known from other experimental techniques as well. These molecules (1OCB, 5OCB, 7CB, 5PCH, 3CCH, 5CCH and 7CCH) have rigid or flexible cores, strongly polar -CN group at one side and the alkyl or alkoxy tail at the other side. A common analysis of the results coming from different experimental methods show large differences between data. Using the Haller-type formula a re-scaling of the S-values was proposed which reduced the scatter of experimental sets of points to a reasonable range. It was established that the Coffey et al. approach leads to the order parameter being consistent with other data if the molecular cores are sufficiently rigid.

The pressure-temperature phase diagram of 4′-tetradecyl-4-cyanobiphenyl (14CB) up to 220 MPa (2.2 kbar) and between 320–400 K was established using DTA. The temperature range of the smectic A (SmA) phase slightly increases with pressure. The layer spacing d at 1 atm was determined as a function of temperature using X-ray diffraction. It was related to the molecular length l by the ratio d/l ˜ 1.4. The dielectric relaxation measurements in the isotropic and smectic Ad phases of 14CB at 1 atm were performed in the frequency range 10 kHz-3 GHz. Contributions from both principal rotational motions, i.e. around the short and long molecular axes, were separated. The relaxation measurements under high pressure in the SmA phase covered the low frequency process. The longitudinal relaxation time τ1, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature dependences, giving activation volumes, Δ# V = RT (∂ ln τ1 / ∂p)T, and activation enthalpies, Δ# H = RT(∂ ln τ1 / ∂T -1)p, respectively. Surprisingly, all the activation quantities characterizing the rotational motions of 14CB molecules under different conditions are nearly the same as those determined recently for the much shorter homologue, 8CB. This indicates that the 14CB molecule is in fact relatively short due to conformational motions of the alkyl tail.

For the first time the low frequency relaxation process in two smectic phases (smectic A and smectic C) was studied at elevated pressures with the aid of DTA and dielectric spectroscopy. The substance studied, 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine (6OPB8 in short) exhibits the nematic (N) - S A - S C phase sequence. The p-T phase diagram was established with DTA. However, the S A - S C transition was not observed in the DTA, but could be detected by dielectric relaxation measurements. The dielectric relaxation time measured as function of temperature and pressure, τ ∥ (p,T), enabled us to calculate the activation volume, Δ # V =RT(∂lnτ /∂p) T , and activation enthalpy, Δ # H = R(∂lnτ /∂T −1 ) p . It was found that Δ # V(SA) > Δ # V(SC) and Δ # H(N) ≫ Δ # H(SA) > Δ # H(SC), indicating that the molecular rotations around the short axes are more feasible in the tilted SC than in the orthogonal S A phase.

The pressure-temperature (p - T) phase diagrams for four smectogenic members of the 4′-alkyl-4-cyanobiphenyl homologous series (nCB, n = 9, 10, 11 and 12) over the temperature range 320–410 K and pressure range 0.1–300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic Ad phases, while the other members show only the smectic Ad phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, dT /dp, depends strongly on the length of the alkyl chain for the nCB series, but does not show a step-wise change between the nematogenic and smectogenic members.

Cm-Cn The high pressure phase behaviour of four 4,4'-dialkylbiphenyl compounds (, m = 5, 6,7, n = 6, 7) has been studied with differential thermal analysis. The pressure dependence of the phase transitions has been determined up to 200 MPa. In one substance a pressure limited and in another a pressure induced phase was observed. Volume changes accompanying the transitions to the isotropic phase were calculated using the Clausius-Clapeyron equation and the enthalpy changes from DSC measurements at 1 atm. They are compared with the data for other two-ring compounds.

The pressure-temperature phase diagram of n-hexyl-isothiocyanato-biphenyl (6BT) at up to 190 MPa and 250 - 400 K was established with the aid of DTA. The dielectric relaxation measurements in the SmE phase of 6BT were performed in the pressure range of 0.1 - 150 MPa and the temperature range of 320 - 350 K. The Debye-type relaxation process was observed in the frequency range of 100 Hz - 100 kHz. The longitudinal relaxation time characterizing the molecular reorientations around the short axis was analyzed with respect to the pressure and temperature dependencies, yielding the activation volume, Δ#V = RΤ(∂ ln τ/∂p)Τ , and activation enthalpy, Δ#H = R(∂ ln τ/∂Τ-1)p , respectively. The results are compared with the analogous data obtained recently for 8BT and other similar compounds having the nematic and SmA liquid crystalline phases.in the telluride retain their molecular character, with small intercluster interactions.

For the first time pressure-volume-temperature (PVT) measurements for the crystal-like smectic E phase have been performed. The phase diagram of 4'-n-octyl-4-isothiocyanatobiphenyl 8BT has been recently established using differential thermal analysis up to 250 MPa. 8BT exhibits a splitting of the clearing line above 170 MPa. PVT data have been measured in the same pressure range for temperatures between 313 and 393 K. Volume and enthalpy changes accompanying the clearing line of 8BT are also presented. The configurational part of the entropy change at the CrE-I transition of 8BT amounts to approximate to60%. Using the PVT data and recently published dielectric relaxation results, the isochoric activation energy was calculated (giving approximate to50% of the activation enthalpy); this is compared with analogous results for other liquid crystals.

The phase diagram of a substance with two CN groups attached to the benzene ring at lateral positions (CNCN) has been obtained with differential thermal analysis (DTA). The pressure range of the smectic A phase is limited, resulting in a triple point (Cr, S-A, L) at 135 MPa and 371 K. However, the S-A-phase exists also above the triple point as a metastable phase. The transverse relaxation times tau (perpendicular to) were obtained from the dielectric spectra measured for several isotherms as a function of pressure within the S-A-phase of CNCN. The activation volume, Delta (#) V-perpendicular to = RT (partial derivative ln tau (perpendicular to)/partial derivativep)(T), = (52 +/- 3) cm(3)/mol is larger than Delta V-#(parallel to), recently derived from the pressure dependence of the longitudinal relaxation times for other substances in the S-A-phase. It is concluded that due to steric hindrances made by the cyano groups the molecular rotations around the long axes become strongly slowed down by pressure and the smectic phase disappears.

The phase diagrams of three 5-n-alkyl-2-(4′-isothiocyanatophenyl)-1,3-dioxanes (6DBT, 8DBT and 10DBT), have been established using differential thermal analysis (DTA) up to 250 MPa. All three compounds have a smectic A1 phase. For 8DBT the melting curve splits with increasing pressure. 10DBT exhibits a solid–solid transition and a splitting of the clearing line above 170 MPa. Additionally pVT data have been measured for 6DBT in the same pressure range at temperatures between 323 and 373 K. Volume and enthalpy changes accompanying the phase transitions of 6DBT are also presented. The configurational part of the entropy change at the SA–I transition of 6DBT amounts to ≈40%. Using the pVT data and recently published dielectric relaxation results the isochoric activation energy was calculated (being ≈50% of the activation enthalpy) and is compared with analogous results for 6CB, 7CB and 8CB.

A homologous series of banana-shaped compounds (n=1-12) is described. The short-chain derivatives exhibit a B-6 phase, while B-6/B-1 dimorphism is found for the pentyloxy and hexyloxy homologues. The B-1 phase of the compounds having longer alkyloxy chains is changed to a B-2 phase for the compounds with n=11 and 12. The phase characterization was performed using their optical textures and by means of X-ray investigations. Transition enthalpics were measured using DSC. In addition, density measurements were performed on one selected homologue. The compound n=8 exhibiting a B-1 phase was investigated also by high-pressure DTA.

Abstract The phase diagram of the binary system nC16H34 -nC17H36 has been established at ambient pressure using DSC and crystallographic measurements. At low temperatures below the rotator phase RI there exist two crystal forms Op (about x(C17) = 0.25) and Mdci (about x(C17) = 0.67) which are different from the crystal structures of the pure compounds (Tp for C16 and Oi for C17). Furthermore two compositions: (a) C16/C17 = 3:1 and (b) = 1:2, which correspond to the coexistence range of Op and Mdci, were chosen for high pressure DTA and pVT measurements, yielding the following findings: The specific volume of the rotator phase of C17 is distinctly lower than those of the binary systems at the same state point. Assuming the existence of a metastable rotator phase for C16, an excess volume of Δ VE/V ≈ 0.01 can be estimated. Due to the very enlarged coexistence range of RI, the mixtures reach their lower transition point at considerably lower temperatures (in isobaric measurements) or higher pressures (in isothermal measurements), where the specific volume is lower than that of C17 at its transition point. Furthermore, the volume and enthalpy changes of the Φord -RI transition is distinctly smaller for the binary systems than for pure C17. Thus the specific volumes of the phases Op and Mdci are appreciably larger than ν(spec.) of C17. Op and Mdci have practically the same specific volume in accordance with the crystallographic results. Enthalpy values are obtained with the aid of the Clausius-Clapeyron equation which agree well with enthalpies derived from the DSC measurements. Furthermore, pVT data have been established for the liquid and solid phases of nC18H38 in the neighbourhood of the melting curve, allowing to determine volume and enthalpy changes of melting as a function of pressure.

The pressure-temperature phase diagram of n-octyl-isothiocyanato-biphenyl (8BT) in the pressure range up to 250 MPa (2.5 kbar) and the temperature range 250-400 K was established with the aid of DTA. At 1 atm the substance exhibits exclusively CrE polymorphism. At pressures above 190 MPa, the clearing line splits showing an additional phase which is not yet identified. Dielectric relaxation measurements on the CrE phase of 8BT were performed in the pressure range 0.1-120 MPa and the temperature range 304-345 K. A Debye-type relaxation process was observed in the frequency range 100 Hz-1 MHz. The longitudinal relaxation time tau, characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature, yielding the activation volume, Delta V-#= RT(partial derivative 1n tau /p)(T), and activation enthalpy, Delta (#) H = R(partial derivative 1n tau/partial derivativeT(-1))(p), respectively. The results are compared with analogous data obtained recently for similar compounds having other liquid crystalline phases (N, SmA).

Abstract Densities have been determined for two 5-n-alkyl-2-(4-isothiocyanatophenyl)-1,3-dioxanes (6DBT, 10DBT), 4-n-tetradecyl-4'-cyanobiphenyl (14CB), 4-n-octyl-4'-thiocyanatobiphenyl (8BT) and two laterally aryl-branched mesogens 4-cyanobenzyl 5-(4-n-octyloxybenzoyloxy)-2-undecyloxybenzoate (4CNOUB), 3-cyanobenzyl 2,5-bis(4-n-octyloxybenzoyloxy)benzoate (3CN-BOB). The densities have been measured in the isotropic, nematic (only 3CNBOB) and smectic phases with a vibrating tube densimeter. The densities decrease with increasing alkyl chain length within a homologous series. Furthermore the enthalpy changes accompanying the phase transitions have been determined by DSC.

The dielectric permittivity components, ε∥ and ε⊥, in the nematic phase of 8PCH (trans-4-n-octyl(4-cyanophenyl)cyclohexane) were measured at 1 atm as a function of temperature (T), and at two temperatures as a function of pressure (p). A close similarity of the temperature and pressure behaviours of the dielectric anisotropy, δε = ε∥ - ε⊥, was established. It is argued that p and T are equivalent quantities in the formation of the nematic state. The well known Maier and Meier equations describe the dielectric parameters under both p = constant and T = constant conditions fairly well.