P. Cusmin

Bio: P. Cusmin is an academic researcher from Polytechnic University of Catalonia. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 4, co-authored 6 publications receiving 94 citations.

Thermodynamic assessment of smectic A-nematic tricritical points through the equal gibbs energy analysis.

Abstract: A complete thermodynamic analysis on four two-component phase diagrams between liquid crystals belonging to the nCB and nOCB series of compounds, the so-called cyanobiphenyls, has been performed through the Oonk's equal Gibbs energy method. The binary systems dealt with in this paper show as a common feature the existence of a tricritical point at the SmA-to-N phase transition, all of which reported elsewhere. As a singular finding of the work proposed in this paper, the Oonk's method is extended in order to account for second-order phase transitions. Likewise, this extension allows performing calculations of the tricritical temperature.

Dielectric, calorimetric and mesophase properties of 1′′-(2′,4-difluorobiphenyl-4′-yloxy)-9′′-(4-cyanobiphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase

Abstract: This paper reports a novel liquid crystal phase having the characteristics of a twist-bend nematic phase formed by a non-symmetric ether-linked liquid crystal dimer. The dimer 1''-(2',4-difluorobiphenyl-4'-yloxy)-9''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB) exhibits two liquid-crystalline phases on cooling at a sufficiently high rate from the isotropic phase. The high temperature mesophase has been reported in the literature as nematic and confirmed in this study. The other mesophase is metastable and can be supercooled giving rise to a glassy state. Its identification and characterization are based on optical textures, broadband dielectric spectroscopy, calorimetry, measurements of both splay and bend elastic constants in the nematic phase and miscibility studies. It is concluded that the low temperature mesophase exhibits the characteristics of a twist-bend nematic phase. Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the isotropic phase, in the nematic mesophase and across the isotropic-to-nematic phase transition. Two orientations, parallel and perpendicular to the director, have been investigated. In the high temperature nematic mesophase, the dielectric anisotropy is found to be positive. Measurements of the parallel component of the dielectric permittivity are well-explained by the molecular theory of dielectric relaxation in nematic dimers (M. Stocchero, A. Ferrarini, G. J. Moro, D. A. Dunmur and G. R. Luckhurst, J. Chem. Phys., 2004, 121, 8079). The dimer is modelled as a mixture of cis and trans conformers and the model allows an estimate of their relative populations at each temperature. The nematic-to-isotropic phase transition has been exhaustively studied from the accurate evolution of the heat capacity and the static dielectric permittivity data. It has been concluded that the transition is first order in nature, but close to tricritical. The nature of the nematic-to-the novel liquid crystal phase transition is difficult to analyze to the same extent because of insufficient precision. Only observations at cooling rates of 10 K min(-1) or higher were possible because on heating from the glassy state, the twist-bend nematic mesophase crystallizes at temperatures far below the nematic-nematic phase transition.

Dielectric and Thermodynamic Study on the Liquid Crystal Dimer α-(4-Cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py)

Abstract: Broadband dielectric spectroscopy (103 to 1.8 × 109 Hz) and specific heat measurements have been performed on the odd nonsymmetric liquid crystal dimer α-(4-cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py), as a function of temperature. The mesogenic behavior is restricted to a nematic mesophase which can be supercooled down to its corresponding glassy state if the cooling rate is fast enough (no less than 15 K·min–1). Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the nematic mesophase as well as in the isotropic phase and across the isotropic-to-nematic phase transition. Two orientations (parallel and perpendicular) of the molecular director with regard to the probe electric field have been investigated. In the nematic mesophase, the dielectric anisotropy is revealed to be positive. Measurements of the parallel component of the dielectric permittivity are well explained by means of the molecular theory of di...

Phase and dielectric behaviors of a polymorphic liquid crystal doped with graphene nanoplatelets

Abstract: We report on the phase behavior and dielectric properties of the liquid crystal (LC) 4′-n-octyloxy-4-cyanobiphenyl dispersed with graphene nanoplatelets (GNPs). The temperature-dependent dielectric permittivity at 104 Hz and its derivative with respect to the temperature reveal that the incorporation of GNPs in a LC cell leads to the modification of crystalline polymorphism and shift in phase transition temperature owing to the enhanced positional and orientational order. Additionally, the dielectric data between 1 and 103 Hz show that the dopant reduces the ionic concentration and alters the diffusivity in the LC mesophases.

Critical behavior of a nonpolar smectogen from high-resolution birefringence measurements.

Abstract: We report high-sensitivity and high-temperature resolution experimental data for the temperature dependence of the optical birefringence of a nonpolar monolayer smectogen 4-butyloxyphenyl-4'-decyloxybenzoate (10[over ¯].O.4[over ¯]) liquid crystal by using a rotating-analyzer technique. The birefringence data cover nematic and smectic-A phases of the 10[over ¯].O.4[over ¯] compound. The birefringence data are used to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions. For the N-I transition, from the data sufficiently far away from the smectic-A phase, the average value of the critical exponent β describing the limiting behavior of S(T) is found to be 0.2507±0.0010, which is in accordance with the so-called tricritical hypothesis, which predicts β=0.25 and excludes higher theoretical values. The critical behavior of S(T) at the N-I transition is discussed in detail by comparing our results with the latest reports in the literature and we conclude that by comparing with the previously reported results, the isotropic internal field assumption by the Vuks-Chandrasekhar-Madhusudana model is adequate to extract the critical behavior of S(T) from the optical birefringence data. We observe that there is no discontinuous behavior in the optical birefringence, signaling the second-order nature of the N-SmA transition. The effect of the coupling between the nematic and smectic-A order parameters on the optical birefringence near the N-SmA transition is also discussed. In a temperature range of about 4K above and below the N-SmA transition, the pretransitional evidence for the N-SmA coupling have been detected. From the analysis of the optical birefringence data above and below the N-SmA transition by means of various fitting expressions we test the validity of the scaling relation λ=1-α between the critical exponent λ describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent α. We then show that the temperature derivative of the nematic order parameter S(T) near T(NA) exhibits the same power-law divergence as the specific heat capacity with an effective critical exponent of 0.2303±0.0035.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB-N phase transition.

Abstract: We report a calorimetric study of a series of mixtures of two twist-bend liquid crystal dimers, the 1′′,7′′-bis(4-cyanobiphenyl)-4′-yl heptane (CB7CB) and 1′′-(2′,4-difluorobiphenyl-4′-yloxy)-9′′-(4-cyanobiphenyl-4′-yloxy) nonane (FFO9OCB), the molecules of which have different effective molecular curvatures. High-resolution heat capacity measurements in the vicinity of the NTB–N phase transition for a selected number of binary mixtures clearly indicate a first order NTB–N phase transition for all the investigated mixtures, the strength of which decreases when the nematic range increases. Published theories predict a second order NTB–N phase transition, but we have developed a self-consistent mean field Landau model using two key order parameters: a symmetric and traceless tensor for the orientational order and a short-range vector field which is orthogonal to the helix axis and rotates around of the heliconical structure with an extremely short periodicity. The theory, in its simplified form, depends on two effective elastic constants and explains satisfactorily our heat capacity measurements and also predicts a first-order NTB–N phase transition. In addition, as a complementary source of experimental measurements, the splay (K1) and bend (K3) elastic constants in the conventional nematic phase for the pure compounds and some selected mixtures have been determined.