arXiv subjects

George Cordoyiannis

Publications and source records attributed to George Cordoyiannis.

3 recordsLinked to original sources

arXiv2026

The effect of side chain length on the mesomorphic properties of ferroelectric nematogens

The discovery of ferroelectric nematic phase opened new directions in the field of soft matter chemistry and physics. In this paper, we have modified a previously reported ferroelectric nematogen based on molecular structure with dimethylamino-terminated part. We have prepared two new homologues by prolonging a side chain and investigated the effect of its length on the mesomorphic properties. While previously reported homologues exhibited a direct phase transition from the isotropic (Iso) to the ferroelectric nematic phase (NF), for prolonged alkyl chain there is a narrow nematic phase (N) in between. For new compounds, we have established their mesomorphic and material properties. We have confirmed ferroelectricity of the NF phase by the polarization, SHG signal and permittivity. Finally, we have compared these parameters with respect to the side-chain length and discussed new effects, discovered for prolonged homologues due to the presence of an additional N phase.

cond-mat.soft↗

arXiv2024

Calorimetric evidence for the existence of an intermediate phase between the ferroelectric nematic phase and the nematic phase in the liquid crystal RM734

The idea that rod-like molecules possessing an electric dipole moment could exhibit a ferroelectric nematic phase was suggested more than a century ago. However, only recently such a phase has been reported for two quite different liquid crystals: RM734 (4-[(4-nitrophenoxy)carbonyl)]phenyl 2,4-dimethoxybenzoate) and DIO (2.3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2.6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate). For RM734 a direct ferroelectric nematic (NF) to classical nematic N transition was reported, whereas for DIO an intermediate phase Nx was discovered between the NF and the N phases. Here we present high-resolution calorimetric evidence that an intermediate Nx phase also exists in RM734 along a narrow temperature range between the NF and the N phases.

cond-mat.soft↗

arXiv2022

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry

Adiabatic scanning calorimetry (ASC) is capable of providing simultaneously the specific enthalpy h(T) and the specific heat capacity cp(T), and is an important tool to determine the order of transitions and to render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound DIO and on mixtures with RM734. Both compounds exhibit a low-temperature ferroelectric nematic phase (NF) and a high-temperature paraelectric nematic (N). In DIO these two phases are separated by an intermediate phase (Nx). Detailed data of h(T) and cp(T), indicated that the intermediate phase was present in all mixtures over the complete composition range, albeit with strongly decreasing temperature width with decreasing mole fraction of DIO (xDIO). The xDIO dependence of the two transitions could be well described by a quadratic function and both transitions were weakly first order. The true latent heat of the Nx-N transition of DIO was as low as 0.0075 +/- 0.0005 J/g and 0.23 +/- 0.03 J/g for the NF-Nx transition, about twice the previously reported value of 0.115 J/g for the NF-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing xDIO. At all the Nx-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the NF to the higher temperature phase exhibits substantial pretransitional behaviour. Power law analysis of cp(T) resulted in an effective critical exponent of 0.88 +/- 0.10 for DIO and this value decreased in mixtures with decreasing xDIO towards 0.50 +/- 0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior for the total transition enthalpy change

cond-mat.soft↗