M. A. Pérez-Jubindo

Naphthyl benzoates versus phenyl benzoates A molecular structure-liquid crystal and ferroelectric behaviour relationship study

Abstract A comparative study of the liquid crystal and ferroelectric properties of several achiral, racemic and non-racemic 2,6-naphthyl benzoates and their 1,4-phenyl benzoate analogues is reported. In terms of pure compounds, both central core structures give rise to similar mesophase sequences; however, the naphthalene derivatives exhibit broader and more thermally stable liquid crystalline states. On the other hand, both central core units give rise to equivalent electric and optoelectronic properties when the compounds are used as chiral dopants in ferroelectric liquid crystal mixtures.

Dielectric studies on orientationally disordered phases of neopentylglycol ((CH3)2C(CH2OH)2) and tris(hydroxymethyl aminomethane) ((NH2)C(CH2OH)3)

The dielectric spectra up to frequencies of 1 GHz of the substances 2,2-dimethyl-1,3-propanediol (or neopentylglycol, NPG) and 2-amino-2-(hydroxymethyl)-1,3-propanediol (or tris(hydroxymethyl)aminomethane, TRIS) have been determined in the orientationally disordered FCC and BCC phases respectively, in the ranges 305 - 353 K for NPG and 343 - 375 K for TRIS (in this case, in the supercooled BCC orientationally disordered phase). The dielectric relaxation of both substances has been studied by considering modified Debye theories as well as the Jonscher equation. Significant deviations from the single Debye relaxation were found in both cases. The temperature dependence of the relaxation time, assuming Arrh?nius or Eyring activation models, allows one to evaluate of the activation enthalpies and entropies. Large values were obtained for both parameters, showing that there are strong intermolecular interactions via hydrogen bonds, as had been proved previously for solid phases of alicyclic and branched (tetrahedral molecules) alcohols.

Study of the critical behavior and scaling relationships at the N-to-I phase transition in hexyloxycyanobiphenyl.

An exhaustive analysis of the critical behavior of the nematic to isotropic (N-to-I) phase transition on the liquid crystal hexyloxycyanobiphenyl (6OCB) has been performed. To do so, the accurate evolution of various physical magnitudes (static dielectric permittivity data together with specific heat and volumetric determinations) around the N-to-I transition has been required. The specific heat data with the isobaric thermal expansion coefficient and the derivative of the static dielectric permittivity with temperature have been proven to be related to each other by a scaling relationship. However, some discrepancies have been observed for the dielectric data in relation to such a scaling relationship and the critical behavior of the N-to-I phase transition. All information has been used to get some insight on the strength of the first-order N-to-I phase transition of the 6OCB in relation to the other counterparts in the nOCB series of compounds.

Effect of Molecular Flexibility on the Nematic-to-Isotropic Phase Transition for Highly Biaxial Molecular Non-Symmetric Liquid Crystal Dimers

In this work, a study of the nematic (N)–isotropic (I) phase transition has been made in a series of odd non-symmetric liquid crystal dimers, the α-(4-cyanobiphenyl-4’-yloxy)-ω-(1-pyrenimine-benzylidene-4’-oxy) alkanes, by means of accurate calorimetric and dielectric measurements. These materials are potential candidates to present the elusive biaxial nematic (NB) phase, as they exhibit both molecular biaxiality and flexibility. According to the theory, the uniaxial nematic (NU)–isotropic (I) phase transition is first-order in nature, whereas the NB–I phase transition is second-order. Thus, a fine analysis of the critical behavior of the N–I phase transition would allow us to determine the presence or not of the biaxial nematic phase and understand how the molecular biaxiality and flexibility of these compounds influences the critical behavior of the N–I phase transition.

Critical behaviour in liquid-crystalline phase transitions: a comparative study of 9OCB in bulk and Anopore membranes

As a humble contribution to this special commemorative issue to Professor Alfred Saupe, we present an experimental study on the critical behaviour of the nematic-to-isotropic (N–I) and smectic A-to-nematic (SmA–N) phase transitions of the liquid crystal 4-nonyloxy-4′-cyanobiphenyl (9OCB). Measurements of the specific heat and the dielectric permittivity were performed for bulk 9OCB as well as for 9OCB confined to Anopore membranes, and the influence of the slight-restrictive confinement was analysed. The possibility of driving the weakly first-order SmA–N phase transition to second-order in nature by means of the confinement was confirmed.

Influence of cylindrical submicrometer confinement on the static and dynamic properties in nonyloxycyanobiphenyl (9OCB).

Broadband dielectric spectroscopy (10(2)-1.9 x 10(9) Hz) and specific heat measurements have been performed on nonyloxycyanobiphenyl (9OCB) in the isotropic (I), nematic (N), and smectic A (SmA) phases confined to 200 nm diameter parallel cylindrical pores of Anopore membranes. Untreated and HTBA-treated membranes have been found to obtain axial and radial confinements, respectively. However, structural or configurational transitions in untreated membranes have been reported to exist in the SmA-mesophase of 9OCB. Both confinements clearly affect the N-I and SmA-N phase transitions. In the axial confinement, the analysis of the specific heat and static dielectric permittivity data leads to a second order SmA-N phase transition, which is known to be weakly first order for bulk 9OCB. Dynamic dielectric measurements have accounted for the different molecular motions in both confinements. On both mesophases, either N or SmA, the relaxation processes in axial configuration are faster than in the bulk. However, in radial confinement, they are either equal or slower than in the bulk. Additionally, there are no differences in the energy barrier hindering the molecular motions between the axial and radial confinements and even in relation to bulk. Likewise, dielectric results suggest that the extension inside the pores of the surface pinned molecular layer (proved to be temperature-dependent) persists at high enough temperature as a residual-thin layer adjacent to the pore wall.

Relaxation dynamics in orientationally disordered molecular mixed crystal [(CH3)3CCH2OH]0.7[(CH3)2C(CH2OH)2]0.3

The complex dielectric function of the orientationally disordered molecular mixed crystal [(CH3)3CCH2OH]0.7[(CH3)2C(CH2OH)2]0.3 (NPA0.7NPG0.3) covering 11 decades in frequency (10-2-109?Hz) is measured over a broad and continuous temperature range (163-333?K) which is close to the orientational glass transition (160.9?K). For the first time, the dynamics of the orientational disordered (OD), the supercooled orientational disordered and the orientational disordered glass is studied without a temperature breakdown as usually occurs due to the catastrophe of the crystallization. The orientational glass transition in the OD phase exhibits the essential features of the structural glass transition, a non-Arrhenius behaviour for the temperature variation of the relaxation time and a broadening of the width of the relaxation with decreasing temperature. The temperature dependence of the relaxation time is analysed by the empirical Vogel-Fulcher-Tammann law as well as the more recent mode-coupling theory. The suggested increase of the ratio between the crossover temperature Tc and glass transition temperature Tg with decrease of fragility of the system is in accordance with the present observation for this mixed crystal (Tc/Tg?1.45).

Frozen Polarization of Piezoelectric Polyimides

We report a comparative frozen polarization study in three piezoelectric polymers: one polyimide without –CN dipolar groups (poly0CN), and two more that differ in the position of the –CN dipolar group present in the repetition unit (poly2-4 and poly2-6). The values of the dielectric permittivity and frozen polarization show that the participation of N-phenylphthalyimide group depends strongly on the curing process. For totally cured samples this group has little influence on the value of remnant polarization. The study was performed by dielectric measurements and by Thermally Stimulated Depolarization Current (TSDC) technique.

Overall analysis of first‐order phase transitions in some alkyloxycyanobiphenyl liquid crystals

Measurements of pressure, molar volume and specific heat as functions of temperature in the isotropic (I) phase as well as in the smectic A (SmA) and nematic (N) mesophases of some alkyloxycyanobiphenyl compounds (nOCB, n = 6–10) were carried out using differential thermal analysis under pressure, densitometry, X‐ray powder diffraction and modulated differential scanning calorimetry. Thermodynamic properties, such as latent heats and volume jumps at the different phase transitions, were determined. The coherence of this whole set of data was tested using pressure–temperature data through the slopes associated to their phase transitions, extrapolated at normal pressure in the light of the Clausius–Clapeyron equation.

Dielectric Properties of Piezoelectric Polyimides

Polyimides are the most promising amorphous piezoelectric polymers not only because of their excellent thermal, mechanical and dielectric properties, but also and, mainly due to their high glass transition temperatures. Here we report on the synthesis of polyimides with one CN (poly2-6) and two CN groups (poly2CN) in the repetition unit, the latter being synthesised for the first time. The dielectric measurements show that remnant polarisation and piezoelectric coefficients values depend strongly on the number of pendant nitrile dipoles in the repetition unit, on the orientation of the backbone anhydride dipole as well as on the imidisation and the poling processes.Molecular modeling and dielectric measurements are used to identify mechanisms governing piezoelectric behavior in polyimides such as dipole orientation during poling and degree of piezoelectricity achievable. Molecular modeling on polyimides containing pendant, polar nitrile (CN) groups has been completed to determine their remanent polarization. Experimental investigation of dielectric properties evaluated as a function of temperature and frequency has substantiated numerical predictions. With this information, we are able to suggest changes in the molecular structures which will improve the piezoelectric response.