arXiv subjects

Nerea Sebastián

Publications and source records attributed to Nerea Sebastián.

22 records · Page 2Linked to original sources

arXiv2019

Ferroelectric-ferroelastic phase transition in a nematic liquid crystal

Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes formation of recently reported splay nematic liquid-crystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation (SHG) imaging, which additionally allowed for determination of the splay modulation period to be of the order of 5 - 10 microns, also confirmed by polarized optical microscopy. The observations can be quantitatively described by a Landau-de Gennes type of macroscopic theory.

cond-mat.soft↗

arXiv2018

Splay nematic phase

Different liquid crystalline phases with long-range orientational but not positional order, so-called nematic phases, are scarce. It rarely occurs that a new nematic phase is discovered and such event is inevitably accompanied by a great interest. Here, we describe a transition from uniaxial to novel nematic phase characterized by a periodic splay modulation of the director. In this new nematic phase, defect structures not present in the uniaxial nematic are observed, which indicates that the new phase has lower symmetry than the ordinary nematic phase. The phase transition is weakly first order with a significant pretransitional behavior, which manifests as strong splay fluctuations. When approaching the phase transition, the splay nematic constant is unusually low and goes towards zero. Analogously to the transition from the uniaxial nematic to the twist-bend nematic phase, this transition is driven by instability towards splay orientational deformation, resulting in a periodically splayed structure. And, similarly, a Landau-de Gennes type of phenomenological theory can be used to describe the phase transition. The modulated splay phase is biaxial and antiferroelectric.

cond-mat.soft↗

arXiv2018

Interplay between structure and relaxation in polyurea networks: the point of view from a novel method of cooperativity analysis of dielectric response

The influence of structural constraints on the relaxation dynamics of three polyurea networks with varying degree of crosslinking, has been studied by means of a thorough analysis of broadband dielectric spectroscopy measurements. Two different relaxation processes are observed, namely, a fast process involving the soft poly(propylene oxide) chains, and a slower and much broader process associated to the immediate surroundings of the hard crosslinkers. Microphase separation in soft and hard domains characterizes the systems in the presence of hydrogen bondings. In this case, different confinement conditions are explored by varying the soft chain length; overall, the so called 'adsorption' effects dominate. With respect to both cooperativity and rearrangement energy threshold in fast relaxation, it is found that the enhancement of configurational constraints is similar to cooling, but only on qualitative grounds. An upper bound of the hard domain's interface thickness, in which the slow relaxation is believed to take place, is estimated from the analysis of the fast relaxation in the system characterized by the highest degree of confinement, taking into account the results of the structural analysis. Dropping the hydrogen bonding mechanism, phase separation does not occur anymore and the configurational constraints at the ends of the soft chains are reduced, leaving just those imposed by the rigid crosslinkers. This leads to a significant increase in cooperativity, on approaching the glass transition, and to a complex behavior that is thoroughly discussed in comparison with those observed in the micro-segregated systems.

cond-mat.soft↗

arXiv2018

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic odd liquid crystal dimer

In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.Py shows three different dielectric relaxation modes and two glass transition (Tg) temperatures: the higher Tg is due to the freezing of the molecular motions responsible of the relaxation mode with the lowest frequency (μ1L); the lower Tg is due to the motions responsible of the two relaxation modes with highest frequencies (μ1H and μ2), which converge just at their corresponding Tg. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (μ1L and μ1H) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (μ2) is cooperative just below some cross-over temperature. In terms of fragility, at the glass transition, the ensemble (μ1H +μ2) presents a value of the steepness index and μ1L a different one, meaning that fragility is a property intrinsic of the molecular motion itself. Finally, the steepness index seem to have a universal behavior with temperature for the dielectric relaxation modes of liquid crystal dimers, being almost constant at high temperatures and increasing drastically when cooling the compound down to the glass transition from a temperature about (3/4)TNI.

cond-mat.soft↗