Director configuration in the twist-bend nematic phase of CB11CB

The director distribution in the nematic phases exhibited by the 100,1100-bis(4-cyanobiphenyl-40-yl)undecane (CB11CB) liquid crystal has been studied in the bulk with the EPR spin probe technique. EPR spectra confirmed the presence of an higher temperature uniaxial nematic phase and of a lower temperature nematic phase in which the director distribution is not uniform. Spectra recorded in the lower temperature nematic phase were not fully compatible with theoretical EPR spectra calculated according to the recently proposed model for the twist-bend phase in which the local domain director twists around an axis with a fixed tilt angle, θ₀, but were well described by a “distributed-tilt” model in which the director has a relatively narrow distribution, centred at θ₀

EPR study of the polydomain structure of the twist-bend nematic phase of CB9CB in the bulk

EPR spin probe spectra of the liquid crystal phases exhibited, in the bulk, by the 1″,9″-bis(4-cyanobiphenyl-4’-yl)nonane (CB9CB) bent-shape dimer showed that, on cooling from the isotropicphase, this material forms a uniaxial nematic phase with a uniform director macroscopicallyaligned along the spectrometer field. Upon further cooling, a transition into the twist-bendnematic phase is observed after a biphasic region of approximately 4 K. In this lower temperaturenematic phase, the director does not appear to be macroscopically aligned. The non-uniformdirector distribution is modelled as a collection of monodomains tilted with respect to themagnetic field and orientationally distributed around the tilt direction

Sistemi liquido cristallini complessi: simulazioni al calcolatore e studi ESR

The aim of this PhD thesis was to study at a microscopic level different liquid crystal (LC) systems, in order to determine their physical properties, resorting to two distinct methodologies, one involving computer simulations, and the other spectroscopic techniques, in particular electron spin resonance (ESR) spectroscopy. By means of the computer simulation approach we tried to demonstrate this tool effectiveness for calculating anisotropic static properties of a LC material, as well as for predicting its behaviour and features. This required the development and adoption of suitable molecular models based on a convenient intermolecular potentials reflecting the essential molecular features of the investigated system. In particular, concerning the simulation approach, we have set up models for discotic liquid crystal dimers and we have studied, by means of Monte Carlo simulations, their phase behaviour and self­-assembling properties, with respect to the simple monomer case. Each discotic dimer is described by two oblate Gay­Berne ellipsoids connected by a flexible spacer, modelled by a harmonic "spring" of three different lengths. In particular we investigated the effects of dimerization on the transition temperatures, as well as on the characteristics of molecular aggregation displayed and the relative orientational order. Moving to the experimental results, among the many experimental techniques that are typically employed to evaluate LC system distinctive features, ESR has proved to be a powerful tool in microscopic scale investigation of the properties, structure, order and dynamics of these materials. We have taken advantage of the high sensitivity of the ESR spin probe technique to investigate increasingly complex LC systems ranging from devices constituted by a polymer matrix in which LC molecules are confined in shape of nano- droplets, as well as biaxial liquid crystalline elastomers, and dimers whose monomeric units or lateral groups are constituted by rod-like mesogens (11BCB). Reflection-mode holographic-polymer dispersed liquid crystals (H-PDLCs) are devices in which LCs are confined into nanosized (50­-300 nm) droplets, arranged in layers which alternate with polymer layers, forming a diffraction grating. We have determined the configuration of the LC local director and we have derived a model of the nanodroplet organization inside the layers. Resorting also to additional information on the nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross-section, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi- monodomain aligned along the nanodroplet long axis. The results suggest that the molecular organization is dictated mainly by the confinement, explaining, at least in part, the need for switching voltages significantly higher and the observed faster turn-off times in H-PDLCs compared to standard PDLC devices. Liquid crystal elastomers consist in cross-linked polymers, in which mesogens represent the monomers constituting the main chain or the laterally attached side groups. They bring together three important aspects: orientational order in amorphous soft materials, responsive molecular shape and quenched topological constraints. In biaxial nematic liquid crystalline elastomers (BLCEs), two orthogonal directions, rather than the one of normal uniaxial nematic, can be controlled, greatly enhancing their potential value for applications as novel actuators. Two versions of a side-chain BLCEs were characterized: side­-on and end­-on. Many tests have been carried out on both types of LCE, the main features detected being the lack of a significant dynamical behaviour, together with a strong permanent alignment along the principal director, and the confirmation of the transition temperatures already determined by DSC measurements. The end­-on sample demonstrates a less hindered rotation of the side group mesogenic units and a greater freedom of alignment to the magnetic field, as already shown by previous NMR studies. Biaxial nematic ESR static spectra were also obtained on the basis of Molecular Dynamics generated biaxial configurations, to be compared to the experimentally determined ones, as a mean to establish a possible relation between biaxiality and the spectral features. This provides a concrete example of the advantages of combining the computer simulation and spectroscopic approaches. Finally, the dimer α,ω-bis(4'-cyanobiphenyl-4-yl)undecane (11BCB), synthesized in the "quest" for the biaxial nematic phase has been analysed. Its importance lies in the dimer significance as building blocks in the development of new materials to be employed in innovative technological applications, such as faster switching displays, resorting to the easier aligning ability of the secondary director in biaxial phases. A preliminary series of tests were performed revealing the population of mesogenic molecules as divided into two groups: one of elongated straightened conformers sharing a common director, and one of bent molecules, which display no order, being equally distributed in the three dimensions. Employing this model, the calculated values show a consistent trend, confirming at the same time the transition temperatures indicated by the DSC measurements, together with rotational diffusion tensor values that follow closely those of the constituting monomer 5CB

Nematic Director Configuration, Local Order and Microviscosity in a PSLC Cell

The director configuration, the local order and the molecular-level reorientational dynamics (microviscosity) of the 5CB liquid crystal (LC) inside a polymer-stabilized LC (PSLC) cell prepared with the 4,4-bis(6-(acryloyloxy)hexyloxy)biphenyl (BAB6) diacrylic monomer have been studied with the EPR spin probe technique across the nematic and the isotropic phase of the LC. The cells made from glass slides, either clean or coated with an aligning layer, were filled with pure 5CB or with 0.5, 1.0 or 2.0 wt% of BAB6, either in the monomeric state or polymerized. Local orientational order parameter () in the non treated cells was always slightly lower than in the coated ones and both were lower than in the bulk LC. Order appeared to be independent of polymerization state or BAB6 concentration up to 1.0 wt%. Reorientational dynamics was essentially bulk-like in all cases, indicating that in our PSLC cells a full molecular mobility is retained. At the concentration of 2.0 wt%, in the coated cell with polymerized BAB6, the monodomain did not reform after a 90 • rotation of the cell in the magnetic field, suggesting that the polymer network is, as expected, stabilizing the preexisting director configuration

Doping liquid crystals with nanoparticles. A computer simulation of the effects of nanoparticle shape

We have studied, using Monte Carlo computer simulations, the effects that nanoparticles of similar size and three different shapes (spherical, elongated and discotic) dispersed at different concentrations in a liquid crystal (LC), have on the transition temperature, order parameter and mobility of the suspension. We have modelled the nanoparticles as berry-like clusters of spherical Lennard-Jones sites and the NP with a Gay\u2013Berne model. We find that the overall phase behaviour is not affected by the addition of small amounts (xN = 0.1\u20130.5%) of nanoparticles, with the lowest perturbation obtained with disc-like nanoparticles at the lowest concentration. We observe a general decrease of the clearing temperature and a reduction in the orientational order with a change in its temperature variation, particularly in the case of the xN = 0.5% dispersions and with a more pronounced effect when the nanoparticles are spherica

Director configuration in the twist-bend nematic phase of CB11CB

The director distribution in the nematic phases exhibited by the 100,1100-bis(4-cyanobiphenyl-40- yl)undecane (CB11CB) liquid crystal has been studied in the bulk using the EPR spin probe technique. EPR spectra confirmed the presence of a higher temperature uniaxial nematic phase and of a lower temperature nematic phase in which the director distribution is not uniform. Spectra recorded in the lower temperature nematic phase were not fully compatible with theoretical EPR spectra calculated according to the recently proposed model for the twist-bend phase in which the local domain director twists around an axis with a fixed tilt angle, y0, but were well described by a ‘‘distributed-tilt’’ model in which the director has a relatively narrow distribution, centred at y_0