George S. Attard

Molecular dynamics of liquid crystalline side-chain polymers: The dielectric relaxation behaviour of a siloxan polymer in the nematic and isotropic phases

Abstract The dielectric relaxation behaviour of a liquid crystalline side-chain polymer having a siloxan backbone to which is attached an aromatic ester group via a C 5 methylene spacer has been studied in the nematic and isotropic phases. One process was observed in the isotropic phase and was characteristic of that of a glass-forming liquid. A composite process was observed in the liquid crystalline phase. In the transition range the dielectric behaviour changed continuously from that of the liquid crystal to the isotropic material. The characteristic features of the relaxations are described and their mechanisms are discussed in relation to earlier dielectric data for small molecule nematogens and for liquid crystalline side-chain polymers based on acrylate and methacrylate monomers.

Molecular dynamics of a smectic liquid-crystalline side-chain polymer in its unaligned and aligned states as studied by dielectric relaxation spectroscopy

Abstract Dielectric relaxation data are presented for unaligned, partially aligned and aligned thin films of a smectic liquid-crystalline side-chain polymer in the ranges 12 to 105Hz and 299 to 343 K. The unaligned material exhibits a complicated dielectric absorption which has been resolved into two broad components (δ and α processes). The dielectric absorption changes remarkably on alignment of the material such that the δ process is enhanced by a factor of three and the α process is considerably diminished. A consideration is given of the molecular theory for dielectric relaxation in these materials and the variation in dielectric properties is shown to depend on the dipole components μml∥ and μml⊢, the order parameter S for the mesogenic head groups and the order parameter S d for the director alignment.

Molecular dynamics and macroscopic alignment properties of thermotropic liquid-crystalline side chain polymers as studied by dielectric relaxation spectroscopy

Abstract It is shown that dielectric relaxation spectroscopy provides a convenient means of studying the anisotropic reorientational dynamics of the mesogenic head groups in thermotropic liquid-crystalline side chain polymers. Their alignment behaviour in directing a.c. electric fields of different amplitudes and frequencies is examined, and samples having a macroscopic alignment which is fully homeotropic, fully planar or any desired intermediate alignment have been prepared. The nature and extent of alignment in such samples has been determined by dielectric spectroscopy. In addition both the temperature and pressure variations of the average dielectric relaxation times for certain relaxation processes have been determined and a bulk alignment phenomenon in the absence of a directing electric field is reported.

Dielectric relaxation in the planarly-aligned nematic phase of a siloxane side-chain polymer Comparison of theory with experiment

Abstract We show that the dielectric loss spectra of the planarly-aligned nematic phase of a liquid-crystalline siloxane side-chain polymer consist of a single broad curve, as predicted by theory.

Pretransitional behaviour in nematogenic mixtures A molecular field theory

Abstract The isotropic phase of nematogenic liquid crystals exhibits strong pretransitional behaviour which diverges rapidly as the transition to the nematic phase is approached. Similarly nematogenic mixtures also show pronounced pretransitional effects although these can be dramatically reversed on the formation of the biphasic regime when isotropic and nematic phases coexist. Here we develop a theory for such effects using approximations taken from previous molecular field theories of nematogenic mixtures. In particular we consider the composition dependence of the divergence temperature for binary nematogenic mixtures and its relation to those of the pure components. A variety of mixtures with particles of different shape are treated, namely rods and spheres, laths and spheres, rods and rods, as well as rods and laths. We also predict the temperature dependence of the field-induced order parameters, often used to monitor pretransitional behaviour, for certain mixtures in both the homogeneous isotropic...

Experimental observation of a novel phenomenon in the biphasic region of a nematogenic siloxane polymer: the recovery of homeotropic alignment in the absence of external fields

Abstract It is shown that cooling a partially homeotropically aligned liquid crystalline siloxane polymer from the biphasic region into the liquid crystalline (l.c.) region leads to homeotropic alignment of the phase in the absence of a directing electric field. Partially aligned samples were prepared either by heating a homeotropic sample into the biphasic range or by cooling an isotropic sample into the biphasic range in the presence of an a.c. directing electric field. The alignment recovery was monitored using wide frequency dielectric relaxation spectroscopy. The extent of recovery was dependent on the cooling rate and on the number of thermal cycles made to fixed temperatures in the biphasic and l.c. states. The origin of the recovery process is assigned to a ‘template crystallization’ of molecules on pre-existing aligned l.c. phase, and its relevance to the formation of aligned l.c. polymer material in applied a.c. electric fields is discussed.

Dielectric relaxation spectroscopy studies on a polymer with mesogenic side chains

Abstract The dielectric relaxations and the optical transmission of a carefully purified liquid crystalline side chain polysiloxane have been measured in the isotropic and the nematic phases, and an apparent hysteresis has been found in the biphasic region during cooling and warming cycles. In the biphasic region the rate of mesophase formation during cooling was found to be accelerated by an aligning field. Planar alignment was achieved, and the pure α process observed. The sample formed a gel upon lengthy heating, but it remained a liquid crystal. In the mesophase of the gel the α relaxation process was restricted in amplitude and the δ process was absent.

Template-induced recovery of homeotropic alignment in the biphasic region of a nematogenic siloxane polymer: a simple phenomenological model

Abstract A simple empirical theory is described which models the partial melting behaviour of a homeotropically aligned film of liquid crystalline side-chain polymer and its subsequent thermal cycling through the biphasic region.