Nematic dielectric properties

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Ordered polymer films made from poly benzthiazole (PBZT) and poly benzoxazole (PBO) can be used as substrates for multilayer printed circuit boards and advanced interconnects to fill the current need for high speed, high density packaging. Foster-Miller, Inc. has made thin substrates (0.002 in.) using biaxially oriented liquid crystal polymer films processed from nematic solutions. PBZT films were processed and laminated to make a substrate with dielectric constant of 2.8 at 1 MHz, and a controllable CTE of 3 to 7 ppm/°C. The films were evaluated for use in multilayer boards (MLBs) which require thin interconnect substrates with uniform controllable coefficient of thermal expansion (CTE), excellent dielectric properties, low moisture absorption, high temperature capability, and simple reliable processing methods. We found that ordered polymer films surpass the limitations of fiber reinforced resins and meet the requirements of future chip-to-chip interconnection. 

The mononuclear, o/ r o-palladated 4,4 -bishexyloxyazoxybenzene acetylacetonate, (255), showed a nematic phase between 90 °C and 105 C, with temperatures very comparable to that of the free azoxy ligand (Cr SON 126 I). This complex, dubbed Azpac, has been subjected to a vast number of studies in order to analyze some specific properties such as electric and dielectric " properties, conductivity, viscoelasticity, and dynamics. ... 

In this chapter a new series of mesomorphic monomers is presented -- the -cyano- -biphenyl (n+2)-alkenoates. Their bulk polymerization in the nematic state has been investigated, and thin layers obtained by thermal polymerization with or without an orienting electric field have been studied. Dielectric properties are discussed also. 

In the nematic phase the typical temperature dependence of dielectric constants and e are given in Fig. 2.4. Similar temperature curves are observed in the chiral nematic phase. The only difference comes from a specific definition of the main dielectric constants in the chiral nematic phase the components e h and e h are taken into consideration, denoting the dielectric properties parallel and perpendicular to the helical axis... 

In this section, we describe the dynamic features of EOM effects. In particular, we focus on the response times to field-on and field-off these are the rise and decay times, respectively. In the case of the EO effects of nematic liquids, the rise and decay times exhibit the characteristic dependencies on voltage, and these characteristic times reflect the elastic, viscous, and dielectric properties of the materials. They have fully characterized the dynamics of the electric-field responses of LMM-LCs in experiments and established the theoretical background [6]. The dynamic features of the EOM effects in nematic gels are expected to differ from those of the EO effects in LMM-LCs, because the gels possess rubber elasticity and also have a different origin of the memory of the initial director. In addition, the dynamic properties give important information about the applicability of EOM effects in practical applications. 

However, the number of liquid crystals that have been studied under pressure is very limited. In most cases neither the equation of state nor the pressure dependence of the order parameter is known. Only the mean-field theory of Maier and Saupe was extended to explain the dielectric properties of liquid crystalline phases. However, a recent approach by Photinos et al. analyzed the nematic reentrance and phase stability based on the variational cluster method. The lack of a full theoretical description as well as insufficient experimental data should stimulate further high-pressure investigations in this field. 

For such materials, which may be smectic or nematic liquid crystals, the dielectric properties of the LC phase are anisotropic. Fora uniaxial LC phase, the dielectric tensor is diagonal such that... 

S. Urban, Static dielectric properties of nematics, in Physical Properties of Liquid Crystals Nematics, D. A. Dunmur, A. Fukuda, and G. R. Luckhurst Eds. lEE, London, 2001, pp. 267-276, and references therein. 

M. Schadt, Dielectric properties of some nematics with strong positive dielectric anisotropy, J. Chem. Phys. 1972, 56, 1494. 

---