Hysteresis loop, liquid crystals

MHPOBC, ii--(l-methylheptyl oxycarbony )phenyl 4 -octyloxybiphenyl-k-carboxylate (LB Number 71B-1 (A)) (Liquid Crystai). Seven phases are known for this liquid crystal. It exhibits an antiferroelectric hysteresis loop at low frequency, as shown in Fig. 4.5-96a, between... 

The porous structure of MCM-41 materials was characterized by their BET surface area, mesopore volume and diameter (Tab.1 and 2). The mesopore volume Vme was obtained as the adsorption (converted to liquid volume) corresponding to the filling of the MCM-41 porous system proper. The external surface area of crystals was estimated from the desorption branch of the hysteresis loop as the surface area of pores larger than the MCM-41 pores proper. The size (diameter) of mesopores was calculated using a tubular model as 4V S where S stands for the BET surface area minus the external surface area. 

The ceramic materials used consisted of lead zirconate—lead titanate mixtures. These materials have relatively square hysteresis loops and can be prepared in large thin plates. Tlie thickness of the ceramic layer was of the order of 100 jum, that of the liquid crystal about 10 Mm. With a thickness of about 50 jum, these ceramic mixtures have a sufficient transparency, so that a matrix which also operates in transmitted light can be realized. 

Presumed ferroelectric effects in liquid crystals were reported by Williams at RCA in Princeton, U. S. A., as early as 1963, and thus at the very beginning of the modern era of liquid crystal research [5]. By subjecting nematics to rather high dc fields, he provoked domain patterns that resembled those found in solid ferroelectrics. The ferroelectric interpretation seemed to be strengthened by subsequent observations of hysteresis loops by Kapustin and Vistin and by Williams and Heilmeier [7]. However, these patterns turned out to be related to electrohydrodynamic instabilities, which are well understood today (see, for instance, [8], Sec. 2.4.3 or [9], Sec. 2.4.2), and it is also well known that certain loops (similar to ferroelectric hysteresis) may be obtained from a nonlinear lossy material (see [10], Sec. 2.4.2). As we know today, nematics do not show ferroelectric or even polar properties. In order to find such properties we have to lower the symmetry until we come to the tilted smectics, and further lowering their symmetry by making them chiral. The prime example of such a liquid crystal phase is the smectic C. ... 

---