Calamitic liquid crystal phases

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Figure 5.1 Liquid crystal phases formed by rod-like (calamitic) molecules.

The liquid crystalline properties of 43a-d were interesting as all derivatives showed very stable mesophases with phase widths of 57-109 K. It is remarkable that Schiff bases 43c,d are stable at temperatures above 300 °C. It also strikes that, in contrast to conventional calamitic liquid crystals, the Schiff bases 43c,d possess significantly higher phase transition temperatures compared to azo linked 43a,b. Additionally, the trans compounds tend to possess higher clearing temperatures due to the elongated shape of the molecules. 

Calamitic phases are essentially defined as a set of mesophases that occur between the breakdown of the long-range periodic, translational ordering of the crystal and the loss of long range orientational order at the transition to the isotropic liquid. In this context six mesophases exist the nematic phase and five smectic phases labelled A, B, C, F and I. Other closely related soft crystal phases, in which the molecules have long-range periodic order but are themselves rotationally disordered, also exist in concert with liquid crystal phases. This second set of mesophases are labelled E, J, G, H, and K, and are essentially the crystal versions of the... 

Summary Cyclic siloxanes substituted with mesogenic groups, which are connected to the backbone by aliphatic spacers, exhibit liquid crystalline (LC) phases as the classic calamitic liquid crystals. 

It is evident in Fig. 25b that the ratio has a value close to 3 at high temperatures (T > 1.0) and declines steadily below T 1.0 until it reaches a value nearly equal to unity at low temperatures. While the Debye model of rotational diffusion, which invokes small steps in orientational motion, predicts the ratio ti/t2 to be equal to 3, a value for this ratio close to 1 is taken to suggest the involvement of long angular jumps [146, 147]. The ratio was observed to deviate from the Debye limit at lower temperatures in a recent molecular dynamics simulation study as well [148]. The onset temperature was thus found to mark the breakdown of the Debye model of rotational diffusion [145]. Recently, the Debye model of rotational diffusion was also demonstrated to break down for calamitic liquid crystals near the I-N phase boundary due to the growth of the orientational correlation [149]. 

Goodby J W 1998 Phase structures of calamitic liquid crystals Handbook of Liquid Crystals Vol 2A. Low Molecular Weight Liquid Crystals I ed D Demus, J Goodby, G W Gray, H-W Spiess and V Vill 1998 (New York Wiley-VCH)... 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

This experimental difficulty was overcome by simulating the behaviour of a model system and a biaxial nematic phase was indeed obtained [122], One way by which the problem of phase separation could be solved experimentally was suggested by Fletcher and Luckhurst and involved covalently linking a rod-like and disc-like unit via a flexible spacer yielding a non-symmetric discotic-calamitic liquid crystal dimer [123] ... 

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