Rheological Theory and Simulation of Surfactant Nematic Liquid Crystals - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 27

decreasing T disrupts fl ow alignment. For lyotropic nematic polymers, fl ow
alignment is dominated by gS (, , ) ( ) ϕγ , such that in the isotropic phase λ > 1 ,

while in the nematic phase at low shear 1. The

actual orientation fi eld is a balance between space-dependent elastic and fl ow
torques, and banded patterns often arise at shear rates less than 1 s − 1 . Thus the
important variables to control fl ow alignment of monomeric nematics is T ,
while for nematic polymers it is γ

λ < 1 and at high shear λ >

.

2.1.4 Diamagnetic Anisotropy

This section is based on the review of diamagnetic susceptibility of lyotropic
liquid crystals by Sonin (1987). Previous reports showed that the nematic
lyophase of composition NaDS-36%, DeOH-7%. D 2 O - 50%. Na 2 SO 2 - 7% has
a positive diamagnetic anisotropy

χ a = χ || − χ ⊥ > 0 (Lawson and Flautt, 1967 ;

Sonin, 1987). Here χ ⊥ are the diamagnetic susceptibilities in the direc-

tion of the director and perpendicular to it, respectively.
Some author established the existence of nematic mesophase having a
negative diamagnetic anisotropy (

χ || and

0) (Radley and Reeves, 1975 ; Radley
et al., 1976). They are formed in the same NaDS-DeOH-H 2 O system, either
upon a small change in the concentrations of the starting components or upon
adding the counterions K*, Li*, or Cs*. Systems having

χ a <

χ a > 0 are called type

I and systems having 0 type II. The hypothesis was advanced (Charvolin

et al., 1979) that the diamagnetic anisotropy in the NaDS-DeOH-H 2 O system
arises from the shape of the micelles: Calamitics have

χ a <

χ a > 0, and discotic

χ a 0. However, the nematic phase in the cesium pentadecafl uorooctanoate

(CsPFO)- H 2 O system with micelles of discotic form has

<

0 Boden et al.
(1979). This contradiction stimulated the study of other systems having poly-
methylene and perfl uoropolymethylene chains (Boden et al., 1981). The former
have negative diamagnetic anisotropy (

χ a >

0).
Consequently, it proved possible to “construct” nematic discotics and calamit-
ics, both with

χ a < 0), and the latter, positive ( χ a >

0. Examples of such systems are presented in
Table 2.2. Thus it was shown that the sign of the diamagnetic susceptibility is
not determined by the type of micelles (Boden et al., 1981). This viewpoint

χ a > 0 and χ a <

TABLE 2.2 Lyotropic Systems Having Different Diamagnetic Anisotropies at a
Temperature of 20°C

χ a Type of Micelles Symbol MTAB r a M T A Φ S O 3 b H 2 O DeOH NH 4 B r

> 0 Cylinder N + 36,0 — 64.0 — —

< 0 Disk N − 26,6 — 63.3 3.8 6.3

Cylinder N − — 38.0 62.0 — —

< 0

N +

> 0 Disk — 14.1 59.2 3.7 7.4

Source : Adapted from Sonin (1987) .
a MTAB r is myristyltrimethylammonium bromide.
b MTA

Φ S O 3 is myristyltrimethyl - ammonium toluensulfonate.