Concentration dependence, pitch

In further temperature and concentration-dependent measurements of the helical pitch, only mixtures with formamide were chosen as the lengthy evolution time necessary for mixtures with water together with the ever present threat of solvent evaporation make such investigations of mixtures with water much more complicated. In Fig. 5.33 the helical pitch p is plotted versus the reduced temperature T - Tq for a sample with 18 wt% of formamide. The pitch shows the typical temperature dependence known from thermotropic SmC phases [30]. Right after the phase transition into the lyotropic SmC analog phase, the pitch increases rapidly to a value of about 5.5 pm and decreases more slowly towards a low temperature value of about 2.5 pm. However, by repeating the measurement with other concentrations of formamide, no significant difference in the value of p could be detected. 

Since the microscopic structure of nematics and cholesterics is similar, these two phases are mixed like two nematics. Thus the thermodynamic behavior of nemato-cholesteric mixtures needs no special discussion (see Section 1.2.1). What is of interest, especially for liquid crystalline materials for technological applications, is the concentration dependence of the helical pitch and the physical parameters relevant to this dependence. 

Analytical approaches to understanding the effect of molecular flexibility on orientational order have concentrated on both the isotropic-nematic and the nematic-smectic transition [61, 62] and mean field theory has shown that cholesteric pitch appears not to depend on the flexibility of the molecule [63]. 

The structures shown in Fig. 4-1 are for B-form DNA, the usual form of the molecule in solution. Different double-helical DNA structures can be formed by rotating various bonds that connect the structure. These are termed different conformations. The A and B conformations are both right-handed helices that differ in pitch (how much the helix rises per turn) and other molecular properties. Z-DNA is a left-handed helical form of DNA in which the phosphate backbones of the two antiparallel DNA strands are still arranged in a helix but with a more irregular appearance. The conformation of DNA (A, B, or Z) depends on the temperature and salt concentration as well as the base composition of the DNA. Z-DNA appears to be favored in certain regions of DNA in which the sequence is rich in G and C base pairs. 

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

When the time dependence takes the form of a periodic perturbation of some parameter, we speak of this as periodic forcing.19 The response will obviously not be a steady state, but can be periodic, quasi-periodic, or chaotic. If the response is periodic, it may be with a period that is a multiple of the period of forcing. It is quasi-periodic if the response winds itself onto the cylinder in a helix whose pitch is an irrational multiple of the forcing period, so that it is never quite truly periodic. An example20 of a forced system is the Gray-Scott autocatalator with the feed concentration sinusoidally perturbed ... 

At temperatures above the softening point, isotropic pitch often displays Newtonian flow characteristics (18,19), but this may well depend upon the concentration of any insoluble particles (i.e., primary QI in the case of coal tar based materials) present within the pitch. A high concentration of QI could lead to non-Newtonian character as a result of the particle-particle attractive forces. Figure 3 shows n -T curves for a variety of pitch materials and their pyrolysis products. Pyrolysis increases the Tg of the system and shifts the viscosity-temperature curve to higher temperatures. 

Figure 9. Pitch size dependence composition cellulose concentration, DP, 210 storage time at 25 C, 17 days.

Cellulose based mesophases have displayed a somewhat steeper inverse dependence of pitch to concentration than that reported by Toriumi (29). A value of x = 3 has been reported for hydroxypropyl cellulose in water (31), for acetoxypropyl cellulose in acetone (32) and for hydroxypropyl cellulose in acetic acid and in methanol (28). Our data for cellulose in NH3/NH4SCN show a reasonably straight line when p l/3 was plotted vs. cellulose concentration (Figure 11), which is in accord with earlier reports (28,29,31,32). 

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