PBLG solutions

FIG. 16.33 Steady shear flow results for 12% PBLG solution in m-cresol at 293K (O) viscosity (A) positive N, (A) negative Nv Reproduced with permission from Mewis J and Moldenaers P (1987) Mol Cryst Liq Cryst 153, 291. Copyright Taylor and Francis Ltd., http //www.informaworld.com. 

The chirality of the pretransition phase is opposite to that of the dilute solution. As the temperature increases, ORD curves reverse sense from predominantly positive to negative for PBLG solutions, and from predominantly negative to positive for PBDG solutions. This effect is observed in dioxane, DMF, tetrahydrofuran, 1,1,2,2-tetrachloroethane, and 1,3-dioxolane. 

The results for five sorts of PBLG solutions are shown in Fig. 13, where L was assumed to be 1370 A for the solute polymer with molecular weight 200,000, and the value of 7,A/D adjusted independently of the concentration for each solvent. Throughout all cases, the theoretical curves reproduce the observed concentration dependence, and the estimates of X.A/D of the order 10seem to be reasonable. 

It is important to note that the theoretical results obtained by Flory about the dependence of the critical concentrations v and vj on x are in good agreement with experimental data. It is sufficient to remember, as an example, the results obtained by Flory for PBLG solutions in dioxane (Fig. 4). The discrepancy between the experimental results (solid curves) and theoretical calculations (dashed curves) looks quite natural on the account of a number of assumptions made when deriving the equation for the free energy on the basis of the lattice model (see also ). 

Aikawa et al. considered the effect of electric field on the phase transition in solutions of rigid-chain polymers for a PBLG solution in dioxane. Theoretical calculations have shown that the application of an electric field must shift the values of v towards lower concentrations. This conclusion was confirmed in experiments. According to the results obtained by Patel et al the application of electric fields also causes a shift in the temperature of the liquid crystalline transitions. 

Quantitative agreement between theory and the rheological behavior of PBLG solutions is not obtained, even when polydispersity is accounted for by the Marrucci-Grizzuti theory. In particular, Larson and Mead (1991) found that the Marrucci-Grizzuti... 

In lyotropic LCPs, the shear-rate range over which Region I exists is sensitive to polymer concentration (Walker and Wagner 1994). In one batch of PBLG, Region I appears suddenly as the polymer concentration is increased above about 37% (see Fig. 11-9). In other PBLG solutions. Region I behavior has been reported at much lower concentrations... 

Steady-state shear viscosity versus shear rate for PBLG solutions (molecular weight = 238,000) in m-cresol for several concentrations. The 38 wt% and 40 wt% samples show Region I behavior. (From Walker et al. 1995, with permission of the Journal of Rheology.)... 

However, in Region III—which includes shear rates above the first, positive maximum in TV —the agreement between theory and experiment for PBLG solutions is very good,... 

In addition, measurements of orientation after cessation of shearing show that flow-induced orientation decreases after cessation of shearing toward an isotropic state of orientation, and the dynamic moduli increase with time (Hongladarom et al. 1994). This behavior is seen in PBLG solutions only under conditions where a pronounced Region I is seen for lower concentrations where only Region II is present, the orientation increases after cessation of shearing (Walker et al. 1995). 

Figure 6.5. The pitch P/Po vs. magnetic field H/Hc for 20% PBLG solution in dioxane. (Modified from Duke and DuPre, 1974b.)...

PBLG solution shows the finger-print texture, seen in Figure 6.20. A pair of neighboring dark-bright stripes corresponds to a half period of the helix, P/2, which is about a few microns. It is a practical means of estimating the pitch of cholesteric liquid crystals. 

Recently DuPre et al reported that, S increases linearly with temperature rise. Qualitatively, their results are consistent with ours. However, the time required to reach the equilibrium pitch, varied with temperature, the concentration of polymer, and also the thermal history. For PBLG solution in dichloro-ethane (EDC), which concentration is 0.12 vol/vol, the variation of pitch with time was measured at a constant temperature by T-jump method. Fig. 1 shows the time dependence of cholesteric pitch by the T-jump method from -2°C to +30°C, 40°C and 50°C respectively. It is clear that the time required to arrive at the equilibrium pitch is shorter at higher temperature but is still over several hours. Therefore, the equilibrium pitch must be measured after prolonged aging at each measuring temperature. It was found that the... 

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