Nematic phase rheology

In addition to their unusual rheological properties, the nematic phases of polymers, like those of simple compounds, can be oriented by the application of magnetic or electrical fields. These properties have been more fully examined for comb-type polymers with mesogenic side-chains than for polymers with the mesogenic groups in the main chain, since in the comb polymers it is possible to influence the side-chain orientation independently of the main-chain orientation. 

The three elastic constants are the Frank elastic constants, called after Frank, who introduced them already in 1958. They originate from the deformation of the director field as shown in Fig. 15.52. A continuous small deformation of an oriented material can be distinguished into three basis distortions splay, twist and bend distortions They are required to describe the resistance offered by the nematic phase to orientational distortions. As an example, values for Miesowicz viscosities and Frank elastic constants are presented in Table 15.10. It should be mentioned that those material constants are not known for many LCs and LCPs. Nevertheless, they have to be substituted in specific rheological constitutive equations in order to describe the rheological peculiarities of LCPs. Accordingly, the viscosity and the dynamic moduli will be functions of the Miesowicz viscosities and/or the Frank elastic constants. Several theories have been presented that are more or less able to explain the rheological peculiarities. Well-known are the Leslie-Ericksen theory and the Larson-Doi theory. It is far beyond the scope of this book to go into detail of these theories. The reader is referred to, e.g. Aciemo and Collyer (General References, 1996). 

We shall now study the flow properties of the nematic phase. From the microscopic viewpoint, no spedal consideration may seem necessary sinoe the constitutive equation (10.75) and eqn (10.78) applies both for the isotropic and the nematic states, llns is not tte case. The rheological properties of solutions of todlike polymers are dian d entirely when the tem becomes nematic. 

Despite their highly successful record, MD or MC simulations are still hardly extended to the direct interpretation of complex set-ups, typical of most rheological experiments. In such cases it is preferable to employ mean-field or continuum descriptions, based of the numerical solution of the constitutive equations describing hydrodynamic properties. Such techniques were for instance applied to the prediction of transient director patterns of liquid crystalline nematic samples [11-14]. Hydrodynamic treatments are algebraically complex and computationally intensive, and their implementation is limited mostly to nematic phases. 

Onsager symmetry relation. For results on the rheological properties in the isotropic and in the nematic phases with stationary flow alignment, following from (2) and (5) see... 

Survey. Next, results are presented for the time-averaged rheological behavior and the time averaged alignment in the nematic phase at a state point where no stable flow alignment is possible. In particular, the temperature = 0 and the parameters Ak = 1.25 and k = 0 are chosen. In Fig. 8 the shear stress and the viscosity are displayed as functions... 

Fig. 8 a Measured at 50 Hz, the storage and loss modulus (G and G") of the sample strongly increased when entering the nematic phase of 5CB. The modulus kept on growing even well below the phase transition temperature, b Cyclic ramping the temperature up and down illustrated the reproducibility of the piezo rheological measurements. No influence of the measurement technique on the evolution could be detected. Reprinted with permission from Roth et al. [62]... 

Novel aromatic-aliphatic biobased polyesters showing thermotropic behavior in the melt have been presented, incorporating different biobased monomers such as 2,5-furandicarbo)ylic acid (2,5-FDCA), suberic acid (SuA), and vanillic acid (VA) in thermotropic L.C. polymers (TLCPs). The chemical structures, molecular weights, phase transitions, thermal behavior, and mechanical performance of the synthesized polymers are studied using polarization optical microscopy, WAXD, DSC, TGA, DMTA, SS NMR spectroscopy, rheology, and tensile tests. These materials show a low temperature transition from the crystalline to the nematic phase, and stable nematic phases up to 300 °C and higher. ... 

Cidade MT, Leal CR, Godinho MH, Martins AF, Navard P (1995) Rheological properties of acetoxypropylcellulose in the thermotropic chiral nematic phase. Mol Cryst Liq Cryst 261 (l) 617-625... 

The viscous properties of a smectic A are characterized by the same five independent viscosities that characterize the nematic. As we shall see, however, the elastic properties of the smectic are very different from those of a nematic, and some flows permitted to the nematic are effectively blocked for the smectic. For smectic C, for which the director is tilted with respect to the layers, there are some 20 viscosities needed to characterize the viscous properties (Leslie 1993). Formulas for these, derived using a method analogous to that used for nematics by Kuzuu and Doi (1983, 1984) can be found in Osipov et al. (1995). The smectic phase for which rheological properties are most commonly measured is smectic A, however, and hereafter we will limit our discussion to it. 

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