Banded textures

Formation of band textures in HPC liquid crystalline solutions was studied by Navard (86) and Takaheshi et al. (94). Upon shearing, some polymeric liquid crystals develop a particular texture which is called "band texture" consisting of fine equidistant lines when viewed under crossed polars. In HPC-H2O mesomorphic solutions these bands are exhibited when the solution is allowed to freely relax after shearing. 

Colloform general term for botryoidal. mamillary, reniform with banded texture... 

P. Navard, Formation of band textures in hydroxypropylcellulose liquid crystals, J. Polym. Sci., Phys. Ed., 24,435 (1986). 

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

The effect of polymer-solvent interactions on the mesophase can be derived from the rigidity of the polymer chain, the critical concentration to form liquid crystalline phase, and relaxation studies. After shearing a rigid polymeric liquid crystal, a banded texture is formed in which the direction of the bands... 

Fig. 5 Banded texture for 45% EC in m-cresol (A) 1 min after shear and (B) 75 min after shear. (View this art in color at www. dekker.com.)...

The reformation of chiral nematic phase from nematic phase sometimes goes through the transient state of band texture. The formation of band texture is also a relaxation phenomenon. For HPC/H2O solutions, the band texture is only observed when the molecules have been well orientated in the shear direction.A critical lower shear rate limit exists because of the stability of chiral nematic textures. 

An upper shear rate limit also exists above which no bands are formed. The upper critical shear rate is associated with the flow-aligning region of molecular dynamics. Band texture formation is driven by the release of energy, which has been stored in the mesophase during shear. The evolution of the band texture depends on the previous shear rate applied to mesophase. The rate of evolution of the band texture... 

Ernst, B. Navard, P. Band textures in mesomorphic (hydroxypropyl) cellulose solutions. 84. 

Harrison, P. Navard, P. Cidade, M.T. Investigation of the band texture occurring in acetoxy-propyl cellulose thermotropic liquid crystalline polymer using rheo-optical, rheological, and light scattering techniques. Rheol. Acta 1999, 38 (6), 594-605. 

Figure 4.19. Banded texture of a mesogen-jacketed liquid crystalline polymer. The shearing direction is perpendicular to the direction of the parallel bands.

As early as 1938, Langmuir observed the phase separation of clay suspensions into an isotropic phase and a birefringent gel at the macroscopic level in test-tubes [9]. However, in the same report, he noted that this property of phase separation was gradually lost with time, which he tentatively explained by the incorporation of impurities diffusing from the glass tubes. He also compared this system to normal liquid crystals. Later, in 1956, Emerson observed a banded texture similar to that displayed by the Tobacco Mosaic Virus [48]. The investigation of clay suspensions from the structural point of view has been recently resumed. However, the study of the nematic order of suspensions of montmorillonite clays is in fact complicated by their gel properties. In spite of sustained efforts to understand its nature, the gelation mechanism has not yet been fully elucidated [49]. 

Figure 22 shows the phase diagram of these suspensions. The suspensions are isotropic at low concentrations. The nematic phase is found at higher concentration and is separated from the isotropic liquid by a biphasic (N/I) region. Moreover, a sol-gel transition also takes place in the middle of the nematic phase. Nematic sols display typical Schlieren textures (Fig. 23) whereas samples of nematic gels sometimes show banded textures if they were submitted to flow on filling the optical capillary tubes. 

Quite recently, Chen et al. [71b, 80] have developed the technique of banded texture decoration. These authors showed that in thick samples, when the disclination density is small enough due to prior annealing. 

The evolution from the static state to different textures observed as a consequence of controlled shearing experiments and the subsequent relaxation behavior at the cessation of flow were first analyzed by Grazia-no and Mackley [96] for a number of thermotropic MCLCPs. This pioneering work introduced and defined the terms used today to classify shear-induced textures worm texture, ordered texture and banded texture. It should be noted that these textures have also been observed in lyotropic nematics. 

Figure 27. Banded texture of polyester 6. The shear direction was horizontal.

The molecular organization in the banded texture is understood in some detail as a result of optical microscopy [49, 50, 83, 110-112, 115-122, 126], transmission electron microscopy [49, 76, 89, 90, 115, 127] and scanning electron microscopy [50]. It has been shown that the orientation variation of the molecules in this texture is near sinusoidal about the shear direction with most of the misorientation within the shear plane. There is a high degree of register of the molecules between each of the layers [50]. The influence of a number of parameters on the formation of bands has been studied the molecular weight [96], the sample thickness [110,120-122], the shear rate [109], the time during which the shear stress was applied [ 122] and the total shear deformation [121]. 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

Phenylacetate and 3-phenylpropionate of HPC [57] Phenylacetoxy, 4-methoxyphenylacetoxy, p-tolylacetoxy cellulose. banded texture... 

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