Shear alignment

The flow properties of other liquid crystals, such as chiral nematics (i.e.. cholesterics). smectics C, and hexagonal phases, are even more poorly understood. 

Bahudur B (ed) (1990). Liquid Crystals Applications and Uses, Vol 1, World Scientific, Singapore. 

Chandrasekhar S (1992). Liquid Crystals, 2nd ed., Cambridge University Press, Cambridge. Chuang I, Durrer R, Turok N, Yurke B (1991). Science 251 1336. 

Cladis P, van Saarloos W (1992). In Solitons in Liquid Crystals, Lam L and Prost J (eds). Springer-Verlag, Berlin. 

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Technora. In 1985, Teijin Ltd. introduced Technora fiber, previously known as HM-50, into the high performance fiber market. Technora is based on the 1 1 copolyterephthalamide of 3,4 -diaminodiphenyl ether and/ -phenylenediamine (8). Technora is a whoUy aromatic copolyamide of PPT, modified with a crankshaft-shaped comonomer, which results in the formation of isotropic solutions that then become anisotropic during the shear alignment during spinning. The polymer is synthesized by the low temperature polymerization of/ -phenylenediamine, 3,4 -diaminophenyl ether, and terephthaloyl chloride in an amide solvent containing a small amount of an alkaU salt. Calcium chloride or lithium chloride is used as the alkaU salt. The solvents used are hexamethylphosphoramide (HMPA), A/-methyl-2-pyrrohdinone (NMP), and dimethyl acetamide (DMAc). The stmcture of Technora is as follows ... 

The pathway and kinetics of the C to S transition have been studied on shear-aligned cylinders of the commercial diblock copolymer of PS and poly(ethylene-co-butylene) (KRATON G 1657 Shell Chemical Company) [143, 144], A complete dissolution of the cylindrical structure before the epitaxial... 

Albderda van Ekenstein G, Polushkin E, Nijland H, Ikkala O, ten Brinke G. Shear alignment at two length scales comb-shaped supramolecules self-organized as cylinders-within-lamellar hierarchy. Macromolecules 2003 36 3684-3688. 

Figure 21.9 2D SAXS (logarithmic scale) from a shear-aligned specimen of the parent ABC block copolymer (a, c). The shear direction is horizontal in (a) and along the X-ray beam direction in (c), while the surface normal of the sample is vertical in both images. Radial averages (b, d) in both cases show a main peak at s = (2.63 + 0.05) x 10 2 nm4 (repeat spacing of 38.0 0.7 nm) and the dotted vertical lines indicate the allowed reflections for a hexagonal lattice.37 (Reprinted with permission from G. E. S. Toombes et al., Chem. Mater. 2008, 20, 3278-3287. Copyright 2008 American Chemical Society.)...

Fig. 56 Shear alignment of a PS-PVP(PDP)x polymer and TEM micrograph of the resulting mixture. Reprinted with permission from [205]...

The shear-alignment behavior of PS-PI block copolymer solutions, where a solvent mixes with both blocks, has been studied by Balsara and coworkers (Balsara and Hammouda 1994 Balsara et al. 1994b). Mostly parallel alignment was observed, apparently with some rippling of the layers. Little else is known about the alignment behavior of solvated block copolymers. 

It would be interesting to test this explanation by determining the shear alignment direction of the inverted triblock, PI-PS-PI, which has a styrene blocks in the middle. Presumably, if the above explanation for the prevalence of perpendicular alignment in PS-PI-PS is correct, then the inverted triblock PI-PS-PI should show parallel alignment at high frequencies. 

From Eqs. (13-7) and (13-8), one can estimate that typical values for B and K for weak to moderately segregated diblocks should be B 10 -10 dyn/cm and K 10 -10 dyn. By compressing shear-aligned diblocks, Hudson et al. (1995) have extracted experimental estimates of B that are around a decade or so lower than these values. 

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