Liquid crystalline polymers banded structure

Banded textures are observed in different oriented main-chain liquid crystalline polymers, e.g. in Kevlar fibres and in other lyotropics and thermotropics. The reason for the banding is that molecules after shearing are oriented according to a serpentine. The average direction of the serpentine is parallel to the shear direction. The exact mechanism for the generation of the serpentine structure is not known. 

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

Fig. 6.18 AFM image of the free surface of a side chain liquid crystal polymer, showing three levels of organization. The polymer contains lamellae 50-300 nm thick. These lamellae are arranged in concentric layers giving a larger scale structure 7 to 10 /xm across which is similar to hedrites that are found in crystalline polymers. Fine banding with a periodicity of about 30 nm appears within the lamellae and runs perpendicular to the long axis of the lamellae. (From Jandt et al [233] reproduced with permission.)...

The Raman spectrum of a polymer is sensitive to the chain conformation since the latter determines the vibrational properties. One expects characteristic differences between a crystalline sequence with a unique regular helical conformation and the wide distribution of different conformations typical for a melt. Figure 5.16 displays spectra measured for polyethylene, in part (a) the spectrum of a crystal, measured for a specially prepared sample of extended chain polyethylene with a crystallinity close to 100%, and in part (b) the spectrum of the melt. The third spectrum (part (c)) was obtained for a partially crystalline sample. The spectrum of the crystal shows sharp bands because restrictive selection rules apply in order to be Raman active, all monomers have to move in-phase. This contrasts with the disordered liquid state, where in the absence of a structural symmetry, selection rules are relaxed and the Raman bands are broadened. On first view, the spectrum of the semicrystalline sample looks like a superposition of the elementary spectra (a) and (b) however. 

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