Chevron formation

Figure 3.12 Schematic drawing of the morphology of a multilayer system with sticks of parallel lamellae in different orientations (globally unoriented system) before deformation (left) and after deformation (right for deformation direction see arrow) chevron formation appears in a lamellar block marked as A ...

These local deformation modes are similar to the situation in semicrystalline polymers with a parallel arrangement of crystalline lamellae (stiff) and amorphous layers (soft) compare Figs. 2.20 and 2.21. Due to the larger number of molecular defects (chain ends, weak entanglements) in the amorphous phase of semicrystalline polymers, cavitation and interlamellar separation often occur. Contrary to this, block copolymers with less molecular defects in the soft layers (PB) can easily deform with chevron formation (6,25). 

Fig. 9—Various threshoid phenomena for nematic fiuids with negative dieiectric anisotropy and perpendicuiar alignment. The dashed horizontal line is the threshold voltage for induced birefringence. The curved solid line describes the frequency dependence of the threshold voltage for domains. The sloped dashed lines are the threshold plots for chevron formation. The material is MBBA at 25 C (Ref. [86]).

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

It is interesting to point out here that with all of the theoretical speculation in the literature about polar order (both ferroelectric and antiferroelectric) in bilayer chevron smectics, and about reflection symmetry breaking by formation of a helical structure in a smectic with anticlinic layer interfaces, the first actual LC structure proven to exhibit spontaneous reflection symmetry breaking, the SmCP structure, was never, to our knowledge, suggested prior to its discovery. 

This description of FLC switching behavior is simplified for the sake of clarity. A small minority of FLC materials behave as described these are termed bookshelf materials in the field. For most FLCs, the formation of a chevron layer structure driven by layer shrinkage at the SmA-SmC transition changes the picture in complex ways. A discussion of this issue, which is not a chirality phenomenon, is outside the scope of this chapter. 

Fig. 11-9. Formation volume factors of saturated black oils. (Copyright 1947 Chevron Oil Field Research Co., with permission.)...

Misra and Finnie (1970) investigating the scribing process used for segmentation of semiconducting silicon wafers established the formation of three-crack systems (Fig. 6.2.6) on the lateral surfaces of the plate, chevron cracks develop along the scratch, whereas beneath the scratch,... 

Ahhou platinum alone or on a variety of neutral supports selectively converts n-hexane to benzene most of these catalysts deactivate rapidly due to coke formation. With the neutral zeolite KL as a support, however, much longer on-stream times are feasible and within a few years of Bernard s origind publication [130] 4e Aromax process had been developed by Chevron [131], Table 6 corrqrares the aromatic selectivity obtained with Pt-Ba KL and Pt Re Sn / AI2O3 - Cl reforming catalysts [58]. Associated with the much hi er aromatic selectivity is a lower amount of light gas production. 

I close this review of the entry of the oil and gas companies into chemicals by merely noting the major mergers among these companies that occurred at the turn into the twenty-first century, the details of which are beyond the scope of this book. Here the strongest of the U.S. companies acquired two of their century-old rivals that failed to enter petrochemicals. In 1999 Exxon acquired Mobil. In the next year Chevron took over Texaco. Two years earlier, Chevron and Phillips merged their petrochemical units. Next, in 2002, Phillips and Conoco, the company Du Pont had acquired after its response to the oil crises of the 1970s, announced their merger and the formation of ConocoPhillips. 

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