Twist Breaks

Except for glass and other brittle materials, twist angles of 20° to 60° are needed to break fibres. In these conditions, the torsional shear stresses are out-weighed by the tensile stresses due to the increased length on the outside of the fibre. Breaks tend to be geometrically distorted forms of tensile breaks, often with. some additional splitting. Although, I do not know of experimental studies, breaks in brittle fibres would be variants of the. standard forms. 

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Standard Test Methods for Tire Yarns, Cords, and Woven Fabrics. ASTM standard D885M-94 includes test methods for characterizing tire cord twist, break strength, elongation at break, modulus, tenacity, work-to-break, toughness, stiffness, growth, and dip pickup for industrial filament yams made from organic base fibers, cords twisted from such yams, and fabrics woven from these cords that are produced specifically for use in the manufacture of pneumatic tires. These test methods apply to nylon, polyester, rayon, and aramid yams, tire cords, and woven fabrics. 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

As other examples one may quote the symmetry-breaking of the CASSCF (4e in 4MO) calculation of the inn twisted excited state of ethylene (G. Trinquier and Malrieu, in "The structirre of Double Bond". Patai ed., John Wiley (1990) p 1, or the symmetry-breaking in electron transfer problems (A. Faradzed, M. Dupuis, E. dementi and A. Aviram, J. Amer. Chem. Soc. 112, 4206 (1992). 

The problem of the preferred conformation of cyclodecane has been extensively studied by Dunitz et al. (46). In the crystals of seven simple cyclodecane derivatives (mono- or 1,6-disubstituted cyclodecanes) the same conformation was found for the ten-membered ring (BCB-conformation, Fig. 9). It follows from this that the BCB-conformation is an energetically favourable conformation, possibly the most favourable one. Numerous force field calculations support this interpretation Of all calculated conformations BOB corresponded to the lowest potential energy minimum. Lately this picture has become more complicated, however. A recent force field calculation of Schleyer etal. (21) yielded for a conformation termed TCCC a potential energy lower by 0.6 kcal mole-1 than for BCB. (Fig. 9 T stands for twisted TCCC is a C2h-symmetric crown-conformation which can be derived from rrans-decalin by breaking the central CC-bond and keeping the symmetry.) A force field of... 

Figure 5.51 Scenario for kinetic evolution of flat membranes into tubules.68,132 (a) When membrane is cooled into tilted phase, it develops stripes in tilt direction and then breaks up along domain walls to form ribbons, (b) Each ribbon twists in solution to form helix, (c) Helical ribbon may remain stable or may grow wider to form tubule. Reprinted with permission from Ref. 139. Copyright 2001 by the American Chemical Society.

Since the walls between heterochiral domains are unacceptable defects in an LC display, enantiomericafly enriched dopants are added to the LC to favor one sign of twist over the other in actual devices, providing a monodomain in the TN cell. It should be noted, however, that the chirality of the structure derives from the interaction of the LC director with the surfaces the molecular chirality serving simply to break the degeneracy between mirror image domains to favor one over the other. 

This twisting allows overlap between the electrons of the double bond and the vacant orbitals forming as a result of the rupture of the carbon-carbon bond, and thus considerably reduces the energy required to break this bond. With this transition state no free (or slightly hindered) internal... 

ISO 3342 1995 Textile glass - Mats - Determination of tensile breaking force ISO 3343 1984 Textile glass - Yarns - Determination of twist balance index ISO 3374 2000 Reinforcement products - Mats and fabrics - Determination of mass per unit area... 

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