Twist defect

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

Kopp VI, Genack AZ (2002) Twist defect in chiral photonic structures. Phys Rev Lett... 

Ozaki M, Ozaki R, Matsui T, Yoshino K (2003) Twist-defect-mode lasing in photopolymerized cholesteric liquid crystal. J Appl Phys II Lett 42 L472... 

The discovery and study of a high-yidd thermal source of conformational defects was followed by an effort to link these drfects to chain diffusion and crystal deformation. An initial simulation was designed to study the fate of a twist defect once it was produced in a crystal [4a]. This is a defect, thought to be... 

Fig. 15. Simulation of a twist defect in a seven dynamic diain, constant volume simulation...

Soon after its introduction, the twist defect was interpreted as a soliton [22, 23], The Hamiltonian for the system was obtained from the sine-Gordon model for a linear chain of harmonically coupled particles, and the solutions of the equation of motion yielded propagating solitons. Both of these models were initially applied only to polyethylene, but were later extended to isotactic and syndiotactic polypropylene and isotactic polystyrene [24]. Both the existence of solitons, as well as the activation energy of the resulting a-relaxation, were predicted for isotactic polypropylene and polyethylene, while the absence of the relaxation is also predicted for the other two cases. Three criteria for the existence of solitons were proposed (1) the energy barriers in the interstem potential should be low, (2) the elongational stiffness of the polymer stem should be high, and (3) the number of bonds per repeat unit should be small. 

Nikolov, S. and Raabe, D. (2006) Yielding of polyethylene through propagation of chain twist defects Temperature, stem length and strain-rate dependence. Polymer,... 

Seguela et al. [170] proposed that the driving force for the nucleation and propagation of screw dislocations across the crystal width relies on chain twist defects that migrate along the chain stems and allow a step>-by-step translation of the stems through the crystal thickness. The motion of such thermally activated defects is responsible for a crystalline relaxation. 

Martin [710] provided evidence of intermolecular twist defects in extended chain fibers, such as PBO, by use of molecular modeling. Spatially resolved electron diffraction also revealed the hierarchical structure in as spun LCP fibers at a resolution of 100-200nm with bimodal orientation, whereas the heat treated fibers were more uniform in orientation [713]. These authors also used low dose HREM diffraction to calculate local orientation order parameters from the same TLCP fibers. 

A TN cell without having reverse twist defect (disclination) will be fabricated by giving the twist angle of less than 90° [16] or by adding a chiral agent [17]. 

We made attempts to use photopolymerized ChLC films to demonstrate the presence of twist defects experimentally [109]. However, even though the principle is straightforward, the realization is not easy. Ryotaro Ozaki, a doctoral student at that time (now at Ehime University) spent half a year to fix the films with a good enough precision to see the localized defects in the transmission spectrum. Furthermore, he could also confirm a laser action from this defect mode, which is interesting because it is a different mechanism to produce laser action [109-111]. 

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