Lamellar twists

The twisting lamellar structure of banded spherulites has been debated for decades without obtaining any satisfactory answer until recently. The nature of the isochiral (certain uniform handedness) lamellar twisting and the synchronic character of the twisting of a group of adjacent dominant lamellae both require an explanation. The permanganic etching technique provided... 

Studies by Ho et al. (2000) point towards lamellar twisting as the reason for the formation of banded spheralites in PTT. The atomic force microscopy (AFM)... 

Another such example is the connection between interface stresses and crystal lattice distortion, and lamellar twisting. The latter phenomenon requires two additional ingredients, in addition to the interface stresses Firstly, the interface stresses must occur asymmetrically at the two opposite lamella surfaces [56,57], which can not be achieved in our simulations by construction of the simulation cell. Reasons for such asymmetries have to be found on different grounds. Secondly, lamellar twisting can only be predicted if also the material properties of the crystalline lamellae are incorporated, either based on experimental [58,59] or simulated data [60]. 

Stem T, Wachtel E and Marom G (1997) Epitaxy and lamellar twisting in transcrystaUine polyethylene, J Polym Sci Part B Polym Phys 35 2429-2433. 

Assouhne E, Wachtel E, Grignll S, Lustiger A, Wagner H D and Marom G (2001) Lamellar twisting in alpha isotactic poljTrropylene transcrystaUinity investigated by synchrotron microbeam X-ray diffraction, Polymer 42 6231-6237. 

Among the molecular mechanisms accotmting for the lamellar twist, the idea of screw dislocation first proposed by... 

Figure 45 (a) Intensities of different diffraction peaks as a function of the radial distance from the spherulite center, (b) Typical PSD of the diffraction intensity reveals the twisting period of 23.25 im. (c) Radial positions corresponding to the maximum intensity of the different diffraction peaks show that the lamellar twist is strictly periodic and identical for all the experimentally obsen/ed diffraction peaks. With permission from Rosenthal, M. Anokhin, D. V. Luchnikov, V. A. etal. lOP Conference Series Materials Science and Engineering-, 2010 Article number 012014. "... 

This section discusses the primary nucleation and subsequent radial growth of spherulites. The latter concentrates on mechanisms by which an increasing number of ribbon-like crystals appear at larger distances from the spherulite center. The intriguing topic of lamellar twist is deferred to Section 3.4.4. 

It is clear that uncompensated chemical (configurational) chirality causes lamellar twist manifested as banded polymer spherulites. A particular enantiomer of a particular chiral polymer almost always has lamellar twist of one hand only, but R enantiomers of different... 

Figure 3.70 The relation between chain tilt and lamellar twist as proposed by Keith and Padden. The top sketch is a view toward advancing growth front that has asymmetry imposed by chain tUt. More compressive stress associated with looser chain folding is indicated by of-i-), resulting in transverse torsions that would bend each ribbon halh if free, as in the middle figure. Because the two halves are connected, the end of the ribbon acquires a left handed twist as in the lower figure. From Lotz and Cheng [23] with permission from Elsevier.

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