Spherulites lamellar twist

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... 

While normally amorphous, and generally featureless on a micron scale, crystallization of polycarbonate was solvent-induced with butyl acetate, generating a disc-like spherulitic structure of ca 10 fina in diameter surroimded by an amorphous matrix. Within the spherulite, the twisted fibrils emanating from the point of nucleation were observed in these afm images, and is consistent with known lamellar growth mechanisms (103). 

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... 

The growth of polymer spherulites involves the segregation of noncrystal-lizable material into the regions between the lamellar ribbons. The components that arc not incorporated into the crystallites include additives like oxidation stabilizers, catalyst residues, and so on. as well as comonomer units or branches. The spherulite structures and interspherulitic boundaries are held together primarily by polymer molecules which run between the twisted lamellar subunits and the spherulites themselves. Slow crystallization at low degrees of supercooling... 

In most spherulitic polymers, touching spherulites occupy whole of the space. Their microstructure is too complex to be completely modelled, especially if there is twisting of lamellar stacks about spherulite radii. Consequently, models simplify the structure, and use composite micromechanics concepts. A stack of parallel lamellar crystals with interleaved amorphous layers (Fig. 3.20) has a similar geometry to a laminated rubber/metal spring (Fig. 4.1). The crystals have different Young s moduli E, Eb and E (Section 3.4.3), and different shear moduli when the... 

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