Spherulites chain folding

Spherulitic chains folded at right angles to main axis... 

The molecular chain folding is the origin of the Maltese cross which identifies the spherulite under crossed Polaroids. The Maltese cross is known to arise from a spherical array of birefringent particles through the following considerations ... 

Usually, crystallization of flexible-chain polymers from undeformed solutions and melts involves chain folding. Spherulite structures without a preferred orientation are generally formed. The structure of the sample as a whole is isotropic it is a system with a large number of folded-chain crystals distributed in an amorphous matrix and connected by a small number of tie chains (and an even smaller number of strained chains called loaded chains). In this case, the mechanical properties of polymer materials are determined by the small number of these ties and, hence, the tensile strength and elastic moduli of these polymers are not high. 

FIGURE 2.16 Spherulite structure showing the molecular-level lamellar chain-folded platelets and tie and frayed chain arrangements (a), and a more complete model of two sets of three lamellar chain-folded platelets formed from polyethylene (PE) (b). Each platelet contains about 850 ethylene units as shown here. 

Figure 1.61 Schematic illustration of chain folding leading to lamellar crystallites (inset) and lamellar stacking to form spherulites.

While, in recent years, many laboratories have demonstrated that nearly every synthetic polymer can crystallize in the form of single crystals (13) consisting of lamellae formed by regular chain folding (Figure 3), it is clear that extreme condi-tions-i.e., very slow crystallization or very dilute solutions, are required for these structures to form. Under normal conditions, such as those encountered in any industrial process, the polymer usually crystallizes in the form of less ordered, large structures, called spherulites. 

Rate of spherulitic crystallization with chain folds in polychlorotri-fluoroethylene. J. Chem. Phys. 37, 1723 — 1741 (1962). 

FIGURE 8-63 Schematic diagram illustrating chain folding in a spherulite. 

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

Khoury, F. The spherulitic crystallization of isotactic polypropylene from solution on the evolution of monoclinic spherulites from dendritic chain-folded crystal precursors. J. Res. Nat. Bur. Stand. 1966, A-70, 29. 

Figure 19.3 Spherulitic structure model. Note the growth direction and points of lamellar ramification, to fill void space with crystals in a uniform way. Source After Hoffman JD, Davis GT, Lauritzen JI. The rate of crystallization of linear polymers with chain folding. In Hannay NB, editor. Treatise Solid State Chemistry, Volume 3, p 418, 508, 520 [8]. Copyright 1976 Plenum Press.

FIGURE 11.8 Fully-developed spherulite grown from the melt, comprising chain-folded lamellae (magnified section) and branching points that help to impart a spherical shape to the structure. Most rapid growth occurs in the direction of the spherulite radius R. (Adapted from McCrum, N.B., Buckley, C.P., and Bucknall, C.B., Principles of Polymer Engineering, Oxford University Press, 1988. With permission.)... 

Shortly afterwards (in 1957) Fischer showed by electron microscopy that the crystallites in melt-grown spherulites of polyethylene and nylon were most likely to be lamellar rather than fibrillar, as would be expected from the fringed-micelle model. It is now accepted that chain-folded lamellar crystallites play an important part in the structure of most ordinary... 

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