Chain folding spherulitic polymers

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. 

While the lamellar structures present in spherulites are similar to those present in polymer single crystals, the folding of chains in spherulites is less organized. Further, the structures that exist between these lamellar structures are generally occupied by amorphous structures including atactic chain segments, low molecular weight chains, and impurities. 

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. 

Spherulites. As a polymer melt solidifies, several folded chain lamellae spherulites form which are up to 0.1 mm in diameter. A typical example of a spherulitic structure is shown in Fig. 1.15. The spherulitic growth in a polypropylene melt is shown in Fig. 1.16. 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

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. 

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.)... 

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

The amorphous component of the crystalline polymer solid contains beyond the amorphous component of single crystals, i.e. crystal defects (linear vacancies, kinks, and interstitials), chain folds and free chain ends, as new elements the rejected non-crystallisable impurities and tie molecules. The former concentrate on the outer boundaries of lamella stacks and spherulites, the latter in the amorphous layers separating the lamellae of the same stack. With the exception of impurities all other components of the amorphous phase are intimately connected with the crystals and cannot be physically separated from them or moved independently of them. 

As discussed in Section 2.7.2 above, in chain-folded linear-chain polymers with spherulitic morphology, where lamellae could have a high concentration of chain folds with adjacent reentry, annealing of the polymer close to the melting... 

Hoffman, J. D. and Lauritzen, J. I. (1961) Crystallization of bulk polymers with chain folding theory of growth of lamellar spherulites, J. Res. Natl. Bur. Standards, 65A, 297-336. 

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