Crystallites with folded chain

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). 

It is supposed that geometry, morphology and crystalline phase nucleation and growth mechanism will be defined to a considerable degree by the nature of heterophase fluctuations in the amorphous state. So, the supposition of heterophase fluctuation from folded chains (Yech model [44]) results in the formation of crystallites with folded chains (CFC). In paper [43], an alternative model of heterophase fluctuation supposes the availability of parallel parts of different chains of macromolecules in it. The use of these models supposes the possibility of formation in the crystallisation process of CFC, crystallites with stretched chains (CSC) or some intermediate morphology... 

Nevertheless, the data of Figure 4.7 demonstrate the accuracy of the identification of clusters as heterophase fluctuations, which later become the nucleus of crystalline phase growth. From the data of Figure 4.7 it also follows that experimental values of X indicate reduction of the fraction of fibrillar crystallites in mixed crystalline morphology at the expense of an increase in the fraction of crystallites with folded chains. The obtained data confirm the conclusion made above, supposing formation of crystallites with mixed morphology at PCP orientational crystallisation [40]. 

PCP showed that this polymer crystalline morphology at orientational crystallisation was a mixed morphology, inclnding both crystallites with folded chains and fibrillar crystallites. An increase in the last fraction resnlts in reduction of the tension critical degree at which the transition from monomolecular nucleation to multimolecular nucleation is observed. 

This distinction can be explained within the frameworks of the cluster model, which is similar to that used in paper [76], excluding the type of local order domains. Slutsker and Filippov [76] supposed that those domains were the analogue of a crystallite with folded chains, whereas clusters are the amorphous analogue of a crystallite with extended chains [9]. Following the reasons in the paper [76] one can say that... 

This type of crystallisation was first suggested by Storks in 1938. He made films of gutta percha 27 nm thick by evaporation from solution. Electron diffraction showed that the films were composed of large crystallites with the chain axes normal to the plane of the film. The only possibility was that the chains folded back and forth upon themselves, so that adjacent segments were parallel and in crystal register. 

Figure 1.12 Crystallites with folded lamellar crystals of thickness f in the direction of the c axis for (a) regular folding and (b) irregular folding of the chain molecules.

The value of angles used in SAXS is from 1° to 5°. SAXS provides information on greater interatomic distances from 50 to 700 A. Consequently, SAXS is useful in detecting larger periodicities in a structure. For example, many polymeric materials crystallize with individual chains folding back and forth within a given crystalline region or crystallite [3]. Other examples of the utility of SAXS are in the study of lamellae crystallites or in the distribution of particles or voids in the material. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Once nucleated, crystallization proceeds with the growlh of folded chain ribbon-like crystallites called lamellae. The arrangement of polymer chains in... 

The ot-L-Ara/-(l->5)-arabinan, oiDP 45-80, covalently attached to rhamnose residues, can be cleaved and isolated, and forms reasonably well-defined crystallites, which, however, cannot be aligned for fibre X-ray. Recent powder X-ray work has indicated that the polymer crystallises in a monoclinic unit cell with one chain per unit cell. It adopts a single two-fold helix with the... 

For polymers manifesting the most common type of crystalline morphology (folded chain lamellae), the "equilibrium" values (asymptotic limits at infinite lamellar thickness) of Tm, of the heat of fusion per unit volume, and of the surface free energy of the lamellar folds, are all lowered relative to the homopolymer with increasing defect incorporation in the crystallites. By contrast, if chain defects are excluded completely from the lamellae, the equilibrium limits remain unchanged since the lamellae remain those of the homopolymer, but the values of these properties still decrease for actual specimens since the average lamella becomes thinner because of the interruption of crystallization by non-crystallizable defects along the chains. 

Fig, 1.12. A two-dimensional representation of folded chains in a crystallite (according to Billmeyer)9 (a) ideal model, without amorphous fraction (b), (c), and (d) models with amorphous fractions of various kinds. 

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