Chain-folded crystallites

Fig. 13.1 Plot of rate of crystallization of CiggHsgg from a 3.85% toluene solution against the crystallization temperature, Tc. extended chain crystallites folded chain crystallites. (From Organ et al. (5))...

Numerous studies of the structure and properties of drawn crystalline polymers have led to the microfibrillar model of fibrous morphology177 179 180. According to Peterlin 179) and Prevorsek et al. 180), the long and thin microfibrils are the basic elements of the fibrous structure. The microfibrils consist of alternating folded chain crystallites and amorphous regions. The axial connection between the crystallites is accomplished by intrafibrillar tie-molecules inside each microfibril and by inter-fibrillar tie-molecules between adjacent microfibrils. 

Roe, R. J., K. J. Smith jr., and W. Krigbaum Equilibrium degrees of crystallisation predicted for "single pass" and folded chain crystallite models. J. Chem. Phys. 35, 1306-1311 (1961). 

Many polymeric solids consist largely of folded chain lamellae and that the breadth of X-ray diffraction lines is caused by the crystallite size distribution and by the disorder within the lamella. 

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). 

Table 19.2 gives a survey of the morphology of polymer crystallisation. The survey is self-explanatory it demonstrates an almost continuous transition from the pure folded chain to the pure extended-chain crystallite. 

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

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. 

The fundamental concept of molecular chains of cellulose extending along the direction of microfibrils, to form crystalline and amorphous or paracrystalline regions, was well accepted and set the trend for interpretation of all relevant data. However, folded-chain conformations have since been advocated as a strong possibility. Also, several other models based on the extended-chain concept have been proposed in order to define better the distribution of crystallites and the disposition of the molecular chain within the microfibrils. 

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