Crystallinity extended-chain crystals

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

The growth of semicrystalline NIF in preference to that of the highly crystalline F2 form is a typical example of crystallization kinetics controlling polymer morphology. The best known example is, of course, the very fact that thin folded-chain crystals form in preference to the more stable extended-chain crystals. 

Extended chain crystals (ECC) (formed when polymer crystallizes under high hydrostatic pressure, or a crystalline polymer is annealed under pressure) (31)... 

A characteristic feature emerges Crystals formed in solution or by melt polymerization are more perfect than those formed upon crystallization of the preformed polyoxymethylene from solution or melt. If extended chain crystals with >95% crystallinity are reprecipitated from hexafluoroacetone sesquihydrate, the degree of crystallinity decreases to 67.5 %76>. 

Melting temperature (Tin) 137 C folded chain crystals strength properties derived from crystallinity low thermal resistance. Extended chain crystals can melt at about 145°C... 

Crystallinity 0.6(L0.95 Properties dependent on crystallinity, with amorphous phase providing flow, flexibility, and toughness. Kinetically stable folded chain crystals thermodynamically stable extended chain crystals can be formed by gel drawing... 

The especially high mobility in the condis crystalline state was first discussed in the 1960 s based on the study of the mechanism of extended-chain crystal growth at elevated pressure from the melt A schematic drawing of the growth-front... 

Figure 5-20. Extended chain crystals of poly(ethylene). Crystallization at 225° C and 4800 bar Mw= 78.300 g/mol, M = 14,800 g/mol, 99% X-ray crystalline (after B. Wunderlich and B. Prime).

Different types of crystalline structures develop Fibrillar or extended-chain crystals and lamellar crystalhtes. Development of the first mentioned stractures is an equilibrium process. We do not have an amorphous phase since both segments and chains are ordered. Extended-chain crystals develop only in polymers of low molecular mass M < 10 kg/mol) and in the range of low undercoolings, that is AT = T - T is small. 

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