Cooling Fold chain

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

Further evidence for development of the folded-chain lamellae in the melt rather than by crystallization during cooling was obtained by reheating a previously well developed lamellar structure to 350 °C for an additional 30 min and then water-quenching it. As shown in Fig. 20, the lamellae remain well developed, with no sign of the fingered edges seen if quenched when first melted,... 

This development started with an observation of Pennings and Kiel (1965) that, when dilute solutions of polyethylene were cooled under conditions of continuous stirring, very fine fibres were precipitated on the stirrer. These fibres had a remarkable morphology a fine central core of extended CH2-chains, with an outer sheath of folded chain material. Electron microscopy revealed a beautiful "shish kebab" structure (see Fig. 19.16). Shish kebabs have also been observed in experiments without any stirring. For example, by washing polyethylene powder with xylene (Jamet and Perret, 1973) and by crystallising nylon 4 from a glycerol/water mixture (Sakaoku et al., 1968). 

By cooling down a dilute solution in xylene, one obtains lamellae with a high crystallinity. In 1957, A. Keller8 showed9 that these lamellae are made of folded chains (see Fig. 1.12) the chains are aligned and nearly perpendicular to the surface of the lamellae. A typical value of the thickness of these lamellae is 10 nm the thickness is an increasing function of the temperature T(T < rF) of the solution during the crystallization process. 

A gel network which involves folded-chain, lamellar crystals can be obtained by cooling a semi-dilute solution of ultra-high molecular weight polyethylene (UHMW-PE). The usual chain-folded crystallization occurs, but, because of the high molecular weight, complete disentanglement with the resulting crystal precipitation is prevented, and a gel network forms, provided that each lamella is connected... 

The lamellar crystal has a preferential orientation along the fibrillar direction (the c-axis of polymer crystal). Thus, it is assumed that the overgrowth of the folded chain crystal (shish kebab type) takes place in the formation of the lamellar crystal formed from the partial melt upon cooling. 

---