Cooling lamellae

When one starts with a liquid of concentration Cg at a high temperature, and cools it down to a temperature slightly below Tg, a solid will grow which consists of two phases, which appear alternatively as lamellae or as fibers of one phase in a matrix of the other phase. 

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. 

The initial melt of Ciooo was cooled down to various crystallization temperatures. Typical developments of crystalline domains at 370 K are shown in Fig. 37 here again the lamellae with the marked tapered shape are observed. Quite surprising is that the crystallization of Ciooo is rather fast in spite of the much longer chains. Any growth of lamellae of Ciooo at the temperature of 370 K, where Cioo did not show any appreciable growth, will be an indication of the molecular weight effect. 

Figure 7.17 AFM image of polyethylene grown at 160 °C and subsequently crystalli/.ed during cooling on the surface of a planar CrOVSiOj catalyst. The left hand inset indicates schematically how polyethylene molecules fold into lamellar structures. The AFM image shows how these lamellae have a tendency to order locally. The right hand inset is a measurement at higher magnification in phase contrast, and shows that lamellae contain substructure, attributed to ordered and amorphous domains (courtesy of J. Loos and P. Thiine [48]).

The system used by Stevens et al. [79] consisted of an air-lift reactor with an external lamella settler. They did not need to pump the cell suspension through the settler, since free flow convection was achieved by cooling the cell suspension (20 °C) before entering the sedimentation device. As also found by other authors, they could achieve selective retention of viable cells by varying the perfusion rate. 

Figure 1.2. Electron micrograph of a RI FE fracture surface using the two-stage replica method. Large lamellae are visible with smaller lamellae perpendicular to them. Specimen was cooled from the melt at 0.12 deg/min. Scale bar = 1 pm.

It is clear that the quenched material has lamellae that are both thinner in the chain direction and smaller in lateral extent. Both these parameters increase with slower rates of cooling. So, too, does density. The microstructure of PTFE prepared this way may be likened to a pack of rods (the helical polymer chains), which in turn comprise lamellar crystals. Consider a hexagonal poker chip as a macroscopic analogue of one lamella. The diameter of die poker chip is the lamellar breadth and its thin dimension is the lamellar thickness. The fully extended chain length of a 10-million-MW PTFE macromolecule would be ca. 26 pm while our thickest observed lamellae are ca. 0.5 pm thick. It therefore... 

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