Single crystals lamellar polyethylene

In 1957, owning to the inventirMi of Ziegler-Natta catalysts to produce sequence-regular polyethylene, Keller prepared the lamellar single crystals of polyethylene from dilute solutions, and observed that the lamellar thickness with its direction... 

M. Hikosaka, K. Amano, S. Rastogi, and A. Keller. Lamellar thickening growth of an extended chain single crystal of polyethylene. 1. pointers to a new crystallization mechanism of polymers. Macromolecules, 30(7) 2067-2074, March 1997. 

Hikosaka M, Okada H, Toda A, Rastogi S, Keller A (1995) Dependence of the lamellar thickness of an extended-chain single crystal of polyethylene on the degree of supercooling and the pressure. J Chem Soc Faraday Trans 91(16) 2573-2579 Hoffman JD (1983) Regime III crystallization in melt-crystallized polymers the variable cluster model of chain folding. Polymer 24(l) 3-26... 

Fig. 4.9 Schematic diagrams of two forms of lamellar single crystals of polyethylene, (a) Crystal with 110 sectors, (b) Crystal with 100 sectors as well.

One of the most remarkable features of polymer crystallization is that such chain molecules can form lamellar crystals that contain heavily folded polymer chains. In experiments, the structural analysis of these lamellar crystals became possible when polyethylene single crystals were first prepared from a solution [100-102]. It was found that the orientation of the polymer chains... 

In the classical Lauritzen-Hoffman theory for the mechanism of polymer crystal growth [106], it is assumed that the observed lamellar thickness corresponds to those crystallites that happen to have the largest growth velocity. However, this picture is hard to reconcile with the experimental observation that the thickness of polyethylene single crystals can be modulated by varying the temperature at which they are grown [117,118]. In fact, simulations by Doye et al. [119,120] suggest that the observed lamellar thickness does... 

Figure 17. Schematic lepiesentation of a lamellar polyethylene single ciystal. (a) and (b) show enlaiged features of its stnicture in (o) the zigzag conformation of the macromolecular chain inside the crystal in (b) a hypothetical model for the regular folding, drawn according to the calculation reported in ref. 224a.

Hocquet, S. et al.. Lamellar and crystalline layer thickness of single crystals of narrow molecular weight fractions of linear polyethylene, Macromolecules 35 (13), 5025-5033, 2002. 

At the initial stage of extrusion, the parallel lamellae rotated or tilted to form inclined lamellar stacks and the perpendicular lamellae tended to increase in number. The perpendicular lamellae increased up to around EDR 4.6 in the second stage where the inclined lamellae started to transform steadily into fibrils. This perpendicular lamellar orientation characteristic for extrusion drawing is likely caused by the compressive force applied for extrusion. Such an effect of the compressive force has bran more clearly observed in our previous study of crystalline-state extrusion of polyethylene single crystal powder (21). 

Since the work with polyethylene, the phenomenon of single-crystal formation from dilute solution has been shown to be quite general (Anderson, 1964 Eppe et al, 1959 Fischer, 1957 Geil, 1960 Keith, 1969 Keller, 1957 Price, 1958 Schlesinger and Leeper, 1953 Till, 1957) and lamellar crystals have been obtained with many different polymers (see, for example. Figure 1.8). Also, chain folding appears to occur in the bulk as well as in solution. 

Single crystals of many polymers may be grown from dilute solution [8]. Linear polyethylene, for example, forms single-crystal lamellae with lateral dimensions of the order of 10-20 /Electron diffraction shows that the molecular chains are oriented approximately normal to the lamellar... 

Fig. 16-9. Storage and loss moduli in extension at 110 Hz, plotted against temperature, for single crystal mats of linear polyethylene crystallized at five different temperatures with the following lamellar thicknesses in A 1, 94 2, 104 3, 114 4, 137 5, 167. Dashed curves are for the polymer crystallized from the melt in bulk. (Takayanagi. )...

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