Crystal lamella hollow pyramide, polyethylene

Ridges are formed by pleats of extra material deposited along one of the diagonals of each diamondshaped crystal. Obviously, therefore, crystals of polyethylene are not simply flat lamellae. Experimental evidence has shown that they may exist in solution as hollow pyramids. These pyramids may or may not be corrugated depending on the crystallization conditions. 

When they are grown at sufficient dilution, the crystallites approximate to lamellae with a uniform thickness of about 12 nm, the precise value depending on the temperature of growth. Electron diffraction shows that the chain axes are approximately perpendicular to the planes of the lamellae. The crystals are not exactly flat, but have a hollow-pyramidal structure, with the chain axes parallel to the pyramid axis. This pyramidal structure is seen clearly in fig. 5.5, which shows a single crystal of polyethylene floating in solution. This should be compared with fig. 5.3(b), which shows similar crystals flattened on an electron-microscope grid. The dark lines on the crystals in fig. 5.3(b) show where the pyramid has broken when the crystal flattened. 

There are two other principal nonplanar habits of polyethylene lamellae chair and ridged crystals. Chair crystals (30) form simultaneously with, and are related to, hollow pyramids their shape may be described as resulting from a hollow pyramid divided in two along the short diagonal, the two halves rotated 180° aroimd their long diagonal and then constrained to rejoin. The proportion of chair to hollow pyramidal crystals appears to vary with crystallization conditions. 

The hollow pyramid shape typical of a single crystal of polyethylene indicates that the chain axis is not parallel with the normal of the lamella. The chain axis is generally at an angle, about 30°, with respect to the lamella normal. The reason for the chain tilt is essentially that a certain type of regular chain fold requires a small vertical displacement of the linear chain in the adjacent position (Fig. 7.14). 

Polyethylene crystallites are generally not planar rather, they have a hollow-pyramidal shape [134, 135]. A pleat or a trapezoid-shaped structure is observed in the center of the lamellae. By precipitation of crystals from highly diluted solutions, mats are obtained which are very brittle because of the scarcity of interlamellar connections and high crystallinity. 

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