Polyethylene chair crystal shapes

Figure 3.33 AFM image of a polyethylene chair crystal similar to that in Figure 3.13b, but viewed directly from above. Note the right-handed spiral growth on the upper half, and the left-handed spiral on the lower half. A lozenge-shaped crystal with a centrally located spiral growth is on the far right. From Toda et al. [19] with permission from Elsevier.

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. 

Figure 3.13 Atomic force microscopy images of M = 32 kDa polyethylene crystals grown from 0.01% solution in xylene at 83°C and sedimented onto a compliant substrate made of 7% poly (vinyl alcohol) dissolved in water. Both (a) hollow pyramid and (b) chair crystals retain their shapes. FromToda et al. [19] with permission from Elsevier.

Figure 5.3 Morphologies of polyethylene single crystals (a) lozenge-shaped tent type, (b) truncated-lozenge-shaped tent type, (c) two halves of truncated lozenge-shaped chair type, and (d) truncated lozenge-shaped chair type with spiral terraces from screw dislocations [11].

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