FIGURE 7 Electron micrograph of polyethylene “shish kebabs” formed by shear of a moderately concentrated solution. Note the break in the central thread in the lower unit. From Clark.14

than steel. The strength, of course, is due to the “shish.” Although shish kebabs themselves are not commercially important, they have been the impetus behind worldwide efforts to produce fibers, sheets, and rods commercially whose strengths take advantage of the molecular orientation of the shish kebab structure. This area of processing is known to polymer scientists as stress-induced crystallization (SIC).

Briefly, one the approaches used to exploit the shish kebab effect is that of solid-state extrusion of polymers in the absence of solvents (for a recent review, see note 17). Figure 9 shows the results for polyethylene.18 The thick, opaque rod at the upper right of this figure consists of common, spherulitic, melt-crystallized polyethylene, as discussed earlier. The extrusion process breaks up the spherulitic structure and produces optically clear fibers, as depicted in the lower portion of the figure. Such processes are being commercialized in laboratories in many countries, for example, by the Allied Corporation in the United States. The structure of such extruded polymers proposed by Zachariades and Porter19 is shown schematically in Figure 10 (others have presented similar pictures). The resemblance to the



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