Sheaf-like aggregation

A preferentially and a sheaf-like aggregation with random in-plane orientation are observed for the thinner films (thicknesses of 0.1, 0.2 and 0.4 pm in panels a-c). By contrast thick films (0.6 pm and thicker, panel d) show a morphology that resembles the well known (bulk) spherulitic form with a banded structure, characteristic of linear polyethylene crystallized from the melt at moderately high undercooling. 

Recent advances in catalysis have allowed the production of polyolefins with low crystallinity. Spherulitic structures (see next section) only occur in propylene-ethylene copolymers when the crystallinity exceeds 45% (Fig. 3.22). Sheaf-like structures occur when the crystallinity is between 30 and 45%, whereas axialites and isolated lamellae occur between 15 and 30% crystallinity. Axialites are multi-layer aggregates of lamellar crystals which splay out from a common edge. Embryonic axialites occur for crystallinity from 5 to 15%. Therefore, as the crystallinity is reduced, the microstructures become simpler. 

Figure 5.7 Sheaf-like lamellar aggregates crystallized from the melt in a blend of linear and low-density polyethylene at 125...

In most circumstances, multilayered lamellar aggregates result from the action of screw dislocations. Such defects constitute an important phenomenon in polymer morphology since they give rise to the proliferation of layers which, together with splaying, can generate sheaf-like structures and ultimately spherulites. However, here discussion will be limited to the simple structures, composed of only a few layers, that develop readily on crystallization from solution, or from the melt under certain conditions. 

Figure 20 Micrographs of sheaf-like structures in polystyrene crystallized at 210 °C for an etched surface which intersects with the lamellar aggregates well away from the centre of the axialite (a) etched plane parallel to the hexagonal view shown in Figure 18(b) (b) etched plane parallel to the sheaf view shown in Figure 18(a)...

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