Sheaf-like structures

Much effort has been devoted to investigating the detailed architectures and the construction of spherulites. Early investigations of the crystallization of polymers through optical microscopy (OM) [7,8] posited that polymer spherulites consisted of radiating fibrous crystals with dense branches to fill space. Later, when electron microscopy (EM) became available, spherulites were shown to be comprised of layer-like crystallites [9,10], which were named lamellae. The lamellae are separated by disordered materials. In the center of the spherulites, the lamellae are stacked almost in parallel [5,6,11-15]. Away from the center, the stacked lamellae splay apart and branch, forming a sheaf-like structure [11,13-15]. It was also found that the thicknesses of lamellae are different [5,6,11,12]. The thicker ones are believed to be dominant lamellae while the thinner ones are subsidiary lamellae. 

With continued solvent evaporation, the four-lobe (x type) and the + type patterns merge to form a sheaf-like structure. In... 

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. 

Crystalline lamellae can be arranged in different ways, from a parallel arrangement in stacks or bundles, sheaf-like structures, up to spherulites with a radial arrangement of lamellae (typical of HDPE, (high-density polyethylene), or polypropylene (PP)) or twisted lamellae, arranged in concentric rings (banded spherulites, typical of LDPE (low-density polyethylene), as in Fig. 1.9). Depending on the polymer and thermal treatment, the spherulites can reach diameters of up to several tens or hundreds of microns. 

PBA particles Salami particles Sheaf-like structure Shish kebab structure Skin-core structure Spheres... 

Dendritic structure Bundle of grains, similar sheaf-like structure 2.159... 

Sheaf-like structure A bundle of grains or cornstalks 2.78, 2.84... 

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. 

The term axialite was first coined by Bassett, Keller and Mitsuhashi to describe multilayered objects grown from relatively concentrated solutions of polyethylene in xylene. The lamellae in these objects, which are bounded by low index faces, diverge, to a first approximation, from a single central axis and, as a result, the overall appearance is either polygonal or sheaf-like depending upon the direction in which they are viewed. Although similar objects crystallized from the melt were originally termed hedrites, it is the term axialite which is now more commonly applied and it is this which will be used here to describe sheaf-like structures which have not developed a spherical envelope. 

Figure 18 shows three orthogonal projections of a sheaf-like structure in isotactic polystyrene crystallized at 228 °C. This temperature, which is only about 14 °C below the equilibrium melting point, represents a temperature at which crystallization proceeds very slowly, the objects in... 

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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