Lamellae branching

If G is constant, then the rate of volume increase is proportional to the surface area of the growing sphere, a reasonable conclusion since growth occurs at this developing surface. This assumption requires that the radial lamellae branch at a rate sufficient to maintain density and that the degree of crystallinity is constant throughout the spherulite. This infers... 

Measurement of relative crystallinity 0 as a function of time, t, can be very simple, and the results are informative. However, some limitation for applying the Avrami equation must be taken into account. The volume is not constant during crystallization, the crystal growth rate may not be constant, the nucleation may vary with time, and the crystal lamellae branching and perfection process may occur over the course of crystallization. 

Blends of linear and branched polyethylene normally crystallize in two stages. The components crystallize separately provided that they are of similar molar mass. Linear polyethylene will crystallizes at the highest temperatures, forming regular shaped crystal lamellae. Branched polymers crystallize at lower temperatures in finer, S-shaped lamellae located between the stacks of the dominant lamellae. Although linear and branched polyethylenes are chemically very similar they can phase separate in the molten state. A characteristic of phase separated behaviour is the observation of a dominant lamella structure (Figure 6.14). ... 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

Figure 18 Schematic model of a iPP and y iPP branching from a parent lamellae. The crystallographic axes are indicated. Adapted from similar schemes in Refs. [245,246], with permission of Elsevier copyright 2004.

Maltese cross (Blanshard, 1979). The crystallinity of starch is caused essentially by amylopectin pol)Tner interactions (Banks and Greenwood, 1975 Biliaderis, 1998 Donald, 2004 Hizukuri, 1996). An illustration of currently accepted starch granule structure is given in Fig. 5.5. It is believed that the outer branches of amylopectin molecules interact to arrange themselves into "crystallites" forming crystalline lamellae within the granule (Fig. 5.5 Tester et al., 2004). A small number of amylose polymers may also interact with amylopectin crystallites. This hypothetical structure has been derived based on the cluster model of amylopectin (Hizukuri, 1986 Robin et ah, 1974 Fig. 5.1). 

Note Space filling is achieved by branching, bending or both, of the constituent fibres or lamellae. 

For linear PE, the initial structure formed is a single crystal with folded chain lamellae. These quickly lead to the formation of sheaflike structures called axialites or hedrites. As growth proceeds, the lamellae develop on either side of a central reference point. They continue to fan out, occupying increasing volume sections through the formation of additional lamellae at appropriate branch points. The result is the formation of spherulites as pictured in Figures 2.15 and 2.16. 

Figure 4.3 The building block structure of potato amylopectin clusters. Branched building blocks (encircled) are mainly found inside amorphous lamellae (A) of semi-crystalline rings in starch granules. Double helices (symbolized as cylinders) extend from the building blocks into the crystalline lamellae (C). Enlargements of a double helix segment, in which the single strands are parallel and left-handed, and a building block are shown to the right.

---