Melt-Crystallized Lamellae

If crystallization is carried out from concentrated solutions, multilamellar aggregates are formed. In particular, melt crystallization of polyethylene gives bunched-up lamellae with an overall spherical symmetry. The space between the lamellae contains uncrystallized amorphous polymer. These objects are called spherulites, and their radii grow linearly with time, in spite of their intricate morphological features [9]. Another remarkable feature of spheruhtes formed by linear polyethylene is that they are gigantically chiral, although the molecules are achiral. 

Where a melt-crystallized polymer has been processed by drawing, rolling or other means to produce an aligned structure in which lamellae as well as polymer chains have discernible order, a pseudocrystalline unit cell is present. Provided that this unit cell contains elements of the crystals as well as the boundaries between crystals and that it is entirely typical of the material as a whole then it could be considered as a RVE within the meaning defined above. The lamella crystal itself sometimes considered as embedded in an amorphous matrix would not seem to be an acceptable RVE for reasons similar to those advanced against the Takayanagi model, namely that its modulus is dependent upon the surface tractions. The boundaries between lamella crystals in the matrix must be included in an acceptable RVE. 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

Figure 4.10. Dependence of crystal microhardness. He upon lamellae thickness, Ic for a series of PE samples ( ) quenched PE (O) PE slowly cooled from the melt and (-I-) melt-crystallized paraffin. (After Balta Calleja Kilian, 1985.)...

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