Semi-crystalline polymers spherulites

A conscious choice of such elements can be made but in general the equilibrium distribution of stress cannot be found except for particular geometries. The assumptions of uniform strain throughout the assembly or of uniform stress were respectively made by Voigt and by Reuss. Returning to the structures actually perceivable in polymers one may consider the spherulite in a semi crystalline polymer as being unsuitable as a RVE because the boundary is not included. However, an assembly of spherulites would be acceptable, since it would contain sufficient to make it entirely typical of the bulk and because such an assembly would have moduli independent of the surface tractions and displacements. The linear size of such a representative volume element of spherulites would be perhaps several hundred microns. 

Deformed crystals. If a semi-crystalline polymer is deformed while undergoing crystallization, oriented lamellae form instead of spherulites. 

Mandelkern et al. (1968) have proved that the WLF formulation, which has had an outstanding success in explaining the segmental mobility and flow properties of completely amorphous polymers, is not applicable to the transport process involved in the growth of spherulites in melts of semi-crystalline polymers. Rather, a temperature-independent energy of activation, specific to a given polymer and dependent on its glass temperature, suffices to explain the experimental data now available. Mandelkern s equation reads ... 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

Figure 1.10. (a) The spherulitic habit of a semi-crystalline polymer on cooling from the melt. (A micrograph of polyethylene glycol viewed under crossed polarizers), (b) A schematic diagram of lamellar fibrils that have nucleated from the points shown and grow to the spherulite boundaries. Tie molecules connecting lamellae are shown. 

Some less-conventional dielectric tests have been presented, including the possibility of following crystallization by monitoring tiie changes in the loss peak. There experiments also allow more information to be obtained about the segmental mobility in semi-crystalline polymers, especially the restricted motions within the spherulitic structures. 

For semi-crystalline polymers, the average orientation function 2 for the crystal c axes can be calculated from X-ray diffraction measurements (Chapter 3). Figure 8.15 shows how 2 increases linearly with the draw ratio, for polypropylene fibres and films, while the spherulitic microstructure survives. At 2 = 0.9, where the spherulites are destroyed and replaced by a microfibrillar structure, there is an increase in the slope of the 2 versus true strain relationship. It is impossible to achieve perfect c axis orientation... 

In Section 9.3 we present experimental studies of plastic deformation in two semi-crystalline polymers, HDPE and Nylon-6, both in tension and in plane-strain compression flow, from initial spherulitic morphologies to large plastic strains. In these studies, the evolving morphological alterations were monitored closely by a complementary array of techniques involving light microscopy and X-ray diffraction and scattering both in the crystalline and in the amorphous components. 

In the plastic response of semi-crystalline polymers the starting material has an initial spherulitic morphology and, in the process of simple extensional flow, either in tension or in plane-strain compression, ends up with a highly perfect... 

This initial microcrack formation is reflected in a stress-strain curve by the deviation from the linear range of the elastic constants. In fact, the failure is analogous to the microcracks that form between spherulites when a semi-crystalline polymer is deformed. (Source Osswald, T.A. and G. Menges, Material Science of Polymers for Engineers, Hanser Publishers, New York, 1996). Refer also to Vulcanization, Peroxides, Peroxide Cross-Linking, Sulfur Vulcanization, and Vulcanizing Agents. 

Figure 5. Comparison of spherulitic centers in (a) semi-crystalline polymers versus liquid crystals growing in a partly cross-linked amorphous polymeric matrix (b and c) for samples confined to thin films between two flat surfaces. (Reproduced from Ref. 11 Copyright American Chemiccd Society).

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