What Are Spherulites And How Can We View Them?


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Syed Asim answered
Upon cooling of the melt, 15-100 nano meter thick single crystals (folding crystallites) are formed which subsequently arrange themselves into larger, polyhedral) structures, so-called spherulites.    Spherulites are centrally symmetric superstructures consisting of many single crystals with their macrornolecules preferentially arranged tangentially to the radius. Spherulites, in general, have a diameter between 0.1-1.0 mm. Typical spherulitic structures as seen under a polarizing microscope. With some spherulites, so-called Maltese crosses and concentric rings can be observed under polarized light; with others, these are absent. The Maltese crosses indicate that there is central symmetry in the arrangement of the folded structures, i.e. Of the single crystals. Rings are observed with PolyEthylene (PE), PolyPropylene (PP), and no rings with PolyAmide, PolyEster, and Polybutyrate. With PE, Poly vinyldiene flouride (PVDF), and certain PP types, both phenomena can be observed.    If the spherulites form in bundles in a plane, like for polypropylene, there are no light deflections and, therefore, no rings are observed. Most photographs under polarized light are made with 10 to 15 micro meter thick microtomed sections so that the planes with folds dominate.Etched surfaces of sections of freshly formed spherulites of PP  show the radial growth of the folding planes very clearly.    With polarized light, when the polarizing filters are positioned 90° to each other, the light passing through the first filter is oscillating in only one direction and then the second filter blocks it completely. If a polymer sample is placed between the two filters, the polarized light that passes through the first filter is deflected (depolarized)

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