It’s a well-known fact that some polymers, depending on their backbone chemistry, will crystallize when they are cooled from the melt down to solid temperatures. You may have observed this without realizing it; when a clear molten polymer turns opaque during cooling, this is crystallization occurring and the crystallites are larger than the wavelength of light, so they make the solid polymer appear cloudy. Controlling this crystallization process is not trivial during polymer and plastics manufacturing. For some polymers, such as Nylons and PEEK, the fraction of the polymer that forms crystals actually drive the mechanical and chemical resistance properties, as well as any shrinking and warpage that might happen to a finished plastics part.

Zooming in, we can see that if polymer crystals form in the absence of flow or stress, they generally form spherulitic super structures that can be seen by electron microscopy:

(The above image was captured on an FEI ESEM by Anne Gohn)

Or you can see them using polarized light microscopy (POM):

However, if the polymers organize during molten flow, we have a different situation. The order induced during polymer flow results in chains that a) nucleate the crystallization of the surrounding chains and b) form different types of crystals, known as shish-kebabs, after polymer flow stops. These types of structures are often formed during injection molding or extrusion processes, and the resulting crystals look something like the POM below (for Nylon 66, our favorite polymer for this type of study):

In our work, we use very fast electronic sensors found in the Flash DSC to quantify the rate that polymers crystallize after they have been subject to shear flow. While this has been studied for quite some number of years, our group is the first to connect the influence of flow to the subsequent crystallization that occurs after very fast cooling rates, such as those you find in plastics manufacturing. The work is under development, but a few of the initial papers have been published (you can find them in the “Publications” tab.

Sensitivity of Polymer Crystallization to Shear at Low and High Supercooling of the Melt

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