Thursday, December 29, 2011

PEO Crystals - The Snowflakes of Polymers

While polymer single crystals have always showed a myriad of shapes, articles on the crystallization of polyethylene oxide (PEO) has always been the most exciting to read. That polymer, more than any other is able to crystallize into a seemingly endless variety of forms depending on reaction conditions. This is largely because of the flexibility of the ether linkages in the backbone. There is minimal hindrance to axial rotations and so getting some type of crystal to form is pretty much a given. It's just that no two crystals seem to be the same, much like snowflakes. Consider these 8 examples:

The above AFM photograph comes from a research letter (open access) published in the new ACS journal Macro Letters. As with all crystallization, you have the competing effects of nucleation and growth occurring. The relative rates determine the shape of the final crystal. Working through a wide range of supercooling conditions and molecular weights, the researchers were able to create a "morphology" diagram - a map of what conditions will lead to what outcomes. Here's a look at it:
where the letters in this figure corresponds to the different morphs in the previous diagram. The general trends is that as the supercooling (or molecular weight) increases, the fiberous nature of the crystals increases as well.

It's a short article with lots more detail in it - I highly recommend reading it if you have an interest in polymer crystallization.

ResearchBlogging.orgZhang, G., Zhai, X., Ma, Z., Jin, L., Zheng, P., Wang, W., Cheng, S., & Lotz, B. (2011). Morphology Diagram of Single-Layer Crystal Patterns in Supercooled Poly(ethylene oxide) Ultrathin Films: Understanding Macromolecular Effect of Crystal Pattern Formation and Selection ACS Macro Letters, 217-221 DOI: 10.1021/mz2001109


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