Thursday, August 22, 2013

Defects in Crystalline Polymers - Part 2

After Monday's discussion of a perfect polymer crystal of polypropylene, it's time to move on to the real world where defects are not only present, but I would argue in some cases desired.

I had these thoughts while reading a new paper in Macromolecules, "Solid-State NMR Characterization of the Chemical Defects and Physical Disorders in α Form of Isotactic Poly(propylene) Synthesized by Ziegler–Natta Catalysts". It's a very well written paper that really shows off how much solid-state NMR can do in characterizing defects in polypropylene. I'm not going to discuss the paper at all. If you are interested in this area, by all means, read the paper. It is well worth the time.

I've not ever seen a theoretical calculation of the ultimate properties for polypropylene [*], but it is apparent that the "defects" present in polypropylene hurt the properties significantly. And the logical suggestion would then be to reduce them in any way possible. But that approach might just backfire if it is not done carefully.

This picture, shows a model of what a typical semicrystalline polymer is like. You can immediately see two different regions, the crystalline regions shown here as multiple, parallel straight lines, and the amorphous regions made up of the squiggly lines. So what defects do we see here? First, the crystallites are scattered about with no preferred orientation. (That defect is usually pretty easy to reduce by heating, stretching and then cooling the polymer.) You can also see another defect, chain ends, in both the crystalline regions as well as the amorphous regions. But looking more carefully, you will see chain ends that run between two crystallites. These are defects too as they certainly don't match up to the "perfect" crystal I described on Monday.

Or maybe not. These molecules running between two (or more) crystallites are called "tie" molecules as they tie together the whole entity. Without them, the sample would have greatly reduced strength as only entanglements and the van der Waals interactions between the crystalline and amorphous regions would be holding things together. When the samples is stressed, that stress is transmitted throughout the sample via the tie molecules, and in particular, the amorphous segment of the molecule that lies between two crystalline regions.

We can't control how the tie molecules run or form, but we can help increase their statistical likelihood by working with longer chains. So while tie molecules are defects, they are necessary defects that give polymers their strength. Until that magical day arrives that we can make perfect polymeric crystals, we ironically need those defects.

Image Source

[*] The theoretical values for polyethylene are still about 4x higher than anything we can currently make, even with gel-spun, ultrahigh-molecular-weight polyethylene. I would expect the case for polypropylene to be that much worse since the molecular weight is lower, there are multiple crystal forms available

No comments: