Monday, August 19, 2013

Defects in Crystalline Polymers - Part 1

Start by imaging a "perfect" crystal of polypropylene, say a fiber that is several centimeters long. Perfection in this case, is clearly unattainable. Perfection would require that the fiber be made up of molecules of polypropylene that are as long as the fiber itself. Not only is such a large polymer unattainable at present [*], but it is even more impossible at present to make all the polymer chains the exact same length (with no branching). To then orient each and every single chain perfectly, line them all up perfectly and then place it in the crystal lattice is really beyond all hope. And since this is my ultimate wish list for Santa, can I have 100% steroregularity too?

So in the real world, all polypropylene crystals have defects. Since the molecules are not long enough to span the entire fiber length, there are end defects where one chain ends and another chains starts. The existence of a distribution of molecular weights also contributes to chain end defects. Our inability to create full orientation of the chains introduces numerous defects, namely chain-folded crystals, where the molecule is folded back-and-forth many times in a crystal. Since this fold length is less than the length of the fully extended polymer, it is possible that one part of the polymer chain can extend into another crystal entirely. And then there are packing defects from numerous sources. Since the viscosity of the polymer is so high, that can limit its ability to get into the lattice in the time needed. Or packing defects can arise from branch points. As well as from losses in stereoregularity.

So while the perfect crystal initially described would a 100% crystalline material, polypropylene in the real world is lucky to reach more than about 60% crystallinity without special processing. The balance of the material are parts of chains that are caught out, unable to line up and crystallize with other chain fragments in their immediate vicinity.

All of this is prelude to tomorrow's the next post about recent research in polypropylene and the "defects" that exist in it.

[*] A C-C bond is about 1.54 Å long, and there is about a 110 degree angle between one bond and the next. With that geometry, each C-C bond has a spans about 1.26 Å of length. A 12.6 cm long fiber would be made up of 107 monomers for a MW of 420,000,000 g/mole - about 500 times larger than what we can currently make. And this is just for a short span of fiber. Imagine a 50 meter rope!

Assuming I did the math correctly, there is still one flaw with geometry presented here, one that is present in polypropylene but not in polyethylene. Anyone care to comment on the little "spin" I should impose here?


Patrick said...

Gauche interactions with the methyl sidechains of PP? You've already hinted at stereoregularity.

John said...


The α-crystals of PP (the most common morph) are a 3-sub-1 helix. I wasn't going to go into the extra geometrical calculations, as the point was already made. The helix "spin" only increases the number of monomers needed to span a given distance.

Brendan McGrail said...

You could always add the requirement for 100% regioregularity, too.

John said...


You're right. I thought I had mentioned it, but it must have been only in my head.

Yes, regioregularity (head-to-head orientation) is of the utmost importance.