Tuesday, June 28, 2016

The mechanical properties of polymers arise from more than just entanglements

I ran across a PR blurb from Stanford yesterday entitled How do you design a better polymer?" and am not sure whether to laugh or cry. Start with this highlight:
"The polymer research process has always followed a similar pattern. Researchers would synthesize a new polymer in the lab and send it off for testing to determine its physical properties – melt temperature, elasticity, tensile strength and so forth. Only then would its creators look for suitable commercial applications."
Sorry, but most polymers are designed with specific applications already in mind. What the researchers envision here occurs very infrequently.

But it get's worse:
" 'All polymers get their mechanical properties not from chemistry, but from the way that the individual molecules are entangled together,' says [Jian] Qin..."
Wow that is riddled with errors, and being a direct quote, we know that it wasn't some PR hack that is just trying to meet a deadline - it's from a professor that should know better.

  • "All polymers..."? How about just glassy ones, or maybe better yet, non-crystalline ones. Crystalline polymers derive much of their strength from being crystalline, which is why the Nobel Prizing winning research of Ziegler and Natta for polymer catalysis was so important. Prior to that, making crystalline polypropylene was extremely difficult. Amorphous polypropylene is a very weak, slightly tacky material. Crystalline polypropylene is a good strong plastic. The difference is from the crystallinity, not "...from the way that the individual molecules are entangled together..."
  • "...not from chemistry..." You can't just throw out inter- and intra-molecular interactions just because polymers get entangled. If that were true, we would be able to blend any polymer with any other polymer. But we can't. Compatible polymer blends are the exception and not the rule. Why? Because of chemistry. If the right van der Waals interactions, hydrogen bonds and other intermolecular forces aren't there, you don't get a blend, exactly the same as with non-polymeric materials. All of this then means that mechanical properties can and do arise from chemistry.
To be clear, entanglements are an important contributor to the mechanical properties of a polymer. But are the only contributor? Not by a long shot.

One more quote before I stop torturing you:
"The sort of knowledge that Qin is imparting to the field is also profoundly important to the multibillion-dollar plastics industry, among others. The manufacture of many well-known products that make up our lives, like a polyethylene water bottle, for instance, requires a complex balance of interrelated molecular stresses and fluid dynamics. This is no easy feat. The maker must create a precise blend of molecules to ensure a uniform and properly formed finished product."
Polyethylene is largely bought and sold on the basis of the melt flow, a single value that kinda resembles a viscosity measurement, but not really. The test is only performed at a single condition. Polymers are non-Newtonian and have viscosities that change in a non-linearly as the test conditions change, so for any melt flow value, there are dozens of different "blends" of molecules that can have the same melt flow index. "Precise blends"? Hahahahahaha.

The point of the blurb is to highlight the new professor's research in computer modeling of polymers.

May I kindly suggest getting some practical, hands-on experience first?

Previous Years

June 28, 2011 - BPA Followup (1/2)

June 28, 2010 - Anomalous Diffusion

June 28, 2010 - Another Blogroll Update


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