Dylan Bialek's Material of the Day Ultra-high-molecular-weight polyethylene

Today's material is ultra-high-molecular-weight polyethylene. It was first commercially produced in the 1950's and has the highest impact strength of any thermoplastic currently made (1). It's molecular weight generally runs from 3.5 - 7 million amu (atomic mass units), and is made up of a recurring chain of hydrocarbons shown below.

Figure 1: Structure of UHMWPE, with n links averaging around 100,000 (source: Wikipedia)

I chose this material because it is used in the base of one of my favorite things: the snowboard. I have skied/snowboarded since I was almost 2 years old, and haven't missed a season since. Because I grew up in Arizona and Colorado, the Rockies were my backyard and it was never too far to go to get in a ski day. As of this past season, I've logged around 320 days on the mountain.

Figure 2: Anatomy of a basic snowboard (source: TheSkiLab.com)

What is UHMWPE?

Figure 3: Molecular chain of UHMWPE (source: Phys.org)

UHMWPE is a polymer with extremely long chains, connected by Van Der Waals forces. Because of its long chains, the material transfers load very effectively over the length of the chain. Overlaps in the chain occur often, and each chain is connected to many others, which is why the material is so strong. The overlaps and abundant connections make up for in numbers what the weak London forces are lacking. According to the Technomic Publishing Company, the material is odorless, tasteless, and nontoxic. It is also highly resistant to corrosive chemicals (except for oxidizing acids), and has an extremely low coefficient of friction. This low coefficient of friction and strength are the two main reasons UHMWPE is used to coat the bottoms of snowboards. The polyethylene also has other uses, especially in the medical field.

Medical

Figure 4: A schema of a hip and knee replacement, respectively, with UHMWPE used in each case as a liner and tibial prosthesis. (source: researchgate.net)

UHMWPE has been known as a stable biomaterial since the 1960s for not only hip and knee replacements, but spine implants as well (spine implants came later, in the 1980s). (1) To improve their resistance to oxidation in the body, the polymers were cross linked and subjected to gamma or electron beam radiation and then thermally processed. Highly cross linked UHMWPE remains the standard of care today for total hip replacements.

References

(1) Stein, H. L. (1998). Ultrahigh molecular weight polyethylenes (uhmwpe). Engineered Materials Handbook, pp. 167-171.

(2) "Ultra-high-molecular-weight Polyethylene (UHMWPE)." Encyclopedic Dictionary of Polymers (n.d.): 1020. Braskem.com. UTEC. Web. 25 Jan. 2017.

(3) Wong, D.W.S.; Camirand, W.M.; (eds.) (1994) "Development of edible coatings for minimally processed fruits and vegetables" pp. 65–88; Edible Coatings and Films, Technomic Publishing Company.

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