There are a staggering number of different of polymers, and their properties are surprisingly varied. There are many mechanisms that create the differences in polymers, but for now only the structure of the molecular chains will be discussed.
Polyethylene is the simplest and most common plastic. It is simply a long repeating chain of ethylene monomers. To avoid many of the complications brought out by more interesting molecular structures, polyethylene is discussed in this video.
There are different types of polyethylene, as many who recycle may have noticed. It ranges from LDPE, or low density polyethylene, to HDPE, or high density polyethylene, and even includes UHMWPE (ultra high molecular weight polyethylene). There are also levels of MDPE (medium density polyethylene). Another form is XLPE or PEX (cross-linked polyethylene).
Most other polymers share many of these same forms being discussed for polyethylene.
LDPE has low density because of the way the polymer chains are attached to one another. The chains are what are known as branched. These branches tend to keep other chains further away than they normally would be, creating more empty space between them.
MDPE is considered medium because it is not highly branched, but does still having branching to a significant degree. HDPE has negligible amounts of branching, consisting almost entirely of straight polymer chains. HDPE is said to have a linear structure.
PEX, as it is commonly referred to in the industry, has branches that connect the long polymer chains, giving it added strength, and heat resistance. Before cross-linking the polyethylene is a thermoplastic, meaning it loses strength when heated and can be melted into a new shape. After cross-linking it is a thermoset, meaning that it will only burn when heated.
UHMWPE is simply polyethylene with extremely long chains. This gives the polymer a lot of added strength, making it a viable material for use in some implanted prosthesis.
This video was created as an extra credit assignment in my Polymers in Infrastructure and the Environment class. I had made another video, but was not confident in the naming conventions used in organic chemistry, so I scrapped it 3 hours before class and made this video... in 3 hours...
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