Chem 421: Introduction to Polymer Chemistry

Classifying Polymers


Like the old story about the elephant and the blind men, the way people classify polymers depends their experience. For example, an organic chemist is interested in the detailed arrangement of atoms in the chain, while a structural engineer only considers a table of physical attributes such as tensile strength or density. There is no uniform system of classification of polymers. The terminology has evolved along with polymer science, and there are numerous exceptions to categories, as well as widely used historical terms or trade names lacking information content. Sorry, but we didn't invent the classification and nomenclature systems, we just try to teach them...


Molecular Classifications

The structures of polymers are conveniently represented by the repeating chain formula, which shows the arrangement of bonds and atoms. The repeat units often contain recognizable functional groups that can be used to describe the polymer. This terminology often emphasizes the functional groups that were involved in the synthesis of the polymer from its monomers, although the usage is seldom exact. Some examples:

Polyester

Polyimide

Polyether

A polymer is often named according to the monomer that was used to form it. This is why the polymer consisting of only a long chain of CH2 groups is called polyethylene, not polymethylene.

Polyethylene

Similarly, the polyamide containing 6 carbons is known as polycaprolactam.

Polycaprolactam

Of course, there are systematic rules for polymer nomenclature set forth by the IUPAC, but for polymers with any complexity, the names are too cumbersome for common usage. Here are two examples:

Poly(vinylbutyral)

Polycarbonate

References to IUPAC Polymer Nomenclature:


Slang and Trade Names

Unfortunately, many slang names have become associated with common polymers. These slang names often contain vestiges of the chemical description of the polymer, but shortened or corrupted.

The two polymers named by the IUPAC systematic rules above are good examples. Poly((2-propyl-1,3-dioxane-4,6-diyl)methylene is known as "polyvinylbutyral," probably because it is synthesized for poly(vinyl alcohol) and butyraldehyde.

Poly(oxycarbonyloxy-1,4-phenylene-isopropylidine-1,4-phenylene) is an even worse case. Because it contains the carbonate linkage (highlighted), it is a kind of polycarbonate, for which there are infinite possible variations. However, because it is by far the most common commercial polymer of its kind, the term "polycarbonate" has come to refer to this particular member of the polycarbonate family. Confusing but true...

Even more maddening are the trade names, which are widely used but convey no structural information at all. The trade names were invented by marketing people in industry, who did not care about chemistry but wanted to have short, snappy-sounding names that could be easily remembered and spelled by their customers (who are often design engineers or even purchasing agents, not chemists or chemical engineers).

Quiz: Match the structures above to the following trade names: (Answers)

Butvar (TMMonsanto)
Dacron (TMDuPont)
Lexan (TM General Electric)
Nylon 6 (TM DuPont)
Noryl (TM General Electric)
Kapton (TM DuPont)
Dowlex (TM Dow)

Compositional Features

Taking a slightly coarser view, one can classify the chain by the number of different repeat units and their sequence (i.e., homopolymer and copolymer).


Chain Architecture Features

Again looking at the chains in decreasing detail, there is an infinite variety of chain architectures (i.e., linear, branched, crosslinked).


Physical Property Classifications

Looking at polymers as materials rather than from a chemical point of view, several classifications are widely used. There are a few polymers that don't fit well into to these categories, but overall the terms are descriptive and useful.

Processability. Some polymers can be readily melted and then molded into any shape. These are known as thermoplastics, and they usually have linear or branched architectures. Most thermoplastics can also be dissolved in suitable solvents. Some other polymers decompose on heating before they can melt. These are known as thermosets, and they are usually crosslinked and therefore insoluble. To form a part out of a thermoset, one usually synthesizes the polymer in the mold itself. Once the polymer has cured, the only way to reshape the part is by machining (e.g., grinding, drilling, etc.)

Physical performance. Polymers that stretch and rebound are called rubbers or elastomers. These materials are usually crosslinked, either by covalent bonds, or, in several modern cases, by noncovalent forces such as H-bonds. Other solid polymers are known simply as plastics, adhesives, or fibers, depending on their application. The word resin is a generic term for polymer, although occasionally it indicates a thermoset.

A common physical measurement that provides distinctive curve shapes for the different physical classes of polymers is called tensile testing. The measurement is carried out by stretching a sample of polymer at a controlled rate, and measuring the amount of force required. The curve stops when the sample breaks, and the area under the curve is a measure of the energy required to make this sample fail.

Tensile test

Stress=Force
  
Strain=Elongation

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