Polymers And Plastics: A Chemical Introduction - Chem1

1 Polymers are different...

Let's begin by looking at an artificial polymer that is known to everyone in the form of flexible, transparent plastic bags: polyethylene. It is also one of the simplest polymers, consisting of random-length (but generally very long) chains made up of two-carbon units.

You will notice some "fuzziness" in the way that the polyethylene structures are represented above. The squiggly lines at the ends of the long structure indicate that the same pattern extends indefinitely. The more compact notation on the right shows the minimal repeating unit enclosed in brackets overprinted with a dash; this means the same thing and is the preferred way of depicting polymer structures.

How polymers are classified

Polymers can be classified in ways that reflect their chemical makeup, or perhaps more importantly, their properties and applications. Many of these factors are strongly interdependent, and most are discussed in much more detail in subsequent sections of this page.

Classification by structure

• Nature of the monomeric units
• Average chain length and molecular weight
• Homopolymers (one kind of monomeric unit) or copolymers;
• Chain topology: how the monomeric units are connected
• Presence or absence of cross-branching
• Method of polymerization

Classification by properties:

• Density
• Thermal properties — can they soften or melt when heated?
• Degree of crystallinity
• Physical properties such as hardness, strength, machineability.
• Solubility, permeability to gases

Classification by applications:

• molded and formed objects ("plastics")
• sheets and films
• elastomers (i.e., elastic polymers such as rubber)
• adhesives
• coatings, paints, inks
• fibres and yarns

Physical properties of polymers

The physical properties of a polymer such as its strength and flexibility depend on:

• chain length - in general, the longer the chains the stronger the polymer;
• side groups - polar side groups (including those that lead to hydrogen bonding) give stronger attraction between polymer chains, making the polymer stronger;
• branching - straight, unbranched chains can pack together more closely than highly branched chains, giving polymers that have higher density, are more crystalline and therefore stronger;
• cross-linking - if polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to melt.

Classification by degree of crystallinity

The crystalline parts of this polymer are shown in blue. [source]

For a very understandable discussion of polymer crystallinity, see this Macrogalleria page.

The spaghetti-like entanglements of polymer molecules tend to produce amorphous solids, but it often happens that some parts can become sufficiently aligned to produce a region exhibiting crystal-like order, so it is not uncommon for some polymeric solids to consist of a random mixture of amorphous and crystalline regions. As might be expected, shorter and less-branched polymer chains can more easily organize themselves into ordered layers than can long chains. Hydrogen-bonding between adjacent chains also helps, and is very important in fiber-forming polymers both synthetic (Nylon 6.6) and natural (cotton cellulose).

Classification by thermal properties: thermoplastics and thermosets

Pure crystalline solids have definite melting points, but polymers, if they melt at all, exhibit a more complex behavior. At low temperatures, the tangled polymer chains tend to behave as rigid glasses. For example, the natural polymer that we call rubber becomes hard and brittle when cooled to liquid nitrogen temperature. Many synthetic polymers remain in this state to well above room temperature.

The melting of a crystalline compound corresponds to a sudden loss of long-range order; this is the fundamental reason that such solids exhibit definite melting points, and it is why there is no intermediate form between the liquid and the solid states. In amorphous solids there is no long-range order, so there is no melting point in the usual sense. Such solids simply become less and less viscous as the temperature is raised.

In some polymers (known as thermoplastics) there is a fairly definite softening point that is observed when the thermal kinetic energy becomes high enough to allow internal rotation to occur within the bonds and to allow the individual molecules to slide independently of their neighbors, thus rendering them more flexible and deformable. This defines the glass transition temperature tg .

See here for a more complete definition of the glass transition temperature.

Depending on the degree of crystallinity, there will be a higher temperature, the melting point tm , at which the crystalline regions come apart and the material becomes a viscous liquid. Such liquids can easily be injected into molds to manufacture objects of various shapes, or extruded into sheets or fibers.

Other polymers (generally those that are highly cross-linked) do not melt at all; these are known as thermosets. If they are to be made into molded objects, the polymerization reaction must take place within the molds — a far more complicated process. About 20% of the commercially-produced polymers are thermosets; the remainder are thermoplastics.