T
he word polymer comes from the Greek roots “poly,” meaning many, and “meros,” meaning units or parts. Polymers are bonded chains of repeated units with covalent bonds from end to end. The building blocks that are repeated in the chain are called monomers. As monomers begin to con-nect together to form chains, they become oligomers. As more monomers add to the oligomer chain, it grows and eventually becomes a polymer when the addition of one more monomer unit would have no discernible effect on the properties of the chain.A polymer might have 10,000 or 1,000,000 repeated monomer units in a chain, which makes it impractical to draw the entire polymer molecule. Instead, the polymer chain is identifi ed by its structural unit (or repeat unit), which is the smallest part of the chain that repeats. The structural unit of polystyrene is shown in Figure 5-1.
The n represents the number of times the structural unit is repeated in the chain. Of course, polymers are not made one chain at a time. Large numbers of chains begin forming simultaneously, and different chains grow to different lengths. The number of repeat units is called the degree of polymerization of the polymer chain and is represented by the symbol DPn.
The covalently bonded atoms (usually carbon) that comprise the long, repeating center of the chain are called the polymer backbone. Atoms attached to the backbone are called side groups or substituents. Hydrogen is the most common side group, but methyl groups, benzene rings, hydroxide molecules, heteroatoms, or even other polymer chains can serve as side groups as well.
When drawing a polymer chain, any side group not specifi cally shown is assumed to be hydrogen.
Polymers are classifi ed based on their ability to be remelted and reshaped.
Thermoplastics fl ow like viscous liquids when heated and continue to do so when reheated and recooled multiple times. Thermoplastics usually are mass produced as pellets that can be dyed, melted, and reshaped by end users. While the individual chains in thermoplastics have covalent bonds along their primary axis, the bonding between chains usually is limited to weak Van Der Waals interactions. In most cases there is no three-dimensional ordering between the chains and often little, if any, two-dimensional order. The apparently random relative positioning of adjacent chains often is described as “spaghetti on a plate,” as shown in Figure 5-2. The lack of bonding between chains tends to reduce the tensile strength of thermoplastics but makes them relatively easy to recycle.
By contrast, when the chemicals that form thermosets are heated, they slowly undergo an irreversible chemical cross-linking reaction that bonds the chains together and causes the liquid to become an infusible solid mass. Once solidifi ed, thermosets cannot be remelted or re-formed. For this reason, the polymerization reaction is performed in a mold or fi ber spinneret so that the thermoset immediately takes on its fi nal shape. The cross-linking between chains, illustrated in Figure 5-2, makes thermosets stronger and more resistant to chemical degradation than thermoplastics, but it also makes them diffi cult to recycle.
Decisions regarding whether to use thermosets or thermoplastics for spe-cifi c applications create interesting ethical dilemmas. Cost and environmen-tal impact tend to favor the use of thermoplastics. Imagine if grocery bags were made stronger using thermosets but could not be recycled and cost a quarter each. By contrast, aviation equipment, ballistic resistance materials, and many military supplies demand the higher performance offered by thermosets.
Many polymers are made of a single structural unit, repeated many times, but this is not the only possibility. When a polymer is formed from the polym-erization of two or more monomers, it is called a copolymer. Four distinct classifi cations for copolymers made from two monomers (A and B) exist that distinguish how the monomers blend. These classifi cations are shown in Figure 5-3.
[ CH2 CH ]n
Figure 5-1 Structural Unit of Polystyrene
| Structural Unit | Smallest repeating unit in a polymer. Also known as a repeat unit.
| Degree of Polymerization | Number of repeat units in a polymer chain.
| Polymer Backbone | Covalently bonded atoms, which are usually carbon, that comprise the center of the polymer chain.
| Side Groups | Atoms attached to the polymer backbone. Also called substituents.
| Thermoplastics | Polymers with low melting points due to the lack of covalent bonding between adjacent chains. Such polymers can be repeatedly melted and re-formed.
| Thermosets | Polymers that cannot be repeatedly melted and re-formed because of strong covalent bonding between chains.
5.1 | Polymer Terminology 151
| Copolymer |
Polymer made up of two or more different monomers covalently bonded together.
Random copolymers add either monomer in any order such that the proba-bility of the next link the chain being monomer A or monomer B is not affected by the identity of the last monomer. By contrast, alternating copolymers always follow monomer A with monomer B and vice versa. The length of the chain may vary, but it will always follow the A-B-A-B pattern. Block copolymers involve long runs of monomer A, followed by long runs of monomer B, followed by
Thermoplastic
• Flows like viscous liquids when heated and continue to do so when reheated and recooled multiple times
• Weak Van Der Waal forces between chains
• Random relative positioning of adjacent chains
Thermoset
• Cannot be remelted or reformed
• Strong cross-linking between chains due to Van Der Waal forces
• Stronger and more resistant to chemical degradation than thermoplastics, but also difficult to recycle
Figure 5-2 Thermoplastic versus Thermoset Polymers
Random: No particular pattern of monomers
Alternating: One monomer is always followed by the other and vice versa
Block: Long runs of one monomer, followed by long runs of the other, followed by
more of the first
Graft: Chain of one monomer is connected as a substituent to a chain of the
other monomer Figure 5-3 Classes of
Copolymers
| Random Copolymers | Polymers comprised of two or more different monomers, which attach to the polymer chain in no particular order or pattern.
| Alternating Copolymers | Polymers comprised of two or more different monomer units that attach to the chain in an alternating pattern (A-B-A-B-A-B).
| Block Copolymers | Polymers comprised of two or more different monomers that attach to the chain in long runs of one type of monomer, followed by long runs of another monomer (AAAAAAABBBBBBBBAAAAA).
more monomer A. Graft copolymers result when a chain of one monomer (B) is connected as a substituent to a chain of the other monomer (A).
Polymer blends are formed by the mechanical mixing of polymers. Blends allow materials to experience a wider range of properties without going through the diffi culty and expense of trying to synthesize a single polymer with the same combination of properties. Blends tend to exhibit properties between those of the original polymers. For example, the toughness of polystyrene can be enhanced by blending small amounts of a rubbery polymer that absorbs impact energy.
Artifi cial surfaces for running tracks are a blend of polyurethane and rubber.
Blends are far more diffi cult to recycle because of the problems in separating out the original polymers.