Polymer chemistry
Rubber, plastics, fibres
Connect polymer-chain structure and intermolecular forces to the contrasting behavior of elastomers, plastics, and fibres, with examples of synthesis, processing, and use.
IntuitionOne family of long molecules, many useful materials
A polymer is a population of long molecules, not a single rigid object. Chains that slide past one another can make a soft plastic; chains tied at sparse points recover their shape like rubber; chains aligned and strongly attracted can form tough fibres.
SchoolStructure, properties, and material classes
Definition: Elastomer
A polymer that can undergo large, mostly reversible deformation. Flexible chains uncoil under load and recoil when it is removed; a sparse network of cross-links prevents permanent flow.
Definition: Thermoplastic and thermoset
Thermoplastics soften on heating and can often be reshaped because their chains are not joined into a permanent covalent network. Thermosets cure into a network that does not melt on reheating; excessive heat eventually causes chemical degradation.
| Class | Chain structure and typical behavior |
|---|---|
| Elastomer | Flexible chains, sparse cross-links; large reversible stretch |
| Thermoplastic | Linear or branched chains; softens and flows on heating |
| Thermoset | Covalent network; rigid, does not melt on reheating |
| Fibre | Long chains aligned along the fibre axis; high tensile strength |
Names describe different things: thermoplastic/thermoset refers to response to heat and network structure; elastomer describes mechanical response; fibre describes shape and orientation. A material may fit more than one description.
In addition polymerization, the C=C bond opens and monomer units join without eliminating a small molecule. Polyethylene contains the repeat unit –CH₂–CH₂–; chain length, branching, and crystallinity strongly affect its properties.
Example: A simple degree-of-polymerization estimate
An ideal polyethylene chain has molar mass 28,000 g mol⁻¹. Estimate the number of ethylene repeat units, using 28.0 g mol⁻¹ per –CH₂–CH₂– unit and neglecting end groups.
Solution
The repeat-unit count is approximately 28,000 ÷ 28.0 = 1,000. Thus the degree of polymerization is about 10³; real samples contain a distribution of chain lengths.
Definition: Natural rubber and vulcanization
Natural rubber is mainly cis-1,4-polyisoprene. Sulfur vulcanization forms cross-links between chains. Too few links allow creep and tack; too many restrict chain motion and make the rubber hard and less extensible.
Fibre strength depends on more than the identity of the polymer: drawing aligns chains, while intermolecular attractions and crystallinity help transfer load. Nylon and polyester are synthetic fibres; regenerated cellulose is made by processing cellulose rather than polymerizing a new monomer.
References
- Textbook of Polymer Science, 3rd Edition · Fred W. Billmeyer, Jr., 1984