Polymers are materials made from long repeating molecular chains, and their structure controls how they bend, stretch, melt, and break. Engineers use polymers in packaging, aircraft parts, medical devices, electronics, textiles, and adhesives because their properties can be tuned over a wide range. Small changes in chain length, branching, crosslinking, and crystallinity can turn a soft rubbery material into a tough plastic or a rigid composite matrix.
A polymer block can contain tangled amorphous regions, ordered crystalline regions, and chemical crosslinks that tie chains together. Thermoplastics soften when heated because their chains can slide past one another, while thermosets keep their shape because crosslinks form a permanent network. The glass transition temperature, Tg, marks the change between a hard glassy state and a softer rubbery state in amorphous regions, while melting temperature, Tm, applies to crystalline regions.
Key Facts
- Degree of polymerization: DP = molecular mass of polymer chain / molecular mass of repeat unit.
- Thermoplastics can be melted and reshaped because their chains are not permanently crosslinked.
- Thermosets do not melt on reheating because covalent crosslinks lock chains into a 3D network.
- Crystallinity increases density, stiffness, strength, and chemical resistance, but often lowers transparency and impact toughness.
- Below Tg, amorphous polymer chains have limited motion and the material is glassy and stiff.
- Above Tg, amorphous chain segments move more freely and the material becomes softer and more rubbery.
Vocabulary
- Polymer
- A polymer is a large molecule made of many repeating units called monomers bonded into long chains.
- Thermoplastic
- A thermoplastic is a polymer that softens when heated and hardens when cooled, allowing it to be reshaped.
- Thermoset
- A thermoset is a polymer that forms permanent crosslinks and cannot be remelted after curing.
- Crystallinity
- Crystallinity is the fraction of a polymer where chains are packed in an ordered, repeating arrangement.
- Glass transition temperature
- The glass transition temperature, Tg, is the temperature where amorphous polymer regions change from glassy and rigid to rubbery and flexible.
Common Mistakes to Avoid
- Treating Tg and Tm as the same temperature is wrong because Tg is a softening transition in amorphous regions, while Tm is melting of crystalline regions.
- Assuming all polymers melt when heated is wrong because thermosets are crosslinked networks that usually degrade before they flow.
- Calling a polymer fully crystalline is usually wrong because most polymers contain both ordered crystalline regions and disordered amorphous regions.
- Ignoring chain structure when predicting properties is wrong because branching, chain length, crosslink density, and crystallinity strongly affect stiffness, toughness, and temperature limits.
Practice Questions
- 1 A polymer chain has a molecular mass of 84,000 g/mol and its repeat unit has a molecular mass of 42 g/mol. Calculate the degree of polymerization.
- 2 A polymer sample is 40 percent crystalline and has a total mass of 250 g. What mass of the sample is in crystalline regions, assuming the percentage is by mass?
- 3 Two polymers have the same chemical repeat unit, but one is lightly branched and mostly amorphous while the other is highly linear and more crystalline. Explain which one is likely stiffer at room temperature and why.