Polymers: Structure and Properties
- Consider the plastic water bottle in your hand or the rubber soles of your shoes.
- These everyday materials owe their incredible versatility to polymers: large molecules made of repeating subunits called monomers.
- But why are polymers so important?
- And how does their structure influence their properties?
What Are Polymers?
Polymers
Polymers are large molecules composed of repeating structural units called monomers, linked together by covalent bonds to form long chains.
- Polymers are macromolecules, meaning they consist of thousands, or even millions, of atoms bonded together in a repeating structure.
- Each polymer is made up of smaller units called monomers that are chemically bonded to form long chains.
Examples of Polymers
- Natural Polymers:
- Cellulose: Found in plants, providing structural support.
- Proteins: Made of amino acids, responsible for countless biological functions.
- Natural Rubber: Harvested from rubber trees.
- Synthetic Polymers:
- Polyethylene (PE): Found in plastic bags and bottles.
- Polypropylene (PP): Used in food containers and automotive parts.
- Polystyrene (PS): Commonly used in packaging and insulation.
Natural rubber is an example of a polymer formed from the monomer isoprene, while synthetic polymers like polyethylene are made by polymerizing ethene molecules.
How Structure Influences Properties
The properties of polymers depend on their chemical structure, including:
- Type of Monomer: The identity of the repeating unit affects physical and chemical properties.
- Length of Polymer Chains: Longer chains usually result in higher strength and melting points.
- Degree of Cross-Linking: The presence of cross-links between chains affects flexibility and durability.
- Branching: Side chains on the main polymer chain influence density and toughness.
Understanding how structural features like cross-linking and chain length impact polymer properties can help in designing materials for specific applications, such as flexible plastics or tough coatings.
Structure of Polymers
Polymers can be classified into two main structural types:
- Linear Polymers:
- Composed of long, straight chains.
- Example: High-density polyethylene (HDPE).
- Branched Polymers:
- Chains have side branches.
- Example: Low-density polyethylene (LDPE).
- Cross-Linked Polymers:
- Chains are linked together at various points, forming a network.
- Example: Vulcanized rubber.
Cross-linking increases strength and elasticity, making cross-linked polymers ideal for applications like car tires.
Thermoplastics vs. Thermosetting Polymers
- Thermoplastics:
- Can be melted and reshaped repeatedly.
- Have weak intermolecular forces that break when heated.
- Examples: Polyethylene, polystyrene.
- Thermosetting Polymers:
- Cannot be reshaped after curing.
- Contain strong covalent bonds that prevent melting.
- Example: Epoxy resin.
- Don’t confuse thermoplastics with thermosetting polymers!
- Thermoplastics soften upon heating, while thermosets retain their shape.
Common Properties of Plastics
Plastics, a subset of synthetic polymers, exhibit a wide range of properties due to their molecular structure.
- Strength and Flexibility:
- Plastics with long, linear chains (e.g., HDPE) tend to be stronger and less flexible.
- Plastics with branching (e.g., LDPE) are more flexible but less strong.
- Thermal Properties:
- Thermoplastics melt upon heating, while thermosetting plastics decompose without melting.
- Chemical Resistance:
- Most plastics are resistant to chemicals because of their stable covalent bonds.
- Lightweight:
- Plastics have low densities, making them useful for transportation and packaging.
Plastics like polytetrafluoroethylene (PTFE) are particularly resistant to chemical attack, which is why they’re used in nonstick cookware.
- Why do some polymers, like thermosetting plastics, resist melting while others soften upon heating?
- How does branching affect the density and strength of polymers?
- Can you think of an application where the elasticity of a polymer is essential? What structural features contribute to this property?
