Bonding in polymers

Explaining Bulk Properties: Melting Point and Strength

Why does it take so much energy to melt a solid block of plastic? The answer lies in the total strength of the intermolecular forces.

Because polymer molecules are exceptionally long, there are countless contact points between adjacent chains. This creates massive total intermolecular forces that require significant thermal energy to overcome, resulting in high bulk properties like a high melting point, high hardness, and high tensile strength.

Unlike pure simple molecules, polymers often melt over a range of temperatures (a softening point) rather than at one sharp temperature. This occurs because the polymer chains within a single sample have varying lengths and different degrees of tangling.

Flexibility and Stiffness

Every time you bend a flexible plastic cable, the polymer chains inside are physically moving. Flexibility depends entirely on the ability of polymer chains to slide over each other.

If the intermolecular forces are relatively weak, or if the chains are not tightly packed, they can easily slip past one another. Conversely, stiffness increases when chains become heavily entangled (like tangled string) or highly ordered, a property known as crystallinity.

Manufacturers can modify these properties by adding a plasticiser. These are small molecules that wedge themselves between the polymer chains, pushing them further apart. This reduces the effectiveness of the intermolecular forces, allowing the chains to slide much more easily and making the material highly flexible.

Thermosoftening vs. Thermosetting Polymers

Understanding polymer bonding explains why a plastic milk bottle melts in a fire, but a plastic electrical plug does not. Polymers are divided into two main categories based on their thermal behaviour.

Thermosoftening polymers consist of individual, tangled chains held together only by weak intermolecular forces. When heated, these forces are easily overcome, allowing the chains to slide over one another so the plastic melts and can be remoulded upon cooling.

Thermosetting polymers, on the other hand, contain cross-links between their chains. A cross-link is a strong covalent bond that forms a physical bridge connecting adjacent polymer chains together.

These cross-links create a rigid three-dimensional network that physically prevents the chains from sliding. Because covalent bonds require immense energy to break, thermosetting polymers do not melt when heated; they remain stiff and rigid until the temperature is so high that they char and decompose.

Worked Example: Comparing Polymer Properties

Explain why poly(ethene) is flexible enough for plastic bags, but melamine is stiff enough for hard plastic plates.

Step 1: Identify the type of polymer and its specific forces.

• Poly(ethene) is a thermosoftening polymer consisting of long chains held together only by weak intermolecular forces.

Step 2: Link the forces to the mechanism of flexibility.

• Because the intermolecular forces are weak and there are no covalent bridges, the polymer chains can easily slide over each other when a force is applied, making it flexible.

Step 3: Identify the opposing polymer's structure.

• Melamine is a thermosetting polymer that contains strong covalent cross-links between its adjacent chains.

Step 4: Link the structure to the property of stiffness.

• These cross-links form a rigid 3D network that physically prevents the chains from sliding past each other, resulting in a stiff, hard material that resists deformation.