Classification of Polymers based on Molecular Forces
Understand why rubbers stretch, fibers are strong, and plastics melt.
Have you ever wondered why a rubber band stretches while a nylon rope does not? Why does a plastic water bottle melt when heated, but the black handle of a frying pan remains rigid? The answer lies in the intermolecular forces (van der Waals forces, hydrogen bonds, and covalent cross-links) operating between the long polymer chains.
Based on the magnitude of these intermolecular forces, polymers are divided into four main categories, which govern their mechanical properties like tensile strength, elasticity, and toughness.
1. Elastomers
These are rubber-like solids with highly elastic properties. You can stretch them under force, and when the force is removed, they snap back to their original shape.
- Intermolecular Forces: They have the weakest intermolecular forces of attraction among all polymers. This allows the polymer chains to stretch out easily.
- Structure: A few 'cross-links' are introduced between the chains (e.g., Sulfur bridges in vulcanized rubber). These cross-links act like chemical anchors, preventing the chains from slipping past each other permanently, helping the polymer retract to its original position.
- Examples: Natural rubber, Buna-S, Buna-N, Neoprene, Vulcanized rubber.
2. Fibers
Fibers are thread-forming solids which possess high tensile strength and high modulus (they are stiff and don't stretch easily). They are used in textiles, ropes, and fabrics.
- Intermolecular Forces: They possess the strongest intermolecular forces, such as strong hydrogen bonding or strong dipole-dipole interactions.
- Structure: Because of these strong forces, the polymer chains are closely packed in a highly ordered, crystalline nature. This dense packing gives them incredible strength.
- Examples: Polyamides (Nylon 6,6), Polyesters (Terylene/Dacron), Polyacrylonitrile (Orlon).
3. Thermoplastic Polymers
These are your everyday "plastics". They become soft and viscous on heating and become hard rigid solids on cooling. This process can be repeated infinitely, meaning they can be easily remolded and recycled.
- Intermolecular Forces: The forces of attraction are intermediate between those of elastomers (weakest) and fibers (strongest).
- Structure: They are typically linear or slightly branched long-chain molecules. There are no cross-links between the chains, which allows them to slide past one another when heated (melting).
- Examples: Polythene (PE), Polyvinyl chloride (PVC), Polystyrene, Teflon.
4. Thermosetting Polymers
These polymers undergo a permanent chemical change upon heating. Once they are heated and molded into a shape, they become hard, infusible, and insoluble mass. They cannot be softened again on heating, meaning they cannot be remolded or recycled.
- Intermolecular Forces / Structure: They are heavily branched and contain extensive covalent cross-links between the polymeric chains, forming a rigid 3D network. Heating them just creates more cross-links until the material breaks down (chars) instead of melting.
- Examples: Bakelite (used in electrical switches and frying pan handles), Melamine-formaldehyde resin (unbreakable crockery), Urea-formaldehyde resin.
Figure 1: Visual comparison of polymer micro-structures based on molecular forces.
Summary: Order of Intermolecular Forces
| Property | Elastomers | Thermoplastics | Fibers |
|---|---|---|---|
| Intermolecular Forces | Weakest | Intermediate | Strongest |
| Nature of Forces | van der Waals | van der Waals / Dipole | H-bonding / Strong Dipole |
| Order of Strength | Elastomers < Thermoplastics < Fibers | ||
*Note: Thermosetting polymers are generally not placed in this direct linear comparison because their properties are dictated by permanent chemical covalent bonds (cross-links) rather than physical intermolecular forces.
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