Classifying Polymers

Introduction

There are thousands of commercial polymers, so chemists need ways to sort them. A single polymer can be described by where it comes from, by how it was made, by the shape of its chains and by what happens when you heat it or pull it. This page concentrates on two practical schemes: classification by origin and classification by thermal and mechanical behaviour . The second is especially important because it decides how a material can be shaped, used and recycled.

Core explanation

By origin. Natural polymers are made by living things: cellulose and starch (polysaccharides), proteins such as silk, wool and collagen, natural rubber (cis-poly(isoprene)) and the nucleic acids. Semi-synthetic polymers are natural polymers that have been chemically modified — cellulose acetate, used in film and spectacle frames, is cellulose with many hydroxyl groups converted to ester groups. Synthetic polymers such as polyethylene, poly(styrene), PVC, nylons and polyesters are made entirely from small monomers, most of them derived at present from petroleum.

By thermal behaviour. Heating separates polymers into three broad classes.

Thermoplastics consist of separate linear or branched chains held together only by intermolecular forces and entanglement. On heating these forces are overcome, the chains slide past one another and the material softens and flows; on cooling it hardens again. The cycle can be repeated, so thermoplastics can be moulded, extruded and in principle recycled by remelting. Polyethylene, polypropylene, PET, poly(styrene) and PVC are thermoplastics, and they make up the great majority of plastic produced.

Thermosets start as small or branched molecules (a resin) that react during curing to form a three-dimensional covalent network. Once cured, every chain is bonded to its neighbours, so heating cannot make them flow; strong heating eventually breaks covalent bonds and the material chars. Epoxy resins, phenol–methanal (Bakelite) resins, melamine resins and vulcanised hard rubbers are examples. They are hard, dimensionally stable and heat-resistant but cannot be remelted.

Elastomers are rubbery materials that can be stretched to several times their length and snap back. They are polymers used above their glass-transition temperature, so their chain segments move freely, and they carry a light network of cross-links — perhaps one cross-link per few hundred backbone atoms. The cross-links stop chains flowing permanently past each other, so the material recovers its shape when released. Vulcanised natural rubber, in which sulfur bridges join poly(isoprene) chains, is the classic example.

The underlying pattern. The three classes differ mainly in cross-link density. No cross-links: thermoplastic. Light cross-linking of flexible chains: elastomer. Dense cross-linking: rigid thermoset. Thermoplastic elastomers blur the boundary: they use physical rather than covalent cross-links (for example hard glassy domains in a block copolymer), so they are rubbery at room temperature yet can be melted and reshaped.

Other schemes. Polymers are also classified by mechanism of formation (step or chain growth), by architecture (linear, branched, network) and by use (fibres, plastics, coatings, adhesives). These schemes overlap: nylon-6,6 is synthetic, step-growth, linear, a thermoplastic and a fibre.

Step-by-step reasoning

To decide the class of an unknown polymer from its behaviour:

1. Heat a sample gently in thought: does it soften and flow? If yes, it is a thermoplastic. 2. If it does not flow and eventually chars, it is a network: a thermoset or an elastomer. 3. At room temperature, is it rubbery and highly extensible with full recovery? Then it is an elastomer. 4. If it is hard and rigid, it is a thermoset.

Visual explanation

Draw three boxes of spaghetti. In the first the strands are loose and can be pulled apart one by one (thermoplastic). In the second a few strands are tied together at occasional points (elastomer). In the third the strands are knotted to each other everywhere into a single rigid mat (thermoset).

Real-world analogy

Thermoplastics are like chocolate: melt it, pour it into a new mould and it sets again. Thermosets are like a boiled egg or a baked cake: once the chemistry has happened on heating, reheating will never return it to a runny liquid.

Real-world example

A PET drinks bottle can be shredded, melted and re-formed into fibres for fleece clothing because PET is a thermoplastic. A circuit board made from glass-fibre-reinforced epoxy cannot be remelted; it is ground up or processed chemically instead, which is one reason thermosets are harder to recycle.

Why?

Why can a thermoplastic be remelted but a thermoset cannot? Melting a thermoplastic only overcomes intermolecular forces, which reform on cooling. In a thermoset the chains are linked by covalent bonds, and flow would require breaking those bonds — which happens only at temperatures where the polymer decomposes.

Common misconception

"Natural polymers are always soft and synthetic ones always hard." Cellulose in wood and silk fibroin are very strong natural polymers, whereas synthetic silicone rubbers are soft. Origin does not determine mechanical behaviour; structure does.

Worked example

Question: A material is rubbery at 20 °C, recovers after stretching, does not melt when heated and chars at high temperature. Classify it and explain the structure.

Reasoning: Recovery after large stretching means flexible chains above their glass transition. Not melting means covalent cross-links. Rubberiness means the cross-links are sparse.

Answer: It is a covalently cross-linked elastomer, such as vulcanised rubber: flexible chains joined by a light network of cross-links.

Quick check

1. Why does a lightly cross-linked rubber return to its original shape after stretching? Answer: The cross-links stop the chains flowing permanently past each other, so they pull back when the force is removed.

Exam focus

Link each class to structure: thermoplastic means no covalent cross-links, elastomer means light cross-linking above Tg, thermoset means dense network. Be ready to explain recyclability and processing (moulding versus curing) in terms of those structures.

Advanced insight

Vitrimers are a newer class of networks whose cross-links are dynamic covalent bonds, such as esters that exchange partners on heating in the presence of a catalyst. The total number of cross-links stays constant, so the material behaves as a thermoset in use but can be reshaped and repaired at high temperature, combining the advantages of both classes.

Summary

Polymers can be natural, semi-synthetic or synthetic. By thermal behaviour they are thermoplastics (separate chains, remeltable), thermosets (dense covalent networks, not remeltable) or elastomers (lightly cross-linked flexible chains that stretch and recover). Cross-link density is the key structural variable connecting these classes.

Practice questions

1. Classify cellulose acetate by origin and explain your answer. Answer: Semi-synthetic: it is made by chemically modifying a natural polymer, cellulose, converting hydroxyl groups into ester groups. 2. Explain why polypropylene can be injection moulded but a cured epoxy adhesive cannot. Answer: Polypropylene chains are held only by intermolecular forces, so on heating they flow and can be moulded; the epoxy is a covalent network that cannot flow without breaking bonds. 3. State the structural difference between an elastomer and a rigid thermoset. Answer: Both are cross-linked, but an elastomer has a low cross-link density and flexible chains above Tg, whereas a thermoset has a high cross-link density giving a rigid network. 4. Suggest why thermosets are more difficult to recycle than thermoplastics. Answer: They cannot be melted and re-formed because their chains are covalently linked, so they must be ground as filler or chemically broken down.