Polymer Chemistry

Classification of Polymers

The most common way of classifying polymers is to separate them into three groups - thermoplastics, thermosets, and elastomers5. The thermoplastics can be divided into two types - those that are crystalline and those that are amorphous. You may click on the words in the diagram below to learn more about these classifications.


Return to Polymer Morphology

Return to Main Polymer Menu



Molecules in a thermoplastic are held together by relatively weak intermolecular forces so that the material softens when exposed to heat and then returns to its original condition when cooled16. Thermoplastic polymers can be repeatedly softened by heating and then solidified by cooling - a process similar to the repeated melting and cooling of metals. Most linear and slightly branched polymers are thermoplastic. All the major thermoplastics are produced by chain polymerization17.

Thermoplastics have a wide range of applications because they can be formed and reformed in so many shapes. Some examples are food packaging, insulation, automobile bumpers, and credit cards.

    Return to Top



A thermosetting plastic, or thermoset, solidifies or "sets" irreversibly when heated. Thermosets cannot be reshaped by heating. Thermosets usually are three-dimensional networked polymers in which there is a high degree of cross-linking between polymer chains. The cross-linking restricts the motion of the chains and leads to a rigid material.

A simulated skeletal structure of a network polymer with a high cross-link density is shown at the right.

Thermosets are strong and durable. They primarily are used in automobiles and construction. They also are used to make toys, varnishes, boat hulls, and glues16.

    Return to Top



Elastomers are rubbery polymers that can be stretched easily to several times their unstretched length and which rapidly return to their original dimensions when the applied stress is released.

Elastomers are cross-linked, but have a low cross-link density. The polymer chains still have some freedom to move, but are prevented from permanently moving relative to each other by the cross-links.

To stretch, the polymer chains must not be part of a rigid solid - either a glass or a crystal. An elastomer must be above its glass transition temperature, Tg, and have a low degree of crystallinity.

Rubber bands and other elastics are made of elastomers.

    Return to Top

Return to Polymer Morphology

Return to Main Polymer Menu

Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grants DUE #9950809 and DUE #9950296. Additional support was provided by the Camille and Henry Dreyfus Foundation. PCOL faculty also acknowledge the National Science Teachers Association which awarded the PCOL Faculty Consortium the 1998 Gustav Ohaus Award for Innovation in College Science Teaching.

Links: Main Menu Composition Molec. Weight Topology Isomerism Morphology
    Transitions Mechanical Experimental References  

This page was last updated on July11, 2000