Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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4.9 Thermoplastic and Thermosetting Polymers • 115

Molecular

characteristics

Chemistry

(repeat unit

composition)

Size

(molecular weight)

Shape

(chain twisting,

entanglement, etc.)

Structure

Linear Branched

Isomeric states

Stereoisomers

Isotactic Syndiotactic Atactic cis trans

Geometrical isomers

Crosslinked Network

Figure 4.8

Classification scheme

for the characteristics

of polymer

molecules.

specify molecular structure in terms of more than one. For example, a linear polymer

may also be isotactic.

Concept Check 4.3

What is the difference betweenconfigurationandconformationin relation to polymer

chains?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

4.9 THERMOPLASTIC AND THERMOSETTING

POLYMERS

The response of a polymer to mechanical forces at elevated temperatures is related to

its dominant molecular structure. In fact, one classification scheme for these materials

thermoplastic

polymer

is according to behavior with rising temperature.Thermoplastics(orthermoplastic

polymers) andthermosets(orthermosetting polymers) are the two subdivisions. Ther-

thermosetting

polymer

moplastics soften when heated (and eventually liquefy) and harden when cooled—

processes that are totally reversible and may be repeated. On a molecular level, as

the temperature is raised, secondary bonding forces are diminished (by increased

molecular motion) so that the relative movement of adjacent chains is facilitated

when a stress is applied. Irreversible degradation results when a molten thermoplastic

polymer is raised to too high a temperature. In addition, thermoplastics are relatively