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Callister - An introduction - 8th edition

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550 • Chapter 14 / Polymer Structures<br />

Figure 14.8<br />

Classification<br />

scheme for the<br />

characteristics of<br />

polymer molecules.<br />

Molecular<br />

characteristics<br />

Chemistry<br />

(repeat unit<br />

composition)<br />

Size<br />

(molecular weight)<br />

Shape<br />

(chain twisting,<br />

entanglement, etc.)<br />

Structure<br />

Linear<br />

Branched<br />

Crosslinked<br />

Network<br />

Isomeric states<br />

Stereoisomers<br />

Geometrical isomers<br />

Isotactic Syndiotactic Atactic cis trans<br />

and network structures are all possible, in addition to several isomeric configurations<br />

(isotactic, syndiotactic, atactic, cis, and trans). These molecular characteristics<br />

are presented in the taxonomic chart shown in Figure 14.8. Note that some of the<br />

structural elements are not mutually exclusive, and it may be necessary to specify<br />

molecular structure in terms of more than one. For example, a linear polymer may<br />

also be isotactic.<br />

Concept Check 14.3<br />

What is the difference between configuration and conformation in relation to<br />

polymer chains?<br />

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

14.9 THERMOPLASTIC AND THERMOSETTING<br />

POLYMERS<br />

The response of a polymer to mechanical forces at elevated temperatures is related<br />

to its dominant molecular structure. In fact, one classification scheme for<br />

thermoplastic polymer<br />

thermosetting<br />

polymer<br />

these materials is according to behavior with rising temperature. Thermoplastics<br />

(or thermoplastic polymers) and thermosets (or thermosetting polymers) are the<br />

two subdivisions. Thermoplastics soften when heated (and eventually liquefy) and<br />

harden when cooled—processes that are totally reversible and may be repeated.<br />

On a molecular level, as the temperature is raised, secondary bonding forces are<br />

diminished (by increased molecular motion) so that the relative movement of

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