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124 • Chapter 4 / Polymer Structures

kinked shapes or contours. With regard to molecular structure, linear, branched,

crosslinked, and network structures are possible, in addition to isotactic, syndiotactic,

and atactic stereoisomers, and the cis and trans geometrical isomers. The copolymers

include random, alternating, block, and graft types.

Thermoplastic and Thermosetting Polymers

With regard to behavior at elevated temperatures, polymers are classified as either

thermoplastic or thermosetting. The former have linear and branched structures; they

soften when heated and harden when cooled. In contrast, thermosets, once having

hardened, do not soften upon heating; their structures are crosslinked and network.

Polymer Crystallinity

Polymer Crystals

When the molecular chains are packed in an ordered atomic arrangement, the condi-

tion of crystallinity is said to exist. In addition to being entirely amorphous, polymers

may also exhibit varying degrees of crystallinity; that is, crystalline regions are in-

terdispersed within amorphous areas. Crystallinity is facilitated for polymers that

are chemically simple and that have regular and symmetrical chain structures. Many

semicrystalline polymers form spherulites; each spherulite consists of a collection of

ribbon-like chain-folded lamellar crystallites that radiate outward from its center.

IMPORTANT TERMS AND CONCEPTS

Alternating copolymer

Atactic configuration

Bifunctional

Block copolymer

Branched polymer

Chain-folded model

Cis (structure)

Copolymer

Crosslinked polymer

Crystallite

Degree of polymerization

Functionality

Graft copolymer

Homopolymer

Isomerism

Isotactic configuration

Linear polymer

Macromolecule

Molecular chemistry

Molecular structure

Molecular weight

Monomer

Network polymer

Polymer

Polymer crystallinity

Random copolymer

Repeat unit

Saturated

Spherulite

Stereoisomerism

Syndiotactic configuration

Thermoplastic polymer

Thermosetting polymer

Trans (structure)

Trifunctional

Unsaturated

REFERENCES

Baer, E., “Advanced Polymers,”Scientific Ameri-

can, Vol. 255, No. 4, October 1986, pp. 178–190.

Carraher, C. E., Jr.,Seymour/Carraher’s Polymer

Chemistry, 6th edition, Marcel Dekker, New

York, 2003.

Cowie, J. M. G.,Polymers: Chemistry and Physics of

Modern Materials, 2nd edition, Chapman and

Hall (USA), New York, 1991.

Engineered Materials Handbook, Vol. 2, Engi-

neering Plastics, ASM International, Materials

Park, OH, 1988.

McCrum, N. G., C. P. Buckley, and C. B. Bucknall,

Principles of Polymer Engineering, 2nd edition,

Oxford University Press, Oxford, 1997. Chap-

ters 0–6.

Rodriguez, F., C. Cohen, C. K. Ober, and L. Archer,

Principles of Polymer Systems, 5th edition,

Taylor & Francis, New York, 2003.