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

GTBL042-13 GTBL042-Callister-v2 August 29, 2007 8:52

562 • Chapter 13 / Types and Applications of Materials

(a)

CH 2 CH CHCH 2

acb

CH 2 CH CH 2 CH

(b)

CH 2 C CHCH 2

acb

CH 2 CH CH 2 CH

CH 3

Figure 13.13 Representations

of the chain chemistries for

(a) styrene-butadiene-styrene

(S-B-S), and (b) styrene-

isoprene-styrene (S-I-S)

thermoplastic elastomers.

butadiene or isoprene units. These TPEs are frequently termedstyrenic block copoly-

mers,and chain chemistries for the two (S-B-S and S-I-S) types are shown in Fig-

ure 13.13.

At ambient temperatures, the soft, amorphous, central (butadiene or isoprene)

segments impart the rubbery, elastomeric behavior to the material. Furthermore,

for temperatures below theTmof the hard (styrene) component, hard chain-end

segments from numerous adjacent chains aggregate together to form rigid crystalline

domain regions. These domains are “physical crosslinks” that act as anchor points

so as to restrict soft-chain segment motions; they function in much the same way as

“chemical crosslinks” for the thermoset elastomers. A schematic illustration for the

structure of this TPE type is presented in Figure 13.14.

The tensile modulus of this TPE material is subject to alteration; increasing the

number of soft-component blocks per chain will lead to a decrease in modulus and,

therefore, a diminishment of stiffness. Furthermore, the useful temperature range

lies betweenTgof the soft and flexible component andTmof the hard, rigid one. For

the styrenic block copolymers this range is between about− 70 ◦C(− 95 ◦F) and 100◦C

(212◦F).

Soft component

segment

Hard component

domain

Figure 13.14 Schematic representation of

the molecular structure for a thermoplastic

elastomer. This structure consists of “soft”

(i.e., butadiene or isoprene) repeat unit

center-chain segments and “hard” (i.e.,

styrene) domains (chain ends), which act as

physical crosslinks at room temperature.

(FromThe Science and Engineering of

Materials, fifth edition by ASKELAND/

PHULE. 2006. Reprinted with permission of

Nelson, a division of Thomson Learning:

http://www.thomsonrights.com. Fax 800 730-2215.)