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

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

8.19 Deformation of Elastomers • 279

(b)

(a)

 

Crosslinks

Figure 8.30 Schematic representation of crosslinked polymer chain molecules (a)inan

unstressed state and (b) during elastic deformation in response to an applied tensile stress.

(Adapted from Z. D. Jastrzebski,The Nature and Properties of Engineering Materials,3rd

edition. Copyright©c1987 by John Wiley & Sons, New York. Reprinted by permission of

John Wiley & Sons, Inc.)

and straightening, and the resultant elongation of the chains in the stress direction,

a phenomenon represented in Figure 8.30. Upon release of the stress, the chains

spring back to their prestressed conformations and the macroscopic piece returns to

its original shape.

Part of the driving force for elastic deformation is a thermodynamic parameter

calledentropy,which is a measure of the degree of disorder within a system; en-

tropy increases with increasing disorder. As an elastomer is stretched and the chains

straighten and become more aligned, the system becomes more ordered. From this

state, the entropy increases if the chains return to their original kinked and coiled

contours. Two intriguing phenomena result from this entropic effect. First, when

stretched, an elastomer experiences a rise in temperature; second, the modulus of

elasticity increases with increasing temperature, which is opposite to the behavior

found in other materials (see Figure 7.8).

Several criteria must be met for a polymer to be elastomeric: (1) It must not

easily crystallize; elastomeric materials are amorphous, having molecular chains that

are naturally coiled and kinked in the unstressed state. (2) Chain bond rotations must

be relatively free for the coiled chains to respond readily to an applied force. (3) For

elastomers to experience relatively large elastic deformations, the onset of plastic

deformation must be delayed. Restricting the motions of chains past one another by

crosslinking accomplishes this objective. The crosslinks act as anchor points between

the chains and prevent chain slippage from occurring; the role of crosslinks in the

deformation process is illustrated in Figure 8.30. Crosslinking in many elastomers

is carried out in a process called vulcanization, to be discussed below. (4) Finally,

the elastomer must be above its glass transition temperature (Section 11.16). The

lowest temperature at which rubber-like behavior persists for many of the common

elastomers is between –50 and –90◦C (–60 and –130◦F). Below its glass transition

temperature, an elastomer becomes brittle so that its stress–strain behavior resembles

curveAin Figure 7.22.

Vulcanization

vulcanization The crosslinking process in elastomers is calledvulcanization,which is achieved by a

nonreversible chemical reaction, ordinarily carried out at an elevated temperature.