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

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2nd Revised Pages

11.16 Melting and Glass Transition Temperatures • 449

Specific volume

Temperature

Liquid

A

B

C

Tg Tm

Glass

Semicrystalline solid

Crystalline solid

Figure 11.47 Specific volume versus

temperature, upon cooling from the

liquid melt, for totally amorphous (curve

A), semicrystalline (curveB), and

crystalline (curveC) polymers.

11.16 MELTING AND GLASS TRANSITION

TEMPERATURES

Melting and glass transition temperatures are important parameters relative to in-

service applications of polymers. They define, respectively, the upper and lower tem-

perature limits for numerous applications, especially for semicrystalline polymers.

The glass transition temperature may also define the upper use temperature for

glassy amorphous materials. Furthermore,TmandTgalso influence the fabrication

and processing procedures for polymers and polymer-matrix composites. These is-

sues are discussed in other chapters.

The temperatures at which melting and/or the glass transition occur for a poly-

mer are determined in the same manner as for ceramic materials—from a plot of

specific volume (the reciprocal of density) versus temperature. Figure 11.47 is such

a plot, where curvesAandC, for amorphous and crystalline polymers, respectively,

have the same configurations as their ceramic counterparts (Figure 14.16).^8 For the

crystalline material, there is a discontinuous change in specific volume at the melt-

ing temperatureTm. The curve for the totally amorphous material is continuous

but experiences a slight decrease in slope at the glass transition temperature,Tg.

The behavior is intermediate between these extremes for a semicrystalline polymer

(curveB) in that both melting and glass transition phenomena are observed;Tm

andTgare properties of the respective crystalline and amorphous phases in this

semicrystalline material. As discussed above, the behaviors represented in Figure

11.47 will depend on the rate of cooling or heating. Representative melting and glass

transition temperatures of a number of polymers are contained in Table 11.3 and

Appendix E.

(^8) It should be noted that no engineering polymer is 100% crystalline; curveCis included in
Figure 11.47 to illustrate the extreme behavior that would be displayed by a totally
crystalline material.