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

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2nd Revised Pages

11.17 Factors That Influence Melting and Glass Transition Temperatures • 451

Molecular weight

Temperature

101 102 103 104 105 106 107

Tm

Tg

Mobile liquid Viscous

liquid

Rubber

Tough plastic

Partially

crystalline

plastic

Crystalline solid

Figure 11.48

Dependence of polymer

properties as well as

melting and glass

transition temperatures

on molecular weight.

(From F. W. Billmeyer,

Jr.,Textbook of Polymer

Science, 3rd edition.

Copyright©c1984 by

John Wiley & Sons, New

York. Reprinted by

permission of John Wiley

& Sons, Inc.)

Glass Transition Temperature

Upon heating through the glass transition temperature, the amorphous solid polymer

transforms from a rigid to a rubbery state. Correspondingly, the molecules that are

virtually frozen in position belowTgbegin to experience rotational and translational

motions aboveTg. Thus, the value of the glass transition temperature will depend

on molecular characteristics that affect chain stiffness; most of these factors and

their influences are the same as for the melting temperature, as discussed above.

Again, chain flexibility is diminished andTgis increased by the presence of the

following:

1.Bulky side groups; from Table 11.3, the respective values for polypropylene

and polystyrene are –18◦C and 100◦C.

2.Polar groups; for example, theTgvalues for poly(vinyl chloride) and

polypropylene are 87◦C and –18◦C, respectively.

3.Double bonds and aromatic groups in the backbone, which tend to stiffen the

polymer chain.

Increasing the molecular weight also tends to raise the glass transition temper-

ature, as noted in Figure 11.48. A small amount of branching will tend to lowerTg;

on the other hand, a high density of branches reduces chain mobility, and elevates

the glass transition temperature. Some amorphous polymers are crosslinked, which

has been observed to elevateTg; crosslinks restrict molecular motion. With a high

density of crosslinks, molecular motion is virtually disallowed; long-range molecular

motion is prevented, to the degree that these polymers do not experience a glass

transition or its accompanying softening.

From the preceding discussion it is evident that essentially the same molecu-

lar characteristics raise and lower both melting and glass transition temperatures.

Normally the value ofTglies somewhere between 0.5 and 0.8Tm(in Kelvin). Conse-

quently, for a homopolymer, it is not possible to independently vary bothTmandTg.

A greater degree of control over these two parameters is possible by the synthesis

and utilization of copolymeric materials.