Callister - An introduction - 8th edition

594 • Chapter 15 / Characteristics, Applications, and Processing of Polymers
15.14 FACTORS THAT INFLUENCE MELTING AND
GLASS TRANSITION TEMPERATURES
Melting Temperature
During melting of a polymer there will be a rearrangement of the molecules in the
transformation from ordered to disordered molecular states. Molecular chemistry
and structure will influence the ability of the polymer chain molecules to make these
rearrangements and, therefore, will also affect the melting temperature.
Chain stiffness, which is controlled by the ease of rotation about the chemical
bonds along the chain, has a pronounced effect. The presence of double bonds and
aromatic groups in the polymer backbone lowers chain flexibility and causes an
increase in T m . Furthermore, the size and type of side groups influence chain rotational
freedom and flexibility; bulky or large side groups tend to restrict molecular rotation
and raise T m . For example, polypropylene has a higher melting temperature than polyethylene
(175C versus 115C, Table 15.2); the CH 3 methyl side group for polypropylene
is larger than the H atom found on polyethylene. The presence of polar groups
(Cl, OH, and CN), even though not excessively large, leads to significant intermolecular
bonding forces and relatively high T m s. This may be verified by comparing the
melting temperatures of polypropylene (175C) and poly(vinyl chloride) (212C).
The melting temperature of a polymer will also depend on molecular weight.
At relatively low molecular weights, increasing M (or chain length) raises T m (Figure
15.19). Furthermore, the melting of a polymer takes place over a range of
temperatures; thus, there will be a range of T m s, rather than a single melting temperature.
This is because every polymer will be composed of molecules having a
variety of molecular weights (Section 14.5), and because T m depends on molecular
weight. For most polymers, this melting temperature range will normally be on the
order of several degrees Celsius. The melting temperatures cited in Table 15.2 and
Appendix E are near the high ends of these ranges.
Degree of branching will also affect the melting temperature of a polymer. The
introduction of side branches introduces defects into the crystalline material and
lowers the melting temperature. High-density polyethylene, being a predominately
linear polymer, has a higher melting temperature (137C, Table 15.2) than lowdensity
polyethylene (115C), which has some branching.
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
Temperature
Mobile liquid
Viscous
liquid
T m
Tough plastic
T g
Partially
Crystalline solid
crystalline
plastic
10 10 2 10 3 10 4 10 5 10 6 10 7
Molecular weight
Rubber
Figure 15.19
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 © 1984 by
John Wiley & Sons, New
York. Reprinted by
permission of John
Wiley & Sons, Inc.)