16.5 Crystallization, Melting, and Glass Transition Phenomena
●
485
Table 16.2 Melting and Glass Transition Temperatures for
Some of the More Common Polymeric Materials
Material
Polyethylene (low density)
Polytetrafluoroethylene
Polyethylene (high density)
Polypropylene
Nylon 6,6
Polyester (PET)
Polyvinyl chloride
Polystyrene
Polycarbonate
Glass Transition
Temperature
[ ⴗC ( ⴗF )]
⫺110 (⫺165)
⫺97 (⫺140)
⫺90 (⫺130)
⫺18 (0)
57 (135)
69 (155)
87 (190)
100 (212)
150 (300)
Melting
Temperature
[ ⴗC ( ⴗF )]
115
327
137
175
265
265
212
240
265
(240)
(620)
(279)
(347)
(510)
(510)
(415)
(465)
(510)
melting and glass transition temperatures of a number of polymers are contained
in Table 16.2 and Appendix E.
FACTORS THAT INFLUENCE MELTING AND GLASS
TRANSITION TEMPERATURES
Melting Temperature
During melting of a polymer there will necessarily occur 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-chain
bonds and aromatic groups lowers chain flexibility and causes an increase in Tm .
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 Tm . For example, polypropylene has a higher melting temperature than polyethylene (175⬚C versus 115⬚C, Table 16.2); the CH3 methyl side group for polypropylene is larger than the H atom found on polyethylene. The presence of polar
side groups (viz. Cl, OH, and CN), even though not excessively large, leads to
significant intermolecular bonding forces and relatively high Tms. This may be
verified by comparing the melting temperatures of polypropylene (175⬚C) and
polyvinyl chloride (212⬚C).
For a specific polymer, melting temperature will also depend on molecular
weight. At relatively low molecular weights, increasing M (or chain length) raises
Tm (Figure 16.9). Furthermore, the melting of a polymer takes place over a range
of temperatures, and, thus, there will exist a range of Tms, rather than a single
melting temperature. This is because, every polymer will be composed of molecules
having a variety of molecular weights (Section 15.5), and because Tm depends on
molecular weight. For most polymers, this melting temperature range will normally
be on the order of several degrees Celsius. Those melting temperatures cited in
Table 16.2 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