GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages450 • Chapter 11 / Phase TransformationsTable 11.3 Melting and Glass Transition Temperatures for Some of the More
Common Polymeric MaterialsGlass Transition Temperature Melting Temperature
Material [◦C(◦F)] [◦C(◦F)]
Polyethylene (low density) −110 (–165) 115 (240)
Polytetrafluoroethylene −97 (–140) 327 (620)
Polyethylene (high density) −90 (–130) 137 (279)
Polypropylene −18 (0) 175 (347)
Nylon 6,6 57 (135) 265 (510)
Poly(ethylene terephthalate) (PET) 69 (155) 265 (510)
Poly(vinyl chloride) 87 (190) 212 (415)
Polystyrene 100 (212) 240 (465)
Polycarbonate 150 (300) 265 (510)11.17 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 in-
crease inTm. Furthermore, the size and type of side groups influence chain rotational
freedom and flexibility: bulky or large side groups tend to restrict molecular rota-
tion and raiseTm. For example, polypropylene has a higher melting temperature
than polyethylene (175◦C versus 115◦C, Table 11.3); the CH 3 methyl side group for
polypropylene is larger than the H atom found on polyethylene. The presence of
polar groups (viz. Cl, OH, and CN), even though not excessively large, leads to sig-
nificant intermolecular bonding forces and relatively highTms. This may be verified
by comparing the melting temperatures of polypropylene (175◦C) and poly(vinyl
chloride) (212◦C).
The melting temperature of a polymer will also depend on molecular weight. At
relatively low molecular weights, increasingM(or chain length) raisesTm(Figure
11.48). Furthermore, the melting of a polymer takes place over a range of tempera-
tures, and thus there will be a range ofTms, rather than a single melting temperature.
This is because every polymer will be composed of molecules having a variety of
molecular weights (Section 4.5), and becauseTmdepends on molecular weight. For
most polymers, this melting temperature range will normally be on the order of sev-
eral degrees Celsius. Those melting temperatures cited in Table 11.3 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 (137◦C, Table 11.3) than low-density
polyethylene (115◦C), which has some branching.