094 CHAPTER 28 Synthetic Polymers
Table 28.7 Properties of Polyethylene as a Function of Crystallinity
Crystallinity (%) 55 62 70 77 85
Density 0.92 0.93 0.94 0.95 0.96
Melting point (°C) 109 116 125 130 133
(g>cm^3 )
Thermoplastic Polymers
Plastics can be classified according to the physical properties imparted to them by the
way in which their individual chains are arranged. Thermoplastic polymershave
both ordered crystalline regions and amorphous noncrystalline regions. Thermoplastic
polymers are hard at room temperature, but soft enough to be molded when heated, be-
cause the individual chains can slip past one another at elevated temperatures. Ther-
moplastic polymers are the plastics we encounter most often in our daily lives—in
combs, toys, switch plates, and telephone casings, for example. They are the plastics
that are easily cracked.
Thermosetting Polymers
Very strong and rigid materials can be obtained if polymer chains are cross-linked. The
greater the degree of cross-linking, the more rigid is the polymer. Such cross-linked
polymers are called thermosetting polymers. After they are hardened, they cannot be
remelted by heating, because the cross-links are covalent bonds, not intermolecular van
der Waals forces. Cross-linking reduces the mobility of the polymer chains, causing the
polymer to be relatively brittle. Because thermosetting polymers do not have the wide
range of properties characteristic of thermoplastic polymers, they are less widely used.
Melmac®, a highly cross-linked thermosetting polymer of melamine and formaldehyde,
is a hard, moisture-resistant material. Because it is colorless, Melmac®can be made into
materials with pastel colors. It is used to make lightweight dishes and counter surfaces.
PROBLEM 20
Propose a mechanism for the formation of Melmac®.
PROBLEM 21
Bakelite was the first of the thermosetting polymers. It is a highly cross-linked polymer
formed from the acid-catalyzed polymerization of phenol and formaldehyde. It is a much
darker polymer than Melmac®, so the range of colors of products made from Bakelite is
limited. Propose a structure for Bakelite.
Leo Hendrik Baekeland
(1863–1944)discovered Bakelite
while looking for a substitute for
shellac in his home laboratory. He
was born in Belgium and became a
professor of chemistry at the
University of Ghent. A fellowship
brought him to the United States in
1889, and he decided to stay. His
hobby was photography, and he
invented photographic paper that
could be developed under artificial
light. He sold the patent to
Eastman-Kodak.
+
H 2 NNH 2
NH NH
NH 2
H 2 CO
formaldehyde
Melmac
melamine
N
NN
NHCH 2 NH
NH
CH 2
NHCH 2 NH
N
NN
NH
NH
N
NN
NH
CH 2
NH
N
NN
NH
N
NN
−H 2 O
ordered—the polymer is, the denser, harder, and more resistant to heat it is (Table 28.7).
If the polymer chains possess substituents (as does poly[methyl methacrylate], for ex-
ample) or have branches that prevent them from packing closely together, the density of
the polymer is reduced.