Chemistry Reference
In-Depth Information
B
T
g
T
m
Temperature
FIGURE 4.2
Volume
temperature relation for an amorphous (upper line) polymer and semicrystalline
(lowerline) polymer.
Both
T
m
and
T
g
are practically important.
T
g
sets an upper temperature limit
for the use of amorphous thermoplastics like poly(methyl methacrylate) or poly-
styrene and a lower temperature limit for rubbery behavior of an elastomer like
SBR rubber or 1,4-
cis
-polybutadiene. With semicrystalline thermoplastics,
T
m
or
the onset of the melting range determines the upper service temperature. Between
T
m
and
T
g
, semicrystalline polymers tend to be tough and leathery. Brittleness
begins to set in below
T
g
of the amorphous regions although secondary transitions
below
T
g
are also important in this connection. As a general rule, however, semi-
crystalline plastics are used at temperatures between
T
g
and a practical softening
temperature that lies above
T
g
and below
T
m
.
Changes in temperature and polymer molecular weight interact to influence
the nature and consequences of thermal transitions in macromolecules. Warming
of glassy amorphous materials converts them into rubbery liquids and eventually
into viscous liquids. The transition between these latter states is very ill marked,
however, as shown in
Fig. 4.3a
. Enhanced molecular
wei
ghts increase
T
g
up to
a plateau level, which is encountered approximately at DP
n
5
500 for vinyl poly-
mers. The rubbery nature of the liquid above
T
g
becomes increasingly more
pronounced with higher molecular weights. Similar relations are shown in
Fig. 4.3b
for semicrystalline polymers where
T
m
at first increases and then levels
off as the molecular weight of the polymer is made greater.
T
m
depends on the
