Chemistry Reference
In-Depth Information
and more complex molecules must undergo translational and conformational reor-
ganizations to fit into crystal lattices and their crystallization rates may be so
reduced that a rigid, amorphous glass is formed before extensive crystallization
occurs on cooling. In many cases, also, the structure of polymers is so irregular
that crystalline structures cannot be formed. If crystallization does not occur, the
viscosity of the liquid will increase on cooling to a level of 10
14
Ns
/
m
2
(10
15
poises) where it becomes an immobile glass. Conformational changes associated
with normal volume contraction or crystallization can no longer take place in the
glassy state and the thermal coefficient of expansion of the material falls to about
one-third of its value in the warmer, liquid condition.
Most micromolecular species can exist in the gas, liquid, or crystalline solid
states. Some can also be encountered in the glassy state. The behavior of glass-
forming high polymers is more complex, because their condition at temperatures
slightly above the glassy condition is more accurately characterized as rubbery
than liquid. Unvulcanized elastomers described in
Section 4.5
are very viscous
liquids that will flow gradually under prolonged stresses. If they are cross-linked
in the liquid state, this flow can be eliminated. In any case these materials are
transformed into rigid, glassy products if they are cooled sufficiently. Similarly,
an ordinarily glassy polymer like polystyrene is transformed into a rubbery liquid
on warming to a high enough temperature.
The change between rubbery liquid and glassy behavior is known as the glass
transition. It occurs over a temperature range, as shown in
Fig. 4.1
, where the
temperature
volume relations for glass formation are contrasted with that for
crystallization. Line
ABCD
is for a substance that crystallizes completely. Such a
material undergoes an abrupt change in volume and coefficient of thermal expan-
sion at its melting point
T
m
. Line
ABEG
represents the cooling curve for a glass-
former. Over a short temperature range corresponding to the interval
EF
, the ther-
mal coefficient of expansion of the substance changes but there is no discontinu-
ity in the volume
temperature curve. By extrapolation, as shown, a temperature
T
0
g
can be located that may be regarded as the glass transition temperature for the
particular substance at the given cooling rate. If the material is cooled more
slowly, the volume
temperature curve is like
ABEG
0
and the glass transition tem-
perature
T
v
g
is lower than in the previous case. The precise value of
T
g
will
depend on the cooling rate in the particular experiment.
Low-molecular-weight molecules melt and crystallize completely over a sharp
temperature interval. Crystallizable polymers differ in that they melt over a range
of temperatures and do not crystallize completely, especially if they have high
molecular weights.
Figure 4.2
compares the volume
temperature relation for
such a polymer with that for an uncrystallizable analog. Almost all crystallizable
polymers are considered to be “semicrystalline” because they contain significant
fractions of poorly ordered, amorphous chains. Note that the melting region in
this sketch is diffuse, and the melting point is identified with the temperature at
B, where the largest and most perfect crystallites would melt. The noncrystalline
portion of this material exhibits a glass transition temperature, as shown. It
