Chemistry Reference
In-Depth Information
B
E
F
G
F'
G'
C
D
T
g
''
T
g
'
T
m
Temperature
FIGURE 4.1
Volume
temperature relations for a glass-forming polymer and a material that crystallizes
completely on cooling. T
m
is a melting point, and T
0
g
and T v
g
are glass transition
temperatures of an uncrystallized material that is cooled quickly and slowly, respectively.
appears that
T
g
is characteristic generally of amorphous regions in polymers,
whether or not other portions of the material are crystalline.
The melting range of a semicrystalline polymer may be very broad. Branched
(low-density) polyethylene is an extreme example of this behavior. Softening is
first noticeable at about 75
C although the last traces of crystallinity do not dis-
appear until about 115
C. Other polymers, like nylon-6,6, have much narrower
melting ranges.
Measurements of
T
m
and melting range are conveniently made by thermal
analysis techniques like differential scanning calorimetry (dsc). The value of
T
m
is
usually taken to be the temperature at which the highest melting crystallites disap-
pear. This parameter depends to some extent on the thermal history of the sample
since more perfect, higher melting crystallites are produced by slower crystalliza-
tion processes in which more time is provided for the conformational changes
needed to fit macromolecular segments into the appropriate crystal pattern.
The onset of softening is usually measured as the temperature required for a
particular polymer to deform a given amount under a specified load. These values
are known as
heat deflection temperatures
. Such data do not have any direct con-
nection with results of X-ray, thermal analysis, or other measurements of the
melting of crystallites, but they are widely used in designing with plastics.
