Chemistry Reference
In-Depth Information
4.8.1
Relaxations at Temperatures below T
g
In the glass transition region, the storage modulus of an amorphous polymer drops
by a factor of
δ
δ
in
Fig. 4.20a
is less than this because the polymer is oriented and partially crystal-
line.) In addition to
T
g
, minor transitions are often observed at lower tempera-
tures, where the modulus may decrease by a factor of
B
1000, and tan
is generally one or more. (The tan
has maxima
of 0.1 or less. These so-called secondary transitions arise from the motions of
side groups or segments of the main chain that are smaller than those involved in
the displacements associated with
T
g
. Secondary transitions increase in tempera-
ture with increasing frequency in a manner similar to the main glass transition.
They can be detected by dynamic mechanical and also by dielectric loss factor
and nuclear magnetic resonance measurements.
Some amorphous polymers are not brittle at temperatures below
T
g
. Nearly all
these tough glasses have pronounced secondary transitions.
Figure 4.21
is a sketch
of the temperature dependence of the shear storage and loss moduli for polycar-
bonate
[8]
, which is one such polymer. The molecular motions that are responsi-
ble for the ductile behavior of some glassy polymers are probably associated with
limited range motions of main chain segments. Polymers like poly(methyl meth-
acrylate) that exhibit secondary transitions due to side group motions are not par-
ticularly tough.
2 and tan
δ
B
CH
3
10
11
(
O
C
O
C
x
CH
3
O
G'
10
10
10
9
G''
10
8
10
7
−
200
−
200
0
100
200
TEMPERATURE (ºC)
FIGURE 4.21
Storage (G
0
) and loss (Gv
) moduli of polycarbonate polymer
[8]
. The broad low-
temperature peak is probably composed of several overlapping maxima.
