Chemistry Reference
In-Depth Information
Figure 11.9.
DMTA curves for white bread (5% water content) during rewarming (18C/min),
illustrating the changes in mechanical properties in the glass and at the glass transition for food
systems. The
relaxation associated with glass transition is characterized by an abrupt decrease
of the storage modulus (E
0
), from a high value in the glass (typically 10
9
Pa) to the rubbery plateau
(about 10
7
Pa) and a maximum of the loss modulus (E
00
). Measurement frequencies were 5 Hz
(solid lines) and 20 Hz (dotted lines). The temperature of the
relaxation increased with the
frequency, i.e. when the characteristic observation time decreased. A
relaxation is also visible in
the glassy state (LeMeste et al., 1992).
Another manifestation of the kinetic character is the fact that the temperature,
T
, at which the mechanical behaviour shows the structural relaxation, is
dependent on the measurement frequency (Figure 11.9). Moreover, below T
g
,
the relaxation processes continue, the slower the lower the temperature. This
''physical ageing'' is accompanied by a reduction in volume and enthalpy, as the
system comes closer to the metastable equilibrium (Figure 11.8).
As a rule, the temperature of the glass transition is an increasing func-
tion of the molecular weight (M) of the material. The expression
1
T
g
T
g
1
þ
K
1
¼
(15)
DP
is used to describe the molecular weight dependence of T
g
in a homogeneous
polymer series. DP is the degree of polymerization, K a constant and T
g
1
is
the high molecular weight limit of T
g
. It has been proved to apply to carbo-
hydrates (Orford et al., 1989). This is explained by the free volume increase
when M decreases, at a given temperature (Ferry, 1980; Sperling, 1986). For a
blend of two or more compatible components, the temperature of the glass
transition has a value intermediate between the T
g
of the components.
