Biomedical Engineering Reference
In-Depth Information
a low temperature thermostat); the solution freezing point temperature (T
mc
) and
actual initial solvent crystallization temperature (T
c
). While the fi nal freezing
temperature (T
f
) is programmed, both values (T
mc
and T
c
) are estimated from
freezing curves. Analysis of the freezing curves (freezing thermograms) is highly
instrumental for the determination of phase and structural changes during cooling
and crystallization within a system [Akyurt et al., 2002; Liu and Zhou, 2003; Luyet,
1966]. In a typical freezing curve, an abrupt rise of temperature indicates the
appearance of ice crystals and the corresponding temperature is T
c
, while the
maximum temperature induced by the released latent heat of crystallization is
equal to T
mc
.
The freezing curves obtained for the solution of monomers (acrylamide
(AAm),
N,N¢
-methylenebisacrylamide (MBAAm) and allyl glycidyl ether
(AGE)) during the preparation of polyacrylamide MG (pAAm-MGs) at three
different fi nal freezing temperatures T
f
of
30 ° C showed clearly
the different phase state of the system under freezing [Plieva et al., 2006a]
(Figure 14.2 ).
Overcooling up to
−
12,
−
20 and
−
−
11 °C for two minutes was observed for the reaction
mixture frozen at
12 °C followed by an abrupt rise of temperature indicating
that initialized solvent crystallization at T
c
about
−
11 ° C. Once the critical mass of
nuclei was reached (2.6 minutes after the reaction mixture was placed in the low
temperature thermostat), the system nucleated at temperature T
mc
about
−
2.5 ° C,
(Figure 14.2). After crystallization was completed, the temperature dropped
slowly till the fi xed T
f
value of
−
12 ° C was reached. A freeze - concentration process
occurred as water was frozen out from the solution. The increase in viscosity of
the unfrozen liquid phase hindered the further crystallization, which was over
after about eight minutes (Figure 14.2). At lower fi nal freezing temperatures (T
f
−
−
30 °C, respectively), the freezing thermograms revealed practically no
overcooling with a small crystallization plateau near 0 °C. A small peak observed
20 and
−
20
10
0
0
2
4
6
8
10
-10
-12 °C
-20
-20 °C
-30
-30 °C
-40
Time (min)
Figure 14.2.
Thermograms for freezing aqueous solution of acrylamide monomers at
−
12,
30 °C in the presence of 1.2% initiating system (APS/TEMED). Reproduced from
[Plieva et al., 2006a] with permission.
20 and
−
−
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