Biomedical Engineering Reference
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when transformed into xerogels for EM observations. This made it dif
cult to determine
the exact size of the porous channels in the initial cryogel state (shrinkage is about 40% in
diameter for a disc shape). Nevertheless, in the original cryogel it may be considered that
the pore size (or the size of ice crystals) increased when the concentration was lowered.
This trend is consistent with a mechanism in which diffusion plays a major role since, on
a lowering of the concentration of PVA, it will become easier to exclude chains from the
ice crystal growth front, and a larger ice crystal size is then expected. When these EM
images of PVA gels are compared to EM images of PVC gels obtained using the critical-
point drying technique (
Figure 8.4
), we can see that the structure in the cryogels is much
more heterogeneous and the morphology much coarser
the pores are larger and the
polymer network structure is very thick. That said, the drying stage may itself have
partially generated this coarsened structure.
The microstructure of these peculiar physical gels has been investigated by many
authors using various techniques. Willcox et al.(
2000
) performed DSC measurements.
As
Figure 8.18
shows, when N
c
increases, a melting peak appears at 78°C and persists,
while a low-temperature shoulder at 71°C to 72°C emerges and grows. The integrated
heat of fusion for the composite peak increases with N
c
:asN
c
goes from 1 to 24 for
'
-
samples, the heat of fusion rises from 0.63 to 8.2 J g
−
1
(the normalizing
mass is that of the polymer). By comparing with the bulk heat of melting for PVA
(150 J g
−
1
), the gel crystallinity can be inferred to increase from 0.4% to 5% of the total
PVA in the system. However, the crystals in the gels are small and highly hydrated, so
this crystallinity should be regarded as a crude, lower-bound estimate. In addition, the
melting of bulk PVA occurs at 250°C, nearly 170°C above the endotherms under
discussion. On ageing, the low-temperature shoulder grows and the higher-temperature
peak shifts upward to 79
non-aged
'
-
83°C, so eventually a distinct second peak in the range
67
-
72°C replaces the shoulder. The overall area under the two peaks saturates at
24 J g
−
1
, irrespective of N
c
, suggesting that the crystallinity itself saturates at
~15% of the total PVA content.
22
-
0.2
N
c
=
1
0.63 J
g
−
1
0.0
N
c
=
3
2.82 Jg
−
1
N
c
=
6
−
0.2
6.31 Jg
−
1
N
c
=
12
6.84 Jg
−
1
−
0.4
−
0.6
N
c
=
24
8.16 Jg
−
1
−
0.8
60
70
80
90
T
(°C)
Effect of the number of cycles N
c
on the endotherms of freshly prepared PVA cryogels by DSC
heating at a rate of 10°C min
−
1
. Adapted from Willcox et al.(
2000
) with permission from John
Wiley & Sons.
Figure 8.18
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