Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 246

15

N c = 10

N c = 9

10

N c = 8

N c = 7

N c = 5
N c = 6

5

N c = 4

N c = 3

N c = 2

N c = 1

0

0 100

Elongation (%)

200 300

Stress strain curves for c = 15 wt% PVA cryogels during N c consecutive freeze/thaw cycles.

Adapted with permission from Yokoyama et al.( 1986 ) © 1986 Springer.

- Figure 8.16

Scanning electron micrograph of a xerogel obtained from 5 wt% PVA cryogel (N c = 10). Adapted
with permission from Yokoyama et al.( 1986 ) © 1986 Springer.

Figure 8.17

Yokoyama et al.( 1986 ) considered that the repeated freezing stages are nothing but
repeated recrystallizations, and regarded the cycling as a kind of re

nement process for
ice crystals. They therefore expected that polymer chains would be rejected from the
growing ice crystals as an water system was frozen. Thus

the concentration of PVA in the PVA-rich solution phase would increase with the freezing
cycle number, and in the PVA-poor solution phase would decrease, giving rise to the
porous structure. They observed that the pore size in the xerogel increased on lowering of
the concentration of PVA. However, they reported shrinkage of the gels (for N c >3

' impurity ' whenever the PVA -

- 4)