Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 249

of water (xerogels) by collapsing the amorphous chains. The following sections deal with
the swelling or drying and rehydration of xerogels,

important aspects of cryogel

applications.

8.5.2 Swelling

Hassan and Peppas ( 2000 ) examined the long-term morphology changes (i.e. over
several months) of PVA on swelling in water at 37°C, relative to preparation conditions.
They observed that, during swelling, a signi cant fraction of PVA chains were not

incorporated into the overall crystalline structure, and therefore dissolved into solution.
The crystalline structure of PVA gels was examined in terms of the overall degree of
crystallinity as derived from DSC experiments in open pans. During these experiments
the samples started to dehydrate around 100°C, whereas melting of PVA crystals started
around 230°C. The degree of crystallinity was corrected for the residual water content
and calculated for samples with various freezing/thawing cycles and after swelling.
Parameters investigated included the number of cycles, the PVA molecular mass, the
initial concentration in solutions and the time of swelling. Approximately a third of the
initial crystallinity was lost in the

first day of swelling, but enhanced stability was created
upon increasing the number of freezing/thawing cycles. Although an increased number
of cycles did not necessarily increase the overall degree of crystallinity, it appeared that
repeated cycles did reinforce those crystals that already existed. For N c = 7, approxi-
mately 20% more of the chains participated in the overall crystalline structure. The
samples treated for N c = 3 showed a much more swollen structure than those treated for
N c =5or7.

8.5.3 Drying and rehydration of cryogels

Ricciardi et al.( 2004 ) carried out an extensive X-ray diffraction study on PVA dried gels
and PVA cryogels obtained by rehydrating the dried (kept in air) samples or obtained by a
slow drying process, by storing for 2 months in sealed vials at room temperature. The
results of their analysis con rm that highly stable PVA cryogels, with a water uptake

higher than 80%, may be obtained from freeze/thaw cycles of the initial solutions. They
showed that the porous structure formed during freeze/thaw cycles was not greatly
altered upon drying. In fact, during the successive rehydration steps, rehydrated gels
almost completely recovered the volume, shape and physical properties of the as-formed
freeze/thaw PVA cryogels, as if they were a permanently cross-linked network. Thus,
they showed that the outstanding physical and mechanical properties of freeze/thaw PVA
cryogels may be preserved for a long time by drying the samples immediately after the
preparation, and then restored when needed upon rehydration of the dried samples. The
X-ray diffraction pro

les of PVA gels may be considered as arising from the sum of three
contributions: free water, crystalline PVA aggregates and swollen amorphous PVA. The
results also support the hypothesis that pores are mainly occupied by water and that the
porous walls consist of swollen amorphous PVA, while the crystalline domains act as
knots of the gel network, validating the schematic structure proposed in Figure 8.19 . The