of the resulting cryogel. For example, with an increase in the number of FTCs, there
is a decrease in pore size within the polymer matrix [8, 32], an increase in
mechanical properties [7] and a decrease in diffusion rates (see Sect. 3.2). An
increase in freezing rate has been shown to decrease the size and density of
crystallites. The freeze–thaw process is reviewed here in terms of its effect on
PVA-C for applications in biomedicine.
2.4.1 Rate of Freeze–Thaw Cycles
The rate of the thermal cycles has been shown to affect the mechanical properties of
the PVA hydrogel. A stiffer material is produced by using a slower thawing rate
because of the increased amount of time for the reorganization of polymer chains
and squeezing out of water molecules. Hatakeyama et al. showed that the rate of
freezing affects the size of the crystals and, therefore, the number of crystals formed
[33]. Slower thawing increases the period in which the specimen is at temperatures
optimal for gel network formation [13].
Lozinsky et al. found that the more time that the solution spends at temperatures
below 0
C, the more time is available for movement of polymer chains. This
allows for more time for entanglements to occur and increases the crystallinity by
increasing both the number and size of crystallites [34]. This produces PVA-C with
increased tensile strength. The rate of freezing has less of an effect on the properties, but has been shown to affect the formation of ice crystals [33]. Thawing rates,
on the other hand, affect the formation of the PVA-C and its mechanical properties
[7, 18, 35, 36].
It is important to keep thawing rates below 10
C/min, as higher rates are not
acceptable for producing hydrogels [35]. In many studies, precise control of
freezing and thawing rates are not maintained because samples are simply placed
in a freezer for freezing and then removed to room temperature for thawing [37–
39]. This will still result in cryogel formation, but the structure, and therefore the
properties, of the PVA-C will not be reproducible.
2.4.2 Number of Freeze–Thaw Cycles
In work by Hassan et al., the amount of dissolution of PVA cryogels was shown to
decrease as the number of FTCs was increased [40]. This is consistent with the
results that each FTC after the second causes a significant increase in the degree of
crystallinity [37, 41].
It has been shown that the number of FTCs has an effect on mechanical
properties. A maximum number of FTCs, after which the structure and properties
of the cryogel no longer changes, has been demonstrated [8], and several studies
have found this maximum number to be six [39, 42]. The increase in PVA-C
stiffness with increasing number of FTCs has been attributed to the crystallite
formation mechanism and liquid–liquid phase separation [7]. An alternative
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
289
is a decrease in pore size within the polymer matrix [8, 32], an increase in
mechanical properties [7] and a decrease in diffusion rates (see Sect. 3.2). An
increase in freezing rate has been shown to decrease the size and density of
crystallites. The freeze–thaw process is reviewed here in terms of its effect on
PVA-C for applications in biomedicine.
2.4.1 Rate of Freeze–Thaw Cycles
The rate of the thermal cycles has been shown to affect the mechanical properties of
the PVA hydrogel. A stiffer material is produced by using a slower thawing rate
because of the increased amount of time for the reorganization of polymer chains
and squeezing out of water molecules. Hatakeyama et al. showed that the rate of
freezing affects the size of the crystals and, therefore, the number of crystals formed
[33]. Slower thawing increases the period in which the specimen is at temperatures
optimal for gel network formation [13].
Lozinsky et al. found that the more time that the solution spends at temperatures
below 0
C, the more time is available for movement of polymer chains. This
allows for more time for entanglements to occur and increases the crystallinity by
increasing both the number and size of crystallites [34]. This produces PVA-C with
increased tensile strength. The rate of freezing has less of an effect on the properties, but has been shown to affect the formation of ice crystals [33]. Thawing rates,
on the other hand, affect the formation of the PVA-C and its mechanical properties
[7, 18, 35, 36].
It is important to keep thawing rates below 10
C/min, as higher rates are not
acceptable for producing hydrogels [35]. In many studies, precise control of
freezing and thawing rates are not maintained because samples are simply placed
in a freezer for freezing and then removed to room temperature for thawing [37–
39]. This will still result in cryogel formation, but the structure, and therefore the
properties, of the PVA-C will not be reproducible.
2.4.2 Number of Freeze–Thaw Cycles
In work by Hassan et al., the amount of dissolution of PVA cryogels was shown to
decrease as the number of FTCs was increased [40]. This is consistent with the
results that each FTC after the second causes a significant increase in the degree of
crystallinity [37, 41].
It has been shown that the number of FTCs has an effect on mechanical
properties. A maximum number of FTCs, after which the structure and properties
of the cryogel no longer changes, has been demonstrated [8], and several studies
have found this maximum number to be six [39, 42]. The increase in PVA-C
stiffness with increasing number of FTCs has been attributed to the crystallite
formation mechanism and liquid–liquid phase separation [7]. An alternative
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
289