heterophase PVA–solvent systems [308, 332–337]. Features of the microstructure
of PVA cryogels were ascertained using various microscopy techniques [330, 331,
333, 338–346] and other physicochemical analysis methods [237, 326, 347–351].
In these studies, the following five principal facts were established:
1. The nodes in the 3D supramolecular network of PVA cryogels have a
noncovalent nature. Therefore, such cryogels can be fused upon heating above
the gel melting temperature with the formation of a polymer solution without
any change in the characteristics of PVA molecular weight compared to that
before freezing [30, 302–304]. This feature is observed provided that the used
polymer is pure, i.e., it contains no reactive admixtures, frequently present in
industrial PVA specimens.
2. Intermolecular H-bonding via the interactions of OH groups of the neighboring
PVA chains plays a key role in the formation of PVA cryogels [305–309].
3. The junction knots in these cryogels were experimentally proven to be PVA
microcrystallites [312–317]. A series of works conducted by a team of Italian
researchers on this topic [318–325] is very impressive; the same can be said
about the precision study by a Japanese team who found that each junction knot
in PVA cryogels includes, depending on the gel formation conditions, about 2–3
chains with around 24–120 segments [314].
4. The defrosting rate of frozen PVA solutions plays a significant role in the
formation and properties of PVA cryogels [234, 311, 326–331].
5. The molecular weight of PVA, chain tacticity, amount of residual O-acyl groups,
and PVA concentration, as well as the conditions of freezing, frozen storage, and
thawing have a significant effect on the properties and macroporosity of PVA
cryogels [1, 9, 10, 23, 29, 30, 232–236, 254, 258, 299–301, 310–316, 326–331,
333, 338, 346, 352–364, 366–372].
Because the number of publications dealing with the relationships between the
preparation conditions of PVA cryogels and their properties is large, Table 3
summarizes only the most important of them, from the viewpoint of the author of
this review. Apart from PVA cryogels formed from simple two-component feeds
(e.g., from systems composed of PVA dissolved in a neat solvent), a large amount
of data is also available in the literature on PVA cryogels that contain soluble
foreign additives, both of low and high molecular weights. In the former case, the
corresponding cryogels can be classified as those prepared from PVA dissolved in a
mixed solvent, whereas in the latter case a more suitable term is “complex PVA
cryogels.” It should also be noted that there are also extensive studies on “composite PVA cryogels” that contain various discrete fillers. Different soluble and
insoluble additives have been introduced in the initial PVA solutions to obtain
complex and filled PVA cryogels, respectively. It was shown that the type of
additives can affect, more or less, the properties and the microstructure of the
resulting cryogels. Since the character of such an influence is very diverse and
the observed effects can be multidirectional, such examples are not cited in Table 3.
However, readers interested in the principal studies in these fields can find the
corresponding references in the following reviews and recent experimental articles:
A Brief History of Polymeric Cryogels
35
of PVA cryogels were ascertained using various microscopy techniques [330, 331,
333, 338–346] and other physicochemical analysis methods [237, 326, 347–351].
In these studies, the following five principal facts were established:
1. The nodes in the 3D supramolecular network of PVA cryogels have a
noncovalent nature. Therefore, such cryogels can be fused upon heating above
the gel melting temperature with the formation of a polymer solution without
any change in the characteristics of PVA molecular weight compared to that
before freezing [30, 302–304]. This feature is observed provided that the used
polymer is pure, i.e., it contains no reactive admixtures, frequently present in
industrial PVA specimens.
2. Intermolecular H-bonding via the interactions of OH groups of the neighboring
PVA chains plays a key role in the formation of PVA cryogels [305–309].
3. The junction knots in these cryogels were experimentally proven to be PVA
microcrystallites [312–317]. A series of works conducted by a team of Italian
researchers on this topic [318–325] is very impressive; the same can be said
about the precision study by a Japanese team who found that each junction knot
in PVA cryogels includes, depending on the gel formation conditions, about 2–3
chains with around 24–120 segments [314].
4. The defrosting rate of frozen PVA solutions plays a significant role in the
formation and properties of PVA cryogels [234, 311, 326–331].
5. The molecular weight of PVA, chain tacticity, amount of residual O-acyl groups,
and PVA concentration, as well as the conditions of freezing, frozen storage, and
thawing have a significant effect on the properties and macroporosity of PVA
cryogels [1, 9, 10, 23, 29, 30, 232–236, 254, 258, 299–301, 310–316, 326–331,
333, 338, 346, 352–364, 366–372].
Because the number of publications dealing with the relationships between the
preparation conditions of PVA cryogels and their properties is large, Table 3
summarizes only the most important of them, from the viewpoint of the author of
this review. Apart from PVA cryogels formed from simple two-component feeds
(e.g., from systems composed of PVA dissolved in a neat solvent), a large amount
of data is also available in the literature on PVA cryogels that contain soluble
foreign additives, both of low and high molecular weights. In the former case, the
corresponding cryogels can be classified as those prepared from PVA dissolved in a
mixed solvent, whereas in the latter case a more suitable term is “complex PVA
cryogels.” It should also be noted that there are also extensive studies on “composite PVA cryogels” that contain various discrete fillers. Different soluble and
insoluble additives have been introduced in the initial PVA solutions to obtain
complex and filled PVA cryogels, respectively. It was shown that the type of
additives can affect, more or less, the properties and the microstructure of the
resulting cryogels. Since the character of such an influence is very diverse and
the observed effects can be multidirectional, such examples are not cited in Table 3.
However, readers interested in the principal studies in these fields can find the
corresponding references in the following reviews and recent experimental articles:
A Brief History of Polymeric Cryogels
35