Whereas the largest capillary pores are interconnected and, therefore, liquids can
easily leak through blocks of such wide-pore cryogels (Fig. 21a, b), the rounded
pores of other kinds are closed (Fig. 21c, d). The former are replicas of the
continuous GuAr-rich phase in the two-phase system formed as a result of liquid–
liquid separation. The heterogeneous morphology of such a system is fixed by the
cryotropic gelation of the PVA-rich phase. Since the cross-section of pores in
conventional PVA cryogels generated by the ice polycrystals does not usually
exceed 1–3 μm [92, 95, 126, 150, 165, 166, 177, 191], it is evident that the 10–
70-μm pores (Fig. 21b, d) are also replicas of the non-gelling GuAr-rich phase, but
in this case dispersed as liquid droplets in the bulk PVA-rich phase. Moreover, the
smallest pores visible in the light microscopy image of Fig. 21d are, most probably,
left in the gel matter by the ice particles after thawing. The pores of the latter type
have a rounded shape, whereas those observed in the “conventional” PVA cryogels
prepared under the same conditions but without foreign polymeric additives are
usually anisodiametric (prolate) in their shape (see Figs. 5, 6, and 18). We have to
mention that the round-shaped ice-derived pores in PVA cryogels are also inherent
in the gel matrices prepared with the addition of surfactants as pore modifiers,
which are capable of influencing the shape of the ice crystals by decreasing the
surface tension at the solid–liquid interface during crystal growth [192]. Thus, such
“rounding” of the pores inside the gel phase of wide-pore cryogels fabricated from
the water–PVA–GuAr feeds are probably due to the surfactant properties of GuAr
macromolecules present in a small amount in the initial PVA-rich phase.
The last type of auxiliary pore-forming agents that will be considered here are
the gaseous porogens. These agents are constantly being entrapped in the cryogel
matrix and perform as the pores per se. Examples of such porogens are small gas
bubbles, and the resulting gas-filled cryogels can be termed “foamed cryogels.”
Inside the foamed freeze–thaw gels, two sorts of macropores can be distinguished:
the cryogenically induced macropores, i.e., those derived from the thawed solvent
polycrystals, and the closed microbubbles remaining entrapped in the cryogel after
defrosting of the frozen foam. Such gas-filled gel materials, PVA-based gels in
particular, have been prepared, studied, and some promising instances of their
practical application have been reported [192–197]. In these works, the gaseous
porogen was generated either using physical methods, e.g., whipping, barbotage,
microfluidic foaming techniques [192–194, 197], or via the chemical liberation of
gas products, e.g., by the reaction of ammonium chloride with sodium nitrite
(NH 4 Cl + NaNO 2 ! NaCl + 2H 2 O + N 2 "), that were introduced into the feed
prior to its freezing [196].
For instance, foamed cryogels were fabricated by mechanical whipping of
aqueous PVA solutions followed by freezing of the resulting foams, storing the
samples frozen, and then slowly defrosting [193]. The peculiarities of the porous
morphology of such gel foamed matrices are illustrated by the microphotographs in
Fig. 22a, b, which show thin sections of the samples prepared by cryostructuring of
the PVA-based liquid foam. These foamed cryogels possess two sorts of pores
distinguishable by optical light microscopy. The first type are the gross round pores
of ~50 to ~250 μm in diameter formed by the auxiliary pore-forming agent, i.e., by
Basic Principles of Cryotropic Gelation
93
easily leak through blocks of such wide-pore cryogels (Fig. 21a, b), the rounded
pores of other kinds are closed (Fig. 21c, d). The former are replicas of the
continuous GuAr-rich phase in the two-phase system formed as a result of liquid–
liquid separation. The heterogeneous morphology of such a system is fixed by the
cryotropic gelation of the PVA-rich phase. Since the cross-section of pores in
conventional PVA cryogels generated by the ice polycrystals does not usually
exceed 1–3 μm [92, 95, 126, 150, 165, 166, 177, 191], it is evident that the 10–
70-μm pores (Fig. 21b, d) are also replicas of the non-gelling GuAr-rich phase, but
in this case dispersed as liquid droplets in the bulk PVA-rich phase. Moreover, the
smallest pores visible in the light microscopy image of Fig. 21d are, most probably,
left in the gel matter by the ice particles after thawing. The pores of the latter type
have a rounded shape, whereas those observed in the “conventional” PVA cryogels
prepared under the same conditions but without foreign polymeric additives are
usually anisodiametric (prolate) in their shape (see Figs. 5, 6, and 18). We have to
mention that the round-shaped ice-derived pores in PVA cryogels are also inherent
in the gel matrices prepared with the addition of surfactants as pore modifiers,
which are capable of influencing the shape of the ice crystals by decreasing the
surface tension at the solid–liquid interface during crystal growth [192]. Thus, such
“rounding” of the pores inside the gel phase of wide-pore cryogels fabricated from
the water–PVA–GuAr feeds are probably due to the surfactant properties of GuAr
macromolecules present in a small amount in the initial PVA-rich phase.
The last type of auxiliary pore-forming agents that will be considered here are
the gaseous porogens. These agents are constantly being entrapped in the cryogel
matrix and perform as the pores per se. Examples of such porogens are small gas
bubbles, and the resulting gas-filled cryogels can be termed “foamed cryogels.”
Inside the foamed freeze–thaw gels, two sorts of macropores can be distinguished:
the cryogenically induced macropores, i.e., those derived from the thawed solvent
polycrystals, and the closed microbubbles remaining entrapped in the cryogel after
defrosting of the frozen foam. Such gas-filled gel materials, PVA-based gels in
particular, have been prepared, studied, and some promising instances of their
practical application have been reported [192–197]. In these works, the gaseous
porogen was generated either using physical methods, e.g., whipping, barbotage,
microfluidic foaming techniques [192–194, 197], or via the chemical liberation of
gas products, e.g., by the reaction of ammonium chloride with sodium nitrite
(NH 4 Cl + NaNO 2 ! NaCl + 2H 2 O + N 2 "), that were introduced into the feed
prior to its freezing [196].
For instance, foamed cryogels were fabricated by mechanical whipping of
aqueous PVA solutions followed by freezing of the resulting foams, storing the
samples frozen, and then slowly defrosting [193]. The peculiarities of the porous
morphology of such gel foamed matrices are illustrated by the microphotographs in
Fig. 22a, b, which show thin sections of the samples prepared by cryostructuring of
the PVA-based liquid foam. These foamed cryogels possess two sorts of pores
distinguishable by optical light microscopy. The first type are the gross round pores
of ~50 to ~250 μm in diameter formed by the auxiliary pore-forming agent, i.e., by
Basic Principles of Cryotropic Gelation
93