treated. Such feed systems result in the formation of composite cryogels with
incorporated fillers, which are then dissolved by rinsing the composite with aqueous
acid or alkali. The pores formed with the aid of such additives are isolated from
each other and have a size close to that of the filler grains, whereas the system of
interconnected macropores is ensured by cryotropic gel formation. Hydrophobic oil
can also be used as an insoluble porogen in the preparation of hydrophilic thermoor pH-responsive poly(N-isopropylacrylamide)-based copolymer cryogels [174–
176]. After preparation of such wide-pore spongy gel composites, they contain
entrapped microdroplets of hydrophobic liquid. These droplets can be removed
upon the stimulus-induced collapse of the cryogel, i.e., by heating or by changing
pH. Since the collapse is reversible, when the collapsed cryogel is placed in oil-free
water, it swells again so that additional “non-cryotropic” spherical pores filled with
water instead of oil microdroplets can be obtained. By iterative collapse–swelling
cycles it is possible to remove completely such hydrophobic auxiliary porogens.
The soluble auxiliary pore-forming agents are compounds dissolved in the
common solvent together with the gel precursors. The porogenic properties of
these agents may affect the crystallization of the liquid medium so that they are
also called modifiers of solvent crystals. This effect is illustrated by the micrographs in Fig. 20, which show the images of thin sections of PVA cryogels prepared
in the presence of various low molecular weight salts at two different concentrations. All the samples were cryostructured under identical freeze–thaw conditions,
and Table 3 summarizes their porosity parameters [177]. The porous morphology of
PVA cryogels is affected by both the nature and the concentration of low molecular
weight electrolyte additives. The size of the macropores decreases as the salt
concentration is increased, in accord with the well-known influence of salts on
the spatial geometry and size of ice crystals [167, 178, 179]. Moreover, the
influence of other processes like the salting-out effect, which leads to an increase
in the ionic strength of UFLMP, cannot be excluded. As a result, certain integral
changes in the macroporous morphology of cryogels can be registered at the
resolution provided by an optical microscope. In other words, such low molecular
weight salt additives act as powerful pore modifiers for the resulting cryogels, and
their impact is achieved via their influence on the course of solvent crystallization.
Certain soluble auxiliary pore forming agents may induce a liquid–liquid phase
separation. Spongy PVA-based cryogels prepared from “water–PVA–gum arabic”
feeds are a good example of the influence of such phenomenon on the formation of
additional pores in cryogels [180]. Aqueous feed systems containing a gel-forming
agent (PVA) and a polymeric additive (gum arabic) unable to form noncovalent
cryogels are interesting because they demonstrate the effect of thermodynamic
incompatibility of the polymeric components on the texture of the resulting
cryogels. Several types of PVA cryogels prepared from such multicomponent
feeds have been described, where the auxiliary pore-forming agents were both
synthetic polymers and natural biopolymers such as polysaccharides, proteins,
and nucleic acids. All of these additives affect the properties and the porous
structure of the formed PVA cryogels [181–187]. There are potentially two extreme
variants of PVA-based feed systems capable of liquid-phase de-mixing. First, the
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V.I. Lozinsky and O. Okay
incorporated fillers, which are then dissolved by rinsing the composite with aqueous
acid or alkali. The pores formed with the aid of such additives are isolated from
each other and have a size close to that of the filler grains, whereas the system of
interconnected macropores is ensured by cryotropic gel formation. Hydrophobic oil
can also be used as an insoluble porogen in the preparation of hydrophilic thermoor pH-responsive poly(N-isopropylacrylamide)-based copolymer cryogels [174–
176]. After preparation of such wide-pore spongy gel composites, they contain
entrapped microdroplets of hydrophobic liquid. These droplets can be removed
upon the stimulus-induced collapse of the cryogel, i.e., by heating or by changing
pH. Since the collapse is reversible, when the collapsed cryogel is placed in oil-free
water, it swells again so that additional “non-cryotropic” spherical pores filled with
water instead of oil microdroplets can be obtained. By iterative collapse–swelling
cycles it is possible to remove completely such hydrophobic auxiliary porogens.
The soluble auxiliary pore-forming agents are compounds dissolved in the
common solvent together with the gel precursors. The porogenic properties of
these agents may affect the crystallization of the liquid medium so that they are
also called modifiers of solvent crystals. This effect is illustrated by the micrographs in Fig. 20, which show the images of thin sections of PVA cryogels prepared
in the presence of various low molecular weight salts at two different concentrations. All the samples were cryostructured under identical freeze–thaw conditions,
and Table 3 summarizes their porosity parameters [177]. The porous morphology of
PVA cryogels is affected by both the nature and the concentration of low molecular
weight electrolyte additives. The size of the macropores decreases as the salt
concentration is increased, in accord with the well-known influence of salts on
the spatial geometry and size of ice crystals [167, 178, 179]. Moreover, the
influence of other processes like the salting-out effect, which leads to an increase
in the ionic strength of UFLMP, cannot be excluded. As a result, certain integral
changes in the macroporous morphology of cryogels can be registered at the
resolution provided by an optical microscope. In other words, such low molecular
weight salt additives act as powerful pore modifiers for the resulting cryogels, and
their impact is achieved via their influence on the course of solvent crystallization.
Certain soluble auxiliary pore forming agents may induce a liquid–liquid phase
separation. Spongy PVA-based cryogels prepared from “water–PVA–gum arabic”
feeds are a good example of the influence of such phenomenon on the formation of
additional pores in cryogels [180]. Aqueous feed systems containing a gel-forming
agent (PVA) and a polymeric additive (gum arabic) unable to form noncovalent
cryogels are interesting because they demonstrate the effect of thermodynamic
incompatibility of the polymeric components on the texture of the resulting
cryogels. Several types of PVA cryogels prepared from such multicomponent
feeds have been described, where the auxiliary pore-forming agents were both
synthetic polymers and natural biopolymers such as polysaccharides, proteins,
and nucleic acids. All of these additives affect the properties and the porous
structure of the formed PVA cryogels [181–187]. There are potentially two extreme
variants of PVA-based feed systems capable of liquid-phase de-mixing. First, the
88
V.I. Lozinsky and O. Okay