hinders the growth of solvent crystals. As a result, only relatively small porogen
particles can be formed, thus generating relatively small pores in the resulting
cryogels.
In this connection, PVA solutions subjected to multiple freeze–thaw cycles
represent a specific case where the porosity characteristics are governed not only
by the conditions of the first cryogenic cycle, but also by the following cycles [111,
126–130, 133, 149, 165, 166]. As illustrated in Fig. 6, the most significant step-like
changes in the size and shape of the macropores occur during the second cycle.
During this cycle, the free solvent crystallizes mainly within the space of the
already formed “primary” macropores, where the liquid contains only a small
amount of sol-fraction [113]. Therefore, the viscosity within this space is considerably lower than that of the initial PVA solution, and larger ice particles are
formed. In addition to widening of the primary pores by these growing crystals, a
certain compression of the pore walls owing to the physical stresses caused by the
ice crystallization also occurs, thus facilitating compaction of the proper gel phase
in the heterophase material. As a consequence, after the second freeze–thaw cycle,
the cross-section of the macropores increases by a factor of 2–3, whereas further
cryogenic cycles have an insignificant influence on the size and shape of these
pores [133].
The cryogenic processing conditions of PVA solutions also affect significantly
the porous structure of PVA cryogels. To demonstrate this effect, noncovalent PVA
cryogels were prepared starting from aqueous solutions of PVA (100 g/L)
according to two different freezing and frozen-storage procedures [44]:
(a) PVA solution was frozen at À20
C for 24 h
(b) PVA solution was first frozen at À20
C for 1 h, then incubated at a fixed
negative temperature between À5 and À1.1
C for 23 h
The micrographs in Fig. 18a, b represent thin sections of PVA cryogels formed
by the two procedures, respectively, where the latter was incubated at À2
C. The
morphology of the cryogel formed by a single-temperature freezing differs markedly from the two-temperature freezing procedure. As pointed out in Sect. 2.4, at a
storage temperature of À2
C, cryotropic gelation of PVA occurs with the highest
efficiency. Moreover, ice re-crystallization phenomena are also very intensive at
this temperature [167–169]. Since the prolonged incubation of the frozen sample at
À2
C equalizes the temperature fields in all directions, the branched crystals are
formed as a result of the re-crystallization. After defrosting, such secondary crystals
leave a net-like system of intersecting macropores in the body of the PVA cryogel.
The next key feature of the textural morphology of freeze–thaw gels is the
interconnected character of their macropores. The main reason for such a porous
character is the 3D growth of porogen particles (i.e., solvent crystals) during
freezing of the feed system, whereby the growth stops for a particular facet of a
crystal when it comes into tight contact with some facet of a neighboring growing
crystal [8]. During subsequent thawing of the frozen sample, these contact areas are
transformed into the connections between macropores. When the unidirectional
freezing technique is employed, the propagation rates of different facets are
Basic Principles of Cryotropic Gelation
85
particles can be formed, thus generating relatively small pores in the resulting
cryogels.
In this connection, PVA solutions subjected to multiple freeze–thaw cycles
represent a specific case where the porosity characteristics are governed not only
by the conditions of the first cryogenic cycle, but also by the following cycles [111,
126–130, 133, 149, 165, 166]. As illustrated in Fig. 6, the most significant step-like
changes in the size and shape of the macropores occur during the second cycle.
During this cycle, the free solvent crystallizes mainly within the space of the
already formed “primary” macropores, where the liquid contains only a small
amount of sol-fraction [113]. Therefore, the viscosity within this space is considerably lower than that of the initial PVA solution, and larger ice particles are
formed. In addition to widening of the primary pores by these growing crystals, a
certain compression of the pore walls owing to the physical stresses caused by the
ice crystallization also occurs, thus facilitating compaction of the proper gel phase
in the heterophase material. As a consequence, after the second freeze–thaw cycle,
the cross-section of the macropores increases by a factor of 2–3, whereas further
cryogenic cycles have an insignificant influence on the size and shape of these
pores [133].
The cryogenic processing conditions of PVA solutions also affect significantly
the porous structure of PVA cryogels. To demonstrate this effect, noncovalent PVA
cryogels were prepared starting from aqueous solutions of PVA (100 g/L)
according to two different freezing and frozen-storage procedures [44]:
(a) PVA solution was frozen at À20
C for 24 h
(b) PVA solution was first frozen at À20
C for 1 h, then incubated at a fixed
negative temperature between À5 and À1.1
C for 23 h
The micrographs in Fig. 18a, b represent thin sections of PVA cryogels formed
by the two procedures, respectively, where the latter was incubated at À2
C. The
morphology of the cryogel formed by a single-temperature freezing differs markedly from the two-temperature freezing procedure. As pointed out in Sect. 2.4, at a
storage temperature of À2
C, cryotropic gelation of PVA occurs with the highest
efficiency. Moreover, ice re-crystallization phenomena are also very intensive at
this temperature [167–169]. Since the prolonged incubation of the frozen sample at
À2
C equalizes the temperature fields in all directions, the branched crystals are
formed as a result of the re-crystallization. After defrosting, such secondary crystals
leave a net-like system of intersecting macropores in the body of the PVA cryogel.
The next key feature of the textural morphology of freeze–thaw gels is the
interconnected character of their macropores. The main reason for such a porous
character is the 3D growth of porogen particles (i.e., solvent crystals) during
freezing of the feed system, whereby the growth stops for a particular facet of a
crystal when it comes into tight contact with some facet of a neighboring growing
crystal [8]. During subsequent thawing of the frozen sample, these contact areas are
transformed into the connections between macropores. When the unidirectional
freezing technique is employed, the propagation rates of different facets are
Basic Principles of Cryotropic Gelation
85