indicated in the figure (χ is assumed to be independent of temperature). Let us first
consider the solid curve in Fig. 6a, which was calculated for χ ¼ 0.48,
corresponding to polyacrylamide–water system [94]. At T prep ¼ À18
C, as indicated by the arrow, C is predicted to be 86 %. This means that, as the solvent
crystallizes at À18
C, the monomer concentration in the unfrozen phase rises
continuously until attaining its equilibrium value of 86 %. For an initial monomer
concentration (C o ) of 5 %, P becomes 0.94 (Fig. 6b), indicating that 94 % of the
reaction system consists of solvent crystals acting as template for pore formation.
Increasing the initial monomer concentration from 5 to 20 % decreases the volume
fraction of frozen solvent, i.e., porosity from 94 to 77 %. Figure 6a, b also shows
that the lower the T prep , the higher the monomer concentration in the reaction zones
and the larger the porosity.
Moreover, decreasing the χ parameter at a given T prep (i.e., increasing the
solvating power of the solvent) also decreases the monomer concentration in the
unfrozen zones and, thus, the total volume of the pores in the final cryogels. This is
attributed to the increasing amount of uncrystallizable solvent bound to polymer
chains as the quality of the diluent is increased. Figure 6c, d presents the results of
calculations for three different solvents. As the temperature drops below the
freezing temperature of the pure solvent, which is 0, 5.4, and 19
C for water,
benzene, and DMSO, respectively, the monomer concentration in the unfrozen
zones rapidly increases. Cryogels with 95 % porosities could be obtained at T prep
¼ À3, +3, and +17
C using water, benzene, and DMSO as the polymerization
solvent, respectively. Thus, the cryogelation temperature can be adjusted by
selecting a suitable solvent. As will be seen in the following sections, the prediction
of simulation results is in qualitative agreement with the experimental findings.
The theoretical results in Fig. 6 also show that cryo-concentration and the
resulting transition from gelation to the cryogelation regime occurs at temperatures
close to the freezing point of the pure solvent. As a result, a drastic change in the gel
properties has to be expected at these temperatures. For instance, increased monomer concentration due to cryo-concentration would lead to a significant increase in
the elastic modulus and a decrease in the swelling capacity of the gels.
This was indeed observed experimentally, but at lower temperatures. The results
of such measurements are shown in Fig. 7a–c for PAAm hydrogels, where the
elastic modulus G, the equilibrium weight q w , volume swelling ratio q v of the gels,
and their dry and swollen state porosities, P and P s , respectively, are plotted against
T prep [26]. The gels were prepared by free-radical crosslinking copolymerization of
AAm with BAAm in aqueous solution at various T prep between À25
C and +25
C
[26]. The dotted rectangular area in Fig. 7a–c indicates the transition between
homogeneous gelation and cryogelation regimes occurring between À6 and
À10
C. Depending on T prep , two types of gels can be obtained:
1. At T prep ! À6
C: The gels exhibit a relatively low modulus of elasticity around
1.5 kPa. Both the weight q w and the volume swelling ratios q v of gels are equal to
~20, and they are independent of T prep .
2. At T prep À10
C: Decreasing T prep below –6
C results in a fourfold increase
in the elastic modulus of gels. Moreover, the weight swelling ratio q w remains
122
O. Okay and V.I. Lozinsky
consider the solid curve in Fig. 6a, which was calculated for χ ¼ 0.48,
corresponding to polyacrylamide–water system [94]. At T prep ¼ À18
C, as indicated by the arrow, C is predicted to be 86 %. This means that, as the solvent
crystallizes at À18
C, the monomer concentration in the unfrozen phase rises
continuously until attaining its equilibrium value of 86 %. For an initial monomer
concentration (C o ) of 5 %, P becomes 0.94 (Fig. 6b), indicating that 94 % of the
reaction system consists of solvent crystals acting as template for pore formation.
Increasing the initial monomer concentration from 5 to 20 % decreases the volume
fraction of frozen solvent, i.e., porosity from 94 to 77 %. Figure 6a, b also shows
that the lower the T prep , the higher the monomer concentration in the reaction zones
and the larger the porosity.
Moreover, decreasing the χ parameter at a given T prep (i.e., increasing the
solvating power of the solvent) also decreases the monomer concentration in the
unfrozen zones and, thus, the total volume of the pores in the final cryogels. This is
attributed to the increasing amount of uncrystallizable solvent bound to polymer
chains as the quality of the diluent is increased. Figure 6c, d presents the results of
calculations for three different solvents. As the temperature drops below the
freezing temperature of the pure solvent, which is 0, 5.4, and 19
C for water,
benzene, and DMSO, respectively, the monomer concentration in the unfrozen
zones rapidly increases. Cryogels with 95 % porosities could be obtained at T prep
¼ À3, +3, and +17
C using water, benzene, and DMSO as the polymerization
solvent, respectively. Thus, the cryogelation temperature can be adjusted by
selecting a suitable solvent. As will be seen in the following sections, the prediction
of simulation results is in qualitative agreement with the experimental findings.
The theoretical results in Fig. 6 also show that cryo-concentration and the
resulting transition from gelation to the cryogelation regime occurs at temperatures
close to the freezing point of the pure solvent. As a result, a drastic change in the gel
properties has to be expected at these temperatures. For instance, increased monomer concentration due to cryo-concentration would lead to a significant increase in
the elastic modulus and a decrease in the swelling capacity of the gels.
This was indeed observed experimentally, but at lower temperatures. The results
of such measurements are shown in Fig. 7a–c for PAAm hydrogels, where the
elastic modulus G, the equilibrium weight q w , volume swelling ratio q v of the gels,
and their dry and swollen state porosities, P and P s , respectively, are plotted against
T prep [26]. The gels were prepared by free-radical crosslinking copolymerization of
AAm with BAAm in aqueous solution at various T prep between À25
C and +25
C
[26]. The dotted rectangular area in Fig. 7a–c indicates the transition between
homogeneous gelation and cryogelation regimes occurring between À6 and
À10
C. Depending on T prep , two types of gels can be obtained:
1. At T prep ! À6
C: The gels exhibit a relatively low modulus of elasticity around
1.5 kPa. Both the weight q w and the volume swelling ratios q v of gels are equal to
~20, and they are independent of T prep .
2. At T prep À10
C: Decreasing T prep below –6
C results in a fourfold increase
in the elastic modulus of gels. Moreover, the weight swelling ratio q w remains
122
O. Okay and V.I. Lozinsky