charge density f of the network chains would decrease the volume fraction of ice in
the reaction system so that the pores in the final gels would be smaller [36].
4.4 Crosslinker Concentration
In PAAm and PSA cryogels formed using BAAm crosslinker, no appreciable
change in the size of the pores was reported with changes in the crosslinker level
[26, 95]. However, in fibroin cryogels formed at T prep ¼ À18
C and in the
presence of EGDE as a crosslinker, the average pore diameter decreased from
55 Æ 10 to 23 Æ 7 μm and the pore-size distribution became narrower as the
amount of EGDE increased from 10 to 30 mmol/g [50]. Similar results were
reported for rubber cryogels formed using S 2 Cl 2 crosslinker. As S 2 Cl 2 concentration is increased, both the polydispersity and the average diameter of the pores
decrease [52]. This could be due to the increased rigidity of the gel in the unfrozen
domains as the amount of crosslinker is increased.
4.5 Additives and Solvent
The presence of low molecular weight salts significantly affects the microstructure
of the cryogels formed in aqueous solutions. For instance, addition of NaCl or
CaCl 2 in aqueous cryogelation solutions results in materials with thicker pore walls
and smaller pores [102]. As the NaCl content in the reaction mixture is increased
from 0 to 10 % (w/w), the pore size of PAMPS cryogels decreases from 30–50 to
10–20 μm [36]. This is due to the fact that the addition of salts into the reaction
system decreases the bulk freezing temperature so that the volume of the nonfrozen
solution increases. This leads to the formation of gels with smaller pores. Indeed, as
the NaCl content in the reaction mixture is increased from 0 to 10 %(w/v), the bulk
freezing times of AMPS-BAAm reaction mixtures at T prep ¼ À22
C increased
from 5 to 35 min, whereas their gelation times at 0
C remained constant (5 Æ 1
min) [36]. Moreover, addition of a polymerization inhibitor such as hydroquinone
into the reaction solution produces more monodisperse and smaller pores in PAAm
cryogels [23]. Hydroquinone provides homogeneous nucleation of ice crystals due
to the shift of the gel point beyond freezing of the reaction solution, so that the
number of crystals increases while their size decreases, leading to the formation of
small pores in PAAm cryogels.
The addition of micro- or nanoparticles in the cryogelation system produces
hybrid cryogels with increased modulus of elasticity [56, 103–106]. For instance,
such cryogels were prepared by the addition of tetramethoxysilane in aqueous
solutions of PVA before its freeze–thaw treatment [104]. They can also be prepared
by crosslinking of PIB in cyclohexane containing silica nanoparticles and using
S 2 Cl 2 as a crosslinking agent [56]. The microstructure of the hybrid PIB cryogels
134
O. Okay and V.I. Lozinsky
the reaction system so that the pores in the final gels would be smaller [36].
4.4 Crosslinker Concentration
In PAAm and PSA cryogels formed using BAAm crosslinker, no appreciable
change in the size of the pores was reported with changes in the crosslinker level
[26, 95]. However, in fibroin cryogels formed at T prep ¼ À18
C and in the
presence of EGDE as a crosslinker, the average pore diameter decreased from
55 Æ 10 to 23 Æ 7 μm and the pore-size distribution became narrower as the
amount of EGDE increased from 10 to 30 mmol/g [50]. Similar results were
reported for rubber cryogels formed using S 2 Cl 2 crosslinker. As S 2 Cl 2 concentration is increased, both the polydispersity and the average diameter of the pores
decrease [52]. This could be due to the increased rigidity of the gel in the unfrozen
domains as the amount of crosslinker is increased.
4.5 Additives and Solvent
The presence of low molecular weight salts significantly affects the microstructure
of the cryogels formed in aqueous solutions. For instance, addition of NaCl or
CaCl 2 in aqueous cryogelation solutions results in materials with thicker pore walls
and smaller pores [102]. As the NaCl content in the reaction mixture is increased
from 0 to 10 % (w/w), the pore size of PAMPS cryogels decreases from 30–50 to
10–20 μm [36]. This is due to the fact that the addition of salts into the reaction
system decreases the bulk freezing temperature so that the volume of the nonfrozen
solution increases. This leads to the formation of gels with smaller pores. Indeed, as
the NaCl content in the reaction mixture is increased from 0 to 10 %(w/v), the bulk
freezing times of AMPS-BAAm reaction mixtures at T prep ¼ À22
C increased
from 5 to 35 min, whereas their gelation times at 0
C remained constant (5 Æ 1
min) [36]. Moreover, addition of a polymerization inhibitor such as hydroquinone
into the reaction solution produces more monodisperse and smaller pores in PAAm
cryogels [23]. Hydroquinone provides homogeneous nucleation of ice crystals due
to the shift of the gel point beyond freezing of the reaction solution, so that the
number of crystals increases while their size decreases, leading to the formation of
small pores in PAAm cryogels.
The addition of micro- or nanoparticles in the cryogelation system produces
hybrid cryogels with increased modulus of elasticity [56, 103–106]. For instance,
such cryogels were prepared by the addition of tetramethoxysilane in aqueous
solutions of PVA before its freeze–thaw treatment [104]. They can also be prepared
by crosslinking of PIB in cyclohexane containing silica nanoparticles and using
S 2 Cl 2 as a crosslinking agent [56]. The microstructure of the hybrid PIB cryogels
134
O. Okay and V.I. Lozinsky