consists of two types of pores: 10
1
μm large pores due to the cyclohexane crystals
acting as a template during gelation and 10
À1 –10
0
μm small pores between the
aggregates of the nanoparticles. The nanoparticles in hybrid cryogels accumulate
within the large pores where cyclohexane crystals originally resided. The cryogels
exhibit a 300-fold larger modulus of elasticity than those prepared in the absence of
nanoparticles [56]. It was shown that these hybrid cryogels can be converted into
organic cryogels by dissolving the silica component in aqueous hydrofluoric acid,
whereas removing the polymer component by calcination results in porous silica
networks with 10
À1 -μm-sized pores [56].
The internal morphology of cryogels is also dependent on the type of solvent
used in cryogelation reactions. This effect arises due to the different shape of
solvent crystals as well as due to the temperature-dependent variations of the
interactions between the polymer and solvent. For instance, PAAm cryogels prepared in formamide at À20
C have an oriented porous structure with long pores as
a result of the formation of needle-like formamide crystals [25]. Moreover, in
contrast to the interconnected 3D pore structure of PNIPA cryogels formed in
water, those formed in dioxane/water medium have a closed pore structure with
thick (10–40 μm) pore walls [33]. The effect of polymer–solvent interactions on the
morphology of cryogels can be illustrated using PIB cryogels formed in cyclohexane and in benzene under identical conditions. Besides their molecular sizes,
benzene and cyclohexane have similar freezing and melting properties; they both
form high temperature orientationally disordered crystals and their melting temperatures in the bulk are close (5.5 versus 6.5
C for benzene and cyclohexane,
respectively) [107]. However, since the chemical structure of cyclohexane is close
to that of the repeating unit of polyisobutylene, it is a good solvent for PIB. In
contrast to cyclohexane, benzene is a poor solvent for PIB and becomes poorer as
the temperature decreases [53]. SEM images of PIB networks formed in benzene
and in cyclohexane show different morphologies. In contrast to the regular morphology of the cryogels prepared in cyclohexane, those formed in benzene exhibit a
broad size distribution of pores from micrometer to millimeter sizes [52]. Further,
the total volume of pores V p in PIB gels formed in benzene was found to increase
from 6 to 9 mL/g as T prep is decreased from À2 to À18
C, in contrast to the value of
about 2.5 mL/g for all low-temperature gels formed in cyclohexane [52, 53]. The
differences observed in the morphology of the gels are due to the cooling-induced
phase separation of the PIB chains in benzene [52]. Thus, the gel cannot absorb all
the available solvent during gelation so that a macrophase separation should occur
during the initial non-isothermal period of the reactions.
If the cryogelation reactions are conducted in a good solvent/poor solvent
mixture, the mechanism of pore structure formation by cryogelation can be combined with the reaction-induced phase separation. For instance, PAAm cryogels
formed at À12
C in acetone–water mixtures exhibit two types of pores [102]. The
10–80 μm large pores are due to the ice crystals acting as a template during gelation
while the submicrometer-sized pores in the pore walls are due to the χ-induced
phase separation in the unfrozen phase enriched with acetone (a poor solvent for
PAAm) and the monomers [102]. Another interesting example is PAAm cryogels
Synthesis and Structure–Property Relationships of Cryogels
135
1
μm large pores due to the cyclohexane crystals
acting as a template during gelation and 10
À1 –10
0
μm small pores between the
aggregates of the nanoparticles. The nanoparticles in hybrid cryogels accumulate
within the large pores where cyclohexane crystals originally resided. The cryogels
exhibit a 300-fold larger modulus of elasticity than those prepared in the absence of
nanoparticles [56]. It was shown that these hybrid cryogels can be converted into
organic cryogels by dissolving the silica component in aqueous hydrofluoric acid,
whereas removing the polymer component by calcination results in porous silica
networks with 10
À1 -μm-sized pores [56].
The internal morphology of cryogels is also dependent on the type of solvent
used in cryogelation reactions. This effect arises due to the different shape of
solvent crystals as well as due to the temperature-dependent variations of the
interactions between the polymer and solvent. For instance, PAAm cryogels prepared in formamide at À20
C have an oriented porous structure with long pores as
a result of the formation of needle-like formamide crystals [25]. Moreover, in
contrast to the interconnected 3D pore structure of PNIPA cryogels formed in
water, those formed in dioxane/water medium have a closed pore structure with
thick (10–40 μm) pore walls [33]. The effect of polymer–solvent interactions on the
morphology of cryogels can be illustrated using PIB cryogels formed in cyclohexane and in benzene under identical conditions. Besides their molecular sizes,
benzene and cyclohexane have similar freezing and melting properties; they both
form high temperature orientationally disordered crystals and their melting temperatures in the bulk are close (5.5 versus 6.5
C for benzene and cyclohexane,
respectively) [107]. However, since the chemical structure of cyclohexane is close
to that of the repeating unit of polyisobutylene, it is a good solvent for PIB. In
contrast to cyclohexane, benzene is a poor solvent for PIB and becomes poorer as
the temperature decreases [53]. SEM images of PIB networks formed in benzene
and in cyclohexane show different morphologies. In contrast to the regular morphology of the cryogels prepared in cyclohexane, those formed in benzene exhibit a
broad size distribution of pores from micrometer to millimeter sizes [52]. Further,
the total volume of pores V p in PIB gels formed in benzene was found to increase
from 6 to 9 mL/g as T prep is decreased from À2 to À18
C, in contrast to the value of
about 2.5 mL/g for all low-temperature gels formed in cyclohexane [52, 53]. The
differences observed in the morphology of the gels are due to the cooling-induced
phase separation of the PIB chains in benzene [52]. Thus, the gel cannot absorb all
the available solvent during gelation so that a macrophase separation should occur
during the initial non-isothermal period of the reactions.
If the cryogelation reactions are conducted in a good solvent/poor solvent
mixture, the mechanism of pore structure formation by cryogelation can be combined with the reaction-induced phase separation. For instance, PAAm cryogels
formed at À12
C in acetone–water mixtures exhibit two types of pores [102]. The
10–80 μm large pores are due to the ice crystals acting as a template during gelation
while the submicrometer-sized pores in the pore walls are due to the χ-induced
phase separation in the unfrozen phase enriched with acetone (a poor solvent for
PAAm) and the monomers [102]. Another interesting example is PAAm cryogels
Synthesis and Structure–Property Relationships of Cryogels
135