4 Effect of Synthesis Parameters
The properties of cryogels depend on many parameters, including the cryogelation
conditions and the composition of the reaction constituents. As the challenge is to
control the porous structure and, thus, the cryogel properties, this section focuses on
the effect of the synthesis parameters on the morphology of cryogels, with examples selected from the literature.
4.1 Temperature and Freezing Rate
It is known that the size of ice crystals can be controlled by varying the freezing
temperature and the freezing rate. For example, to produce large ice crystals, the
freezing temperature should be as high as possible and the time for crystallization
should be extended. To produce small crystals, freezing should be at a very low
temperature and the freezing rate should be high in order to reduce the time
available for ice crystals to grow. Because solvent crystals in the cryogelation
systems act as templates for the formation of pores, the same relationship usually
exists between the pore size of cryogels and the gelation temperature T prep [25, 39,
68]. Because the lower the T prep , the faster the cooling rate of the gelation solution
and, thus, the shorter the time period until the freezing temperature of the reaction
solution is reached, the T prep and the rate of cooling(or combination of both) do
affect the properties of cryogels.
Ivanov et al. investigated the temperature-dependent variation of pore diameter
in PAAm cryogels [43]. Cryogels were prepared by crosslinking of linear PAAm
chains in aqueous solutions using glutaraldehyde as a crosslinker. It was shown that
the pore diameter decreases from 30–60 μm to 10–20 μm as T prep is decreased from
À5 to À20
C [43]. In PAAm cryogels prepared from AAm and BAAm monomers,
the largest pore size was obtained at T prep ¼ À18
C and decreased as T prep was
further reduced down to À24
C (Fig. 7d) [26]. The average pore diameters were
60, 32, and 22 μm for T prep ¼ À18, À20, and À22
C, respectively. Similar results
were also reported for cryogels derived from aqueous solutions of fibroin chains or
DMA monomer (Fig. 12) [28, 50]. In PSA cryogels, the pore diameter decreases
from 25 to 11 μm as T prep is decreased from À9 to À20
C [95]. Since lower T prep
means faster freezing of the reaction solution, decreasing pore size with decreasing
T prep is consistent with the fact that a larger number of small solvent crystals form as
the freezing rate is increased. Additionally, since solvent in large voids is preferentially frozen relative to that in small capillaries due to a smaller freezing-point
depression, it is thought that, at T prep close to the solvent freezing point, only
solvent in large voids freezes during gelation, leading to large pores. We have to
note that there are also conflicting data regarding the temperature dependence of
pore size, especially at very low temperatures. This is attributed to the weakness of
the cryogel matrices formed at low T prep , which leads to collapse of the porous
structure upon drying.
Synthesis and Structure–Property Relationships of Cryogels
129
The properties of cryogels depend on many parameters, including the cryogelation
conditions and the composition of the reaction constituents. As the challenge is to
control the porous structure and, thus, the cryogel properties, this section focuses on
the effect of the synthesis parameters on the morphology of cryogels, with examples selected from the literature.
4.1 Temperature and Freezing Rate
It is known that the size of ice crystals can be controlled by varying the freezing
temperature and the freezing rate. For example, to produce large ice crystals, the
freezing temperature should be as high as possible and the time for crystallization
should be extended. To produce small crystals, freezing should be at a very low
temperature and the freezing rate should be high in order to reduce the time
available for ice crystals to grow. Because solvent crystals in the cryogelation
systems act as templates for the formation of pores, the same relationship usually
exists between the pore size of cryogels and the gelation temperature T prep [25, 39,
68]. Because the lower the T prep , the faster the cooling rate of the gelation solution
and, thus, the shorter the time period until the freezing temperature of the reaction
solution is reached, the T prep and the rate of cooling(or combination of both) do
affect the properties of cryogels.
Ivanov et al. investigated the temperature-dependent variation of pore diameter
in PAAm cryogels [43]. Cryogels were prepared by crosslinking of linear PAAm
chains in aqueous solutions using glutaraldehyde as a crosslinker. It was shown that
the pore diameter decreases from 30–60 μm to 10–20 μm as T prep is decreased from
À5 to À20
C [43]. In PAAm cryogels prepared from AAm and BAAm monomers,
the largest pore size was obtained at T prep ¼ À18
C and decreased as T prep was
further reduced down to À24
C (Fig. 7d) [26]. The average pore diameters were
60, 32, and 22 μm for T prep ¼ À18, À20, and À22
C, respectively. Similar results
were also reported for cryogels derived from aqueous solutions of fibroin chains or
DMA monomer (Fig. 12) [28, 50]. In PSA cryogels, the pore diameter decreases
from 25 to 11 μm as T prep is decreased from À9 to À20
C [95]. Since lower T prep
means faster freezing of the reaction solution, decreasing pore size with decreasing
T prep is consistent with the fact that a larger number of small solvent crystals form as
the freezing rate is increased. Additionally, since solvent in large voids is preferentially frozen relative to that in small capillaries due to a smaller freezing-point
depression, it is thought that, at T prep close to the solvent freezing point, only
solvent in large voids freezes during gelation, leading to large pores. We have to
note that there are also conflicting data regarding the temperature dependence of
pore size, especially at very low temperatures. This is attributed to the weakness of
the cryogel matrices formed at low T prep , which leads to collapse of the porous
structure upon drying.
Synthesis and Structure–Property Relationships of Cryogels
129