to the conventional gels [42]. For example, in the free-radical crosslinking copolymerization of AMPS and BAAm (17 mol % of the comonomer mixture) at T prep
¼ À22
C, crosslinked polymer starts to form at C o ¼ 0.1 %, as compared to 5 %
when conducted at T prep ¼ 25
C [63]. In the preparation of PIB cryogels in
benzene using S 2 Cl 2 crosslinker (6 % v/w with respect to PIB), critical PIB
concentration for gelation is 0.08 Æ 0.02 % at À18
C, compared to 4 % at T prep
¼ 20
C [52]. Therefore, cryogels can be prepared over a wider concentration range
compared to the conventional gels; however, since the cryogels formed at very low
C o are mechanically weak, the usual concentration range is between 2 and 20 %
(w/v). Because increasing C o or decreasing T prep decreases the pore size of the
cryogels and simultaneously increases their mechanical strength, these synthesis
parameters can be used to tune the properties of cryogels.
2.4 Geometry of Cryogels
Cryogels can be prepared in various shapes such as blocks, sheets, discs, and beads
[64]. Cryogels as continuous-bed columns (monoliths) for chromatographic separation of biomolecules have been prepared by filling the gelation solution in plastic
syringes with a closed outlet at the bottom and then immersing the syringes in a
cryostat at T prep [29, 65–67]. To ensure the reproducibility of the freezing patterns,
reaction mixtures of the same volume, and syringes of the same dimensions, should
be used for every synthesis [68]. After completion of the cryogelation reaction, the
cryogel blocks are thawed at room temperature and then thoroughly washed with a
good solvent to remove unreacted species. Cryogels can also be prepared in the
form of membranes by injecting the reaction solution between two glass plates
separated by a spacer.
In contrast to blocks consisting of a single piece of macroporous material,
cryogels in the form of beads are suitable for packing in columns with different
scales. However, due to a number of preparation difficulties, there are only a few
publications on the preparation of cryogel beads. For instance, if the cryogelation
reactions are carried out under the conditions of the usual suspension polymerization technique, odd-shaped gel particles with a broad size distribution are obtained,
due to collisions between the frozen droplets [69]. It was shown that cryogel
particles can be prepared inside the wall of a hollow plastic carrier to make them
resistant to intensive stirring [70]. Yun et al. utilized a microfluidic flow focusing
technique to generate a suspension of aqueous gelation solution in a waterimmiscible organic phase [71, 72]. After keeping the droplets at subzero temperatures, PAAm-based cryogel beads of 1 mm in diameter were produced. Alginateagarose, PAAm, and DNA-based cryogel beads have been prepared recently by
dropwise addition of the aqueous reaction solution into the paraffin oil as the
continuous phase at temperatures between À15 and À20
C [47, 64, 73]. The
diameter of the cryogel beads could be adjusted by changing the tip diameter of
112
O. Okay and V.I. Lozinsky
¼ À22
C, crosslinked polymer starts to form at C o ¼ 0.1 %, as compared to 5 %
when conducted at T prep ¼ 25
C [63]. In the preparation of PIB cryogels in
benzene using S 2 Cl 2 crosslinker (6 % v/w with respect to PIB), critical PIB
concentration for gelation is 0.08 Æ 0.02 % at À18
C, compared to 4 % at T prep
¼ 20
C [52]. Therefore, cryogels can be prepared over a wider concentration range
compared to the conventional gels; however, since the cryogels formed at very low
C o are mechanically weak, the usual concentration range is between 2 and 20 %
(w/v). Because increasing C o or decreasing T prep decreases the pore size of the
cryogels and simultaneously increases their mechanical strength, these synthesis
parameters can be used to tune the properties of cryogels.
2.4 Geometry of Cryogels
Cryogels can be prepared in various shapes such as blocks, sheets, discs, and beads
[64]. Cryogels as continuous-bed columns (monoliths) for chromatographic separation of biomolecules have been prepared by filling the gelation solution in plastic
syringes with a closed outlet at the bottom and then immersing the syringes in a
cryostat at T prep [29, 65–67]. To ensure the reproducibility of the freezing patterns,
reaction mixtures of the same volume, and syringes of the same dimensions, should
be used for every synthesis [68]. After completion of the cryogelation reaction, the
cryogel blocks are thawed at room temperature and then thoroughly washed with a
good solvent to remove unreacted species. Cryogels can also be prepared in the
form of membranes by injecting the reaction solution between two glass plates
separated by a spacer.
In contrast to blocks consisting of a single piece of macroporous material,
cryogels in the form of beads are suitable for packing in columns with different
scales. However, due to a number of preparation difficulties, there are only a few
publications on the preparation of cryogel beads. For instance, if the cryogelation
reactions are carried out under the conditions of the usual suspension polymerization technique, odd-shaped gel particles with a broad size distribution are obtained,
due to collisions between the frozen droplets [69]. It was shown that cryogel
particles can be prepared inside the wall of a hollow plastic carrier to make them
resistant to intensive stirring [70]. Yun et al. utilized a microfluidic flow focusing
technique to generate a suspension of aqueous gelation solution in a waterimmiscible organic phase [71, 72]. After keeping the droplets at subzero temperatures, PAAm-based cryogel beads of 1 mm in diameter were produced. Alginateagarose, PAAm, and DNA-based cryogel beads have been prepared recently by
dropwise addition of the aqueous reaction solution into the paraffin oil as the
continuous phase at temperatures between À15 and À20
C [47, 64, 73]. The
diameter of the cryogel beads could be adjusted by changing the tip diameter of
112
O. Okay and V.I. Lozinsky