3 Cryogels from Water-Soluble Vinyl Monomers
It has been demonstrated by several teams that the UV irradiation technique can be
used for the synthesis of polymer cryogels from different vinyl monomers (Table 2)
in the presence of a photoinitiator and a crosslinking agent.
The polymer network is formed in the nonfrozen liquid microphase by a free
radical photopolymerization/crosslinking reaction (Fig. 11) at defined negative
temperature. The use of crosslinking agent provides the formation of a threedimensional network instead of linear macrochains. Specifically, the concentration
of reagents in the microphase is much higher than the concentration in the initial
solution, due to the fact that a large amount of the solvent forms crystals.
The incorporation of crosslinks between the polymer chains also contributes to
the mechanical strength of the material. A comparison between two polyacrylamide
cryogels, synthesized from 5 mass% monomer solution containing crosslinking
agent poly(ethylene glycol) diacrylate (PEGDA, 10 mass%) and 5 mass% polymer
solutions without PEGDA, revealed that the latter cryogel has a storage modulus
several times lower than the cryogel synthesized from monomer (Fig. 12). However, the addition of 10 mass% PEGDA to the polymer solution resulted in a
cryogel with G
0 similar to that of AAm/PEGDA-based cryogels.
The choice of a powerful source of UV light seems to be the crucial factor for
fast monomer conversion and formation of polymer network. The results from
experiments carred out with a Dymax 5000-EC UV curing equipment with
400 W metal halide flood lamp emitting full spectrum UV–visible light at an
irradiation dose rate of 5.7 J/cm
2 min (input power 93 mW/cm
2 ) showed that
5 min of irradiation is sufficient for preparation of disk-shaped cryogels of good
quality [16, 21, 22]. Moreover, in the case of NIPAAm, ETEGA, and AAm,
cryogels of extremely high GF yield (nearly quantitative monomer conversion)
were obtained (Table 3). Thus, the as-synthesized materials do not contain undesirable monomer and crosslinking agent and can be directly used without any
extraction procedure. The conversion of HEMA to a PHEMA network within the
studied concentration range was also very high. Only vinyl caprolactam (VCL)
cannot form cryogels of high GF yield, and the possible reason for the low
crosslinking efficiency can be attributed to the pооr solubility of VCL in water.
The use of non-crystallizable co-solvent (ethanol; 10 vol%) seems to affect the
regular cryostructuring and hinder the formation of polymer network.
In general, the time for preparation of polymer cryogels from vinyl monomers
via UV irradiation is much shorter compared to the most often used technique,
which is based on a redox system. Since the reagents can be dissolved quickly in the
solvent and the reaction time is only 5 min, the freezing/cryostructuring of the
system appears to be the longest stage in the preparation procedure. Usually,
depending on the sample size and shape, the solvent can be frozen within several
(up to 120) minutes, making completion of the entire process possible in less than
1 h. It should be noted that all the commercially available monomers listed in
Table 3 can be used without any purification, which further facilitates the synthesis
Cryogels via UV Irradiation
209
It has been demonstrated by several teams that the UV irradiation technique can be
used for the synthesis of polymer cryogels from different vinyl monomers (Table 2)
in the presence of a photoinitiator and a crosslinking agent.
The polymer network is formed in the nonfrozen liquid microphase by a free
radical photopolymerization/crosslinking reaction (Fig. 11) at defined negative
temperature. The use of crosslinking agent provides the formation of a threedimensional network instead of linear macrochains. Specifically, the concentration
of reagents in the microphase is much higher than the concentration in the initial
solution, due to the fact that a large amount of the solvent forms crystals.
The incorporation of crosslinks between the polymer chains also contributes to
the mechanical strength of the material. A comparison between two polyacrylamide
cryogels, synthesized from 5 mass% monomer solution containing crosslinking
agent poly(ethylene glycol) diacrylate (PEGDA, 10 mass%) and 5 mass% polymer
solutions without PEGDA, revealed that the latter cryogel has a storage modulus
several times lower than the cryogel synthesized from monomer (Fig. 12). However, the addition of 10 mass% PEGDA to the polymer solution resulted in a
cryogel with G
0 similar to that of AAm/PEGDA-based cryogels.
The choice of a powerful source of UV light seems to be the crucial factor for
fast monomer conversion and formation of polymer network. The results from
experiments carred out with a Dymax 5000-EC UV curing equipment with
400 W metal halide flood lamp emitting full spectrum UV–visible light at an
irradiation dose rate of 5.7 J/cm
2 min (input power 93 mW/cm
2 ) showed that
5 min of irradiation is sufficient for preparation of disk-shaped cryogels of good
quality [16, 21, 22]. Moreover, in the case of NIPAAm, ETEGA, and AAm,
cryogels of extremely high GF yield (nearly quantitative monomer conversion)
were obtained (Table 3). Thus, the as-synthesized materials do not contain undesirable monomer and crosslinking agent and can be directly used without any
extraction procedure. The conversion of HEMA to a PHEMA network within the
studied concentration range was also very high. Only vinyl caprolactam (VCL)
cannot form cryogels of high GF yield, and the possible reason for the low
crosslinking efficiency can be attributed to the pооr solubility of VCL in water.
The use of non-crystallizable co-solvent (ethanol; 10 vol%) seems to affect the
regular cryostructuring and hinder the formation of polymer network.
In general, the time for preparation of polymer cryogels from vinyl monomers
via UV irradiation is much shorter compared to the most often used technique,
which is based on a redox system. Since the reagents can be dissolved quickly in the
solvent and the reaction time is only 5 min, the freezing/cryostructuring of the
system appears to be the longest stage in the preparation procedure. Usually,
depending on the sample size and shape, the solvent can be frozen within several
(up to 120) minutes, making completion of the entire process possible in less than
1 h. It should be noted that all the commercially available monomers listed in
Table 3 can be used without any purification, which further facilitates the synthesis
Cryogels via UV Irradiation
209