nearly nonporous polymer network. Although freeze-dried hydrogels are also
porous, no distinct plateau is observed in stress–strain curves [50], which is
attributed to the weak network structure due to the absence of cryo-concentration.
Squeezability of the cryogels and their reusability, as well as the continuous
extraction capacity for the removal of pollutants, are determined by subjecting the
cryogel samples to successive sorption–squeezing cycles under identical conditions. In a typical procedure [54], the cryogel sample is first immersed in the test
solution for 1 min and then it is left to drip for 30 s. The cryogel is weighed and put
into a Bu ¨chner funnel and squeezed for 30 s under 50 mm vacuum. Then, it is
weighed again to calculate the amount of solution taken up by one gram of cryogel.
This sorption–squeezing cycle is repeated many times to obtain the recycling
efficiency and continuous extraction capacity of the cryogels.
3 Transition from Gelation to Cryogelation: Gels Versus
Cryogels
Since cryo-concentration is a characteristic phenomenon of cryogelation, it is
necessary first to discuss how increased monomer concentration at gelation affects
the properties of conventional gels. Several studies show that the gel structure and,
thus, the gel properties strongly depend on the initial monomer concentration [11,
13, 85–89]. No continuous network is formed below a critical concentration of
monomer. Increasing the amount of monomer at polymerization causes the polymer
chains to entangle so that the network formed in a semidilute solution can swell
poorly, even when exposed to a good solvent. This increase in the monomer
concentration also decreases the probability of cyclization reactions, so that a
large fraction of the crosslinker is consumed in effective crosslinks [85]. In accord
with the experiments, the statistical model proposed by Bromberg et al. predicts
that the effective crosslink density of gels scales with the second power of the
monomer concentration [89, 90]. As a consequence, the network structure formed
becomes increasingly tight as the monomer concentration increases.
Let us now consider what will happen to the monomer concentration when the
polymerization temperature is decreased below the freezing point of the reaction
system. During freezing, the monomers and the initiator are expelled from the
forming solvent crystals and become entrapped within channels between the crystals. As a consequence, the polymerization reactions can only take place in spatially
restricted reaction fields, which are the unfrozen microchannels of the apparently
frozen system. The reason why solvent does not freeze below the bulk freezing
temperature is attributed to the freezing point depression of the solvent due to the
solutes, e.g., monomers and polymers [91–93]. For instance, about 6 % of the water
in swollen PAAm hydrogels remains unfrozen, even at À24
C [26]. Kirsebom used
1 H NMR to estimate the monomer concentration in the unfrozen microchannels
[28]. The reaction system studied was the crosslinking copolymerization of DMA
Synthesis and Structure–Property Relationships of Cryogels
117
porous, no distinct plateau is observed in stress–strain curves [50], which is
attributed to the weak network structure due to the absence of cryo-concentration.
Squeezability of the cryogels and their reusability, as well as the continuous
extraction capacity for the removal of pollutants, are determined by subjecting the
cryogel samples to successive sorption–squeezing cycles under identical conditions. In a typical procedure [54], the cryogel sample is first immersed in the test
solution for 1 min and then it is left to drip for 30 s. The cryogel is weighed and put
into a Bu ¨chner funnel and squeezed for 30 s under 50 mm vacuum. Then, it is
weighed again to calculate the amount of solution taken up by one gram of cryogel.
This sorption–squeezing cycle is repeated many times to obtain the recycling
efficiency and continuous extraction capacity of the cryogels.
3 Transition from Gelation to Cryogelation: Gels Versus
Cryogels
Since cryo-concentration is a characteristic phenomenon of cryogelation, it is
necessary first to discuss how increased monomer concentration at gelation affects
the properties of conventional gels. Several studies show that the gel structure and,
thus, the gel properties strongly depend on the initial monomer concentration [11,
13, 85–89]. No continuous network is formed below a critical concentration of
monomer. Increasing the amount of monomer at polymerization causes the polymer
chains to entangle so that the network formed in a semidilute solution can swell
poorly, even when exposed to a good solvent. This increase in the monomer
concentration also decreases the probability of cyclization reactions, so that a
large fraction of the crosslinker is consumed in effective crosslinks [85]. In accord
with the experiments, the statistical model proposed by Bromberg et al. predicts
that the effective crosslink density of gels scales with the second power of the
monomer concentration [89, 90]. As a consequence, the network structure formed
becomes increasingly tight as the monomer concentration increases.
Let us now consider what will happen to the monomer concentration when the
polymerization temperature is decreased below the freezing point of the reaction
system. During freezing, the monomers and the initiator are expelled from the
forming solvent crystals and become entrapped within channels between the crystals. As a consequence, the polymerization reactions can only take place in spatially
restricted reaction fields, which are the unfrozen microchannels of the apparently
frozen system. The reason why solvent does not freeze below the bulk freezing
temperature is attributed to the freezing point depression of the solvent due to the
solutes, e.g., monomers and polymers [91–93]. For instance, about 6 % of the water
in swollen PAAm hydrogels remains unfrozen, even at À24
C [26]. Kirsebom used
1 H NMR to estimate the monomer concentration in the unfrozen microchannels
[28]. The reaction system studied was the crosslinking copolymerization of DMA
Synthesis and Structure–Property Relationships of Cryogels
117