preparation of a heterogeneous open porous structure is that the solvent is frozen
before the beginning of crosslinking reactions, i.e., the reaction system is
cryostructured. In the case of chemical crosslinking by a redox system, which is
one of the most frequently used approaches for synthesis of polymer cryogels, the
polymerization rate has to be slow enough to prevent crosslinking of the system
from the time of mixing of the reagents until the complete cryostructuration, i.e.,
any crosslinking in solution must be avoided [3]. Typically, when using a redox
system, the time required for preparation of cryogels varies from 16 to 24 h [4–6].
From this point of view, the most efficient methods for regular cryostructuration
of the system are those based on chemical crosslinking induced by high energy
radiation (gamma-rays, electron beam) [7–9]. Here, the solution of reagents is first
allowed to freeze, forming well-separated large interconnected ice crystals and
nonfrozen liquid microphase. Then, the polymer network is formed. Usually, the
irradiation procedure takes 60–180 min and saves time in the preparation of
cryogels. However, the complex and expensive equipment needed in combination
with the safety requirements seem to limit the wide use of these methods.
Ten years ago, the UV irradiation technique was successfully employed for the
first time by our team for the synthesis of poly(ethylene oxide) (PEO) cryogels
[10]. This method starts with freezing of the solvent and conducts the crosslinking
reaction after the complete structuring of the system. Thus, together with full
control over the formation of large-size crystals and nonfrozen liquid microphase,
the method benefits from the facile procedure and easy access to a UV light source.
By optimizing the experimental conditions, PEO cryogels of very high gel fraction
(GF) yield (95 %) were obtained.
This review summarizes the recent achievements in preparation of various
supermacroporous polymer cryogels via UV-induced crosslinking in partly frozen
systems. The method is equally effective for the formation of cryogels from both
water-soluble high molar mass linear polymers and vinyl monomers. Special
attention is paid to some novel materials based on biodegradable and/or stimuliresponsive polymers and their application in some emerging fields, as well as the
fabrication of nanocomposites with intriguing properties.
2 Cryogels from Water-Soluble High Molar Mass
Polymers
Cryogels from high molar mass polymer precursors were obtained by a simple
procedure involving preparation of semidilute/concentrated polymer solution
(0.5–5 mass%) containing a photoinitiator, followed by freezing, UV-induced
crosslinking, and thawing. Notably, the crosslinking reaction is very fast and
formation of the polymer network can be completed within several minutes. The
accepted mechanism of crosslinking of high molar mass linear polymers induced by
UV light involves generation and subsequent recombination of macroradicals [10]
Cryogels via UV Irradiation
201
before the beginning of crosslinking reactions, i.e., the reaction system is
cryostructured. In the case of chemical crosslinking by a redox system, which is
one of the most frequently used approaches for synthesis of polymer cryogels, the
polymerization rate has to be slow enough to prevent crosslinking of the system
from the time of mixing of the reagents until the complete cryostructuration, i.e.,
any crosslinking in solution must be avoided [3]. Typically, when using a redox
system, the time required for preparation of cryogels varies from 16 to 24 h [4–6].
From this point of view, the most efficient methods for regular cryostructuration
of the system are those based on chemical crosslinking induced by high energy
radiation (gamma-rays, electron beam) [7–9]. Here, the solution of reagents is first
allowed to freeze, forming well-separated large interconnected ice crystals and
nonfrozen liquid microphase. Then, the polymer network is formed. Usually, the
irradiation procedure takes 60–180 min and saves time in the preparation of
cryogels. However, the complex and expensive equipment needed in combination
with the safety requirements seem to limit the wide use of these methods.
Ten years ago, the UV irradiation technique was successfully employed for the
first time by our team for the synthesis of poly(ethylene oxide) (PEO) cryogels
[10]. This method starts with freezing of the solvent and conducts the crosslinking
reaction after the complete structuring of the system. Thus, together with full
control over the formation of large-size crystals and nonfrozen liquid microphase,
the method benefits from the facile procedure and easy access to a UV light source.
By optimizing the experimental conditions, PEO cryogels of very high gel fraction
(GF) yield (95 %) were obtained.
This review summarizes the recent achievements in preparation of various
supermacroporous polymer cryogels via UV-induced crosslinking in partly frozen
systems. The method is equally effective for the formation of cryogels from both
water-soluble high molar mass linear polymers and vinyl monomers. Special
attention is paid to some novel materials based on biodegradable and/or stimuliresponsive polymers and their application in some emerging fields, as well as the
fabrication of nanocomposites with intriguing properties.
2 Cryogels from Water-Soluble High Molar Mass
Polymers
Cryogels from high molar mass polymer precursors were obtained by a simple
procedure involving preparation of semidilute/concentrated polymer solution
(0.5–5 mass%) containing a photoinitiator, followed by freezing, UV-induced
crosslinking, and thawing. Notably, the crosslinking reaction is very fast and
formation of the polymer network can be completed within several minutes. The
accepted mechanism of crosslinking of high molar mass linear polymers induced by
UV light involves generation and subsequent recombination of macroradicals [10]
Cryogels via UV Irradiation
201