point can be a better way to suppress supercooling. Such a physical impact on the
liquid system causes cavitation of gas microbubbles within the whole volume of the
sample, thus resulting in uniform nucleation of the solvent crystallization
[53]. However, no practical examples of implementing this approach for the
preparation of polymeric cryogels are known so far to the authors of this review.
This is probably due to the absence of laboratory-scale ultrasound equipment
suitable for cryotropic gelation procedures.
2.3 Incubation of the Gelation System in a Frozen State
Keeping the gelation system in a frozen state at a moderate negative temperature is
the main stage in the synthesis of covalent cryogels, since the chemical reactions
leading to gelation mainly occur during this time period [8]. In the case of physical
cryogels, the pattern is not so straightforward and will be discussed later. Nonetheless, in both cases, polymeric cryogels form within the volume of the unfrozen
liquid microphase (UFLMP, also called the “nonfrozen liquid microphase”) where
the precursors are concentrated as the solvent starts to crystallize [8]. Therefore, the
concept of UFLMP will be discussed in more detail.
The term “unfrozen liquid microphase” was proposed by Sergeev and co-authors
in 1973 for designating the liquid fraction at a particular negative temperature in a
macroscopically frozen system [54]. This concept was developed in the course of
exploration of various chemical reactions occurring in the non-deeply frozen
multicomponent solutions of low molecular weight reactants. These studies, beginning from the initially empiric observations [55–61], then generalized by Pincock
[62], and leading to a rather harmonious kinetic theory elaborated by Sergeev and
Batyuk with coworkers [54, 63, 64], revealed the following three fundamental
features of such processes and their mechanisms:
1. When the initial molecular or colloidal solution is non-deeply frozen, although
the system looks like a completely solid matter, it is heterophase at a microscopic level and contains, along with the solvent polycrystals, a certain unfrozen
fraction, UFLMP. Thus, the moderately or non-deeply frozen systems are those
where UFLMP is still present. The existence of UFLMP at temperatures both
above and below the eutectic point of the particular system is confirmed by
different physicochemical methods, e.g., by the narrow NMR signals of the
solutes [8, 41, 50, 52, 63–72] and by the shape of ESR spectra characteristic
of the liquid microenvironment of spin probes [8, 63, 73–76]. The temperature
boundary of the existence of UFLMP is stipulated by several factors such as the
nature of the solvent, the amount of solutes, their physicochemical properties
(e.g., molecular weight [77]), the flexibility of the chains in the case of polymeric precursors, thermal prehistory of the system during its freezing if the
phase equilibrium is not yet attained, etc.
58
V.I. Lozinsky and O. Okay
liquid system causes cavitation of gas microbubbles within the whole volume of the
sample, thus resulting in uniform nucleation of the solvent crystallization
[53]. However, no practical examples of implementing this approach for the
preparation of polymeric cryogels are known so far to the authors of this review.
This is probably due to the absence of laboratory-scale ultrasound equipment
suitable for cryotropic gelation procedures.
2.3 Incubation of the Gelation System in a Frozen State
Keeping the gelation system in a frozen state at a moderate negative temperature is
the main stage in the synthesis of covalent cryogels, since the chemical reactions
leading to gelation mainly occur during this time period [8]. In the case of physical
cryogels, the pattern is not so straightforward and will be discussed later. Nonetheless, in both cases, polymeric cryogels form within the volume of the unfrozen
liquid microphase (UFLMP, also called the “nonfrozen liquid microphase”) where
the precursors are concentrated as the solvent starts to crystallize [8]. Therefore, the
concept of UFLMP will be discussed in more detail.
The term “unfrozen liquid microphase” was proposed by Sergeev and co-authors
in 1973 for designating the liquid fraction at a particular negative temperature in a
macroscopically frozen system [54]. This concept was developed in the course of
exploration of various chemical reactions occurring in the non-deeply frozen
multicomponent solutions of low molecular weight reactants. These studies, beginning from the initially empiric observations [55–61], then generalized by Pincock
[62], and leading to a rather harmonious kinetic theory elaborated by Sergeev and
Batyuk with coworkers [54, 63, 64], revealed the following three fundamental
features of such processes and their mechanisms:
1. When the initial molecular or colloidal solution is non-deeply frozen, although
the system looks like a completely solid matter, it is heterophase at a microscopic level and contains, along with the solvent polycrystals, a certain unfrozen
fraction, UFLMP. Thus, the moderately or non-deeply frozen systems are those
where UFLMP is still present. The existence of UFLMP at temperatures both
above and below the eutectic point of the particular system is confirmed by
different physicochemical methods, e.g., by the narrow NMR signals of the
solutes [8, 41, 50, 52, 63–72] and by the shape of ESR spectra characteristic
of the liquid microenvironment of spin probes [8, 63, 73–76]. The temperature
boundary of the existence of UFLMP is stipulated by several factors such as the
nature of the solvent, the amount of solutes, their physicochemical properties
(e.g., molecular weight [77]), the flexibility of the chains in the case of polymeric precursors, thermal prehistory of the system during its freezing if the
phase equilibrium is not yet attained, etc.
58
V.I. Lozinsky and O. Okay