3.6 Mechanism of Formation of PVA Cryogels
The mechanism of formation of the hierarchical structure of PVA cryogels featuring an open porous structure is the result of the occurrence of at least three
concomitant and, at the same time, conflicting processes: crystallization of the
solvent, LL phase separation, and crystallization of PVA. At subzero temperatures,
solutions having the typical concentrations used for preparation of PVA cryogels
are below the spinodal curve shown in Fig. 8 [4–6, 43]. Therefore, during freezing
and successive permanence of the solution at subzero temperatures, a LL phase
separation may occur in addition to the crystallization of solvent and PVA. The
exact mechanism of formation of the macroporous structure (i.e., water crystallization at subzero temperatures or LL phase separation, or the concomitant effect of
both transitions) depends on a number of factors such as the cooling rate at subzero
temperatures, the use of other solvents mixed with water, the presence of additives,
and the PVA concentration [4–6].
Applying the arguments given in Sect. 2, the formation mechanism of PVA
cryogels represents a paradigmatic example of the Ostwald “stage rule” [17, 25,
33]. This means that, upon formation of these gels, a hierarchy of metastability may
arise due to the possible occurrence of several metastable phases via different
processes [17, 25]. Therefore, when a given process is arrested by an agency before
completion, the system is frozen in a phase of circumstantial metastability that,
once formed, has the potential to evolve faster than the transition leading to the
ultimate stable phase and may dominate the whole transformation process [17, 25].
Lozinsky and coworkers [4–6] suggested a general mechanism for cryotropic
gelation, which is illustrated in Fig. 15.
In this model, the initial solution (Fig. 15a), is frozen at temperatures slightly
below the solvent crystallization point, giving rise to an inhomogeneous system that
includes an unfrozen liquid microphase along with crystals of the frozen solvent
(Fig. 15b). Since the polymer (and any other additive) are generally rejected in the
unfrozen liquid microphase, the solute concentration in the unfrozen liquid
microphase is higher than in the initial solution. At this stage, a crosslinking process
(chemical or physical) may easily take place in the unfrozen liquid microphase,
leading to the formation of microgel fractions. If the regions occupied by the
microgel fraction achieve an interconnected structure, a macroscopic gel is
obtained upon defrosting. Thus, Lozinsky considered that cryogels are formed
inside these unfrozen microregions of the frozen system [4–6]. During freezing,
the crystals of frozen solvent act as a porogen and grow until they meet the facets of
other solvent crystals. Upon thawing, the system transforms into a macroporous
cryogel containing large interconnected pores with variable size and geometry
(Fig. 15c). The dimensions and shape of the pores are related to the volume of
the unfrozen liquid microphase, which depends on numerous factors such as the
nature of solvent, initial polymer concentration, the molecular weight of solutes, the
system temperature, and the presence of soluble or insoluble admixtures [4–6].
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
185
The mechanism of formation of the hierarchical structure of PVA cryogels featuring an open porous structure is the result of the occurrence of at least three
concomitant and, at the same time, conflicting processes: crystallization of the
solvent, LL phase separation, and crystallization of PVA. At subzero temperatures,
solutions having the typical concentrations used for preparation of PVA cryogels
are below the spinodal curve shown in Fig. 8 [4–6, 43]. Therefore, during freezing
and successive permanence of the solution at subzero temperatures, a LL phase
separation may occur in addition to the crystallization of solvent and PVA. The
exact mechanism of formation of the macroporous structure (i.e., water crystallization at subzero temperatures or LL phase separation, or the concomitant effect of
both transitions) depends on a number of factors such as the cooling rate at subzero
temperatures, the use of other solvents mixed with water, the presence of additives,
and the PVA concentration [4–6].
Applying the arguments given in Sect. 2, the formation mechanism of PVA
cryogels represents a paradigmatic example of the Ostwald “stage rule” [17, 25,
33]. This means that, upon formation of these gels, a hierarchy of metastability may
arise due to the possible occurrence of several metastable phases via different
processes [17, 25]. Therefore, when a given process is arrested by an agency before
completion, the system is frozen in a phase of circumstantial metastability that,
once formed, has the potential to evolve faster than the transition leading to the
ultimate stable phase and may dominate the whole transformation process [17, 25].
Lozinsky and coworkers [4–6] suggested a general mechanism for cryotropic
gelation, which is illustrated in Fig. 15.
In this model, the initial solution (Fig. 15a), is frozen at temperatures slightly
below the solvent crystallization point, giving rise to an inhomogeneous system that
includes an unfrozen liquid microphase along with crystals of the frozen solvent
(Fig. 15b). Since the polymer (and any other additive) are generally rejected in the
unfrozen liquid microphase, the solute concentration in the unfrozen liquid
microphase is higher than in the initial solution. At this stage, a crosslinking process
(chemical or physical) may easily take place in the unfrozen liquid microphase,
leading to the formation of microgel fractions. If the regions occupied by the
microgel fraction achieve an interconnected structure, a macroscopic gel is
obtained upon defrosting. Thus, Lozinsky considered that cryogels are formed
inside these unfrozen microregions of the frozen system [4–6]. During freezing,
the crystals of frozen solvent act as a porogen and grow until they meet the facets of
other solvent crystals. Upon thawing, the system transforms into a macroporous
cryogel containing large interconnected pores with variable size and geometry
(Fig. 15c). The dimensions and shape of the pores are related to the volume of
the unfrozen liquid microphase, which depends on numerous factors such as the
nature of solvent, initial polymer concentration, the molecular weight of solutes, the
system temperature, and the presence of soluble or insoluble admixtures [4–6].
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
185