1 Introduction
In this chapter, we deal with kinetic and mechanistic aspects associated with the
complex cryotropic gelation of semidilute aqueous solutions of poly(vinyl alcohol)
(PVA). The possibility of obtaining strong physical gels by cryogenic treatments of
PVA/water solutions has been well known since the 1970s [1–6]. The cryogenic
treatment basically consists of freezing an initially homogeneous polymer solution
at low temperatures, storing in the frozen state for a definite time, and defrosting.
Freezing is generally performed at high cooling rates and, in order to improve the
strength of the gels, freezing and thawing are repeated a number of times. The
strength of PVA cryogels prepared by means of freeze–thaw cycles depends on the
initial polymer concentration in the solution to be frozen, the thawing rate, the
permanence time of the solution at low temperatures, and the number of imposed
freeze–thaw cycles. Cryogel strength is virtually independent of the temperature of
freezing and the cooling rate of the initial solution to that temperature [4, 5,
7]. Strong physical gels can be obtained even by imposing a single freeze–thaw
cycle, either with use of slow thawing rates or by the prolonged storage of the
samples at subzero temperatures [3].
The large interest in PVA cryotropic hydrogels is due to a number of unique
properties, including high dimensional stability, large deformability, maintenance
of high conservative elastic modulus even after immersion in water for a long time
[8, 9], and self-healing performance [10]. In addition, the well-tested biocompatibility of freeze–thaw PVA hydrogels and their ability to incorporate and release
large amounts of host molecules of different size in their structure make these
systems particularly attractive for biomedical and biotechnological applications [1,
5, 11].
The outstanding physical properties of freeze–thaw PVA hydrogels derive from
their complex structure, whereby PVA chains and solvent molecules are organized
at different hierarchical length scales. These gels exhibit a structure that includes an
interconnected network of micro- and macropores filled by a polymer-poor phase.
The network scaffolding is ensured by interconnected regions of a polymer-rich
phase [1]. The latter phase is itself organized and consists of small fringed micellelike crystalline aggregates of PVA chains and amorphous domains. The PVA
chains in the amorphous domains are swollen by the solvent and act as tie chains
that connect the PVA crystallites.
Section 2 is aimed at recalling the general aspects of polymer gels. These
concepts turn out to be useful in Sect. 3 for understanding the mechanism of
formation, the structure, and the properties of PVA hydrogels generated by physical
crosslinking processes, first in general terms, then focusing on freeze–thaw PVA
hydrogels in particular. The structure of freeze–thaw PVA hydrogels is discussed in
detail and possible mechanisms leading to their formations are illustrated, emphasizing some aspects that have been investigated using time-resolving analysis with
small angle neutron scattering (TR-SANS). TR-SANS is used to follow the structural transformations occurring during consecutive freeze–thaw cycles, with the
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
161
In this chapter, we deal with kinetic and mechanistic aspects associated with the
complex cryotropic gelation of semidilute aqueous solutions of poly(vinyl alcohol)
(PVA). The possibility of obtaining strong physical gels by cryogenic treatments of
PVA/water solutions has been well known since the 1970s [1–6]. The cryogenic
treatment basically consists of freezing an initially homogeneous polymer solution
at low temperatures, storing in the frozen state for a definite time, and defrosting.
Freezing is generally performed at high cooling rates and, in order to improve the
strength of the gels, freezing and thawing are repeated a number of times. The
strength of PVA cryogels prepared by means of freeze–thaw cycles depends on the
initial polymer concentration in the solution to be frozen, the thawing rate, the
permanence time of the solution at low temperatures, and the number of imposed
freeze–thaw cycles. Cryogel strength is virtually independent of the temperature of
freezing and the cooling rate of the initial solution to that temperature [4, 5,
7]. Strong physical gels can be obtained even by imposing a single freeze–thaw
cycle, either with use of slow thawing rates or by the prolonged storage of the
samples at subzero temperatures [3].
The large interest in PVA cryotropic hydrogels is due to a number of unique
properties, including high dimensional stability, large deformability, maintenance
of high conservative elastic modulus even after immersion in water for a long time
[8, 9], and self-healing performance [10]. In addition, the well-tested biocompatibility of freeze–thaw PVA hydrogels and their ability to incorporate and release
large amounts of host molecules of different size in their structure make these
systems particularly attractive for biomedical and biotechnological applications [1,
5, 11].
The outstanding physical properties of freeze–thaw PVA hydrogels derive from
their complex structure, whereby PVA chains and solvent molecules are organized
at different hierarchical length scales. These gels exhibit a structure that includes an
interconnected network of micro- and macropores filled by a polymer-poor phase.
The network scaffolding is ensured by interconnected regions of a polymer-rich
phase [1]. The latter phase is itself organized and consists of small fringed micellelike crystalline aggregates of PVA chains and amorphous domains. The PVA
chains in the amorphous domains are swollen by the solvent and act as tie chains
that connect the PVA crystallites.
Section 2 is aimed at recalling the general aspects of polymer gels. These
concepts turn out to be useful in Sect. 3 for understanding the mechanism of
formation, the structure, and the properties of PVA hydrogels generated by physical
crosslinking processes, first in general terms, then focusing on freeze–thaw PVA
hydrogels in particular. The structure of freeze–thaw PVA hydrogels is discussed in
detail and possible mechanisms leading to their formations are illustrated, emphasizing some aspects that have been investigated using time-resolving analysis with
small angle neutron scattering (TR-SANS). TR-SANS is used to follow the structural transformations occurring during consecutive freeze–thaw cycles, with the
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
161