3.2 PVA Physical Hydrogels
The formation of physical gels from water solutions of PVA has been widely
studied. In fact, it is well known that aqueous PVA solutions undergo gelation
upon cooling. In these gels, the physical cross-links are generally fringed micellelike PVA crystallites. In particular, Komatsu et al. [43] found that the phase
diagram of this system shows an upper critical solution temperature (UCST), and
that the spinodal curve crosses the sol–gel transition curve. This phase diagram is
redrawn in Fig. 8.
It is apparent that gelation takes place both below and above the spinodal curve,
indicating that gels may be formed either accompanied by spinodal decomposition
or without liquid–liquid phase separation, respectively [43]. In both cases, crystalline gels are formed where small crystals of PVA in the monoclinic form [40, 41]
act as junctions [43, 44]. Statistical gels having a homogeneous structure are formed
in the temperature–concentration region above the spinodal; however, the supramolecular structure of the hydrogels that develop in the temperature–concentration
region below the spinodal is considerably different and more heterogeneous than
the structure of the statistical gels [17, 24, 25, 43]. As discussed in Sect. 2, the
development of an interconnected structure in the polymer-rich phase formed by
spinodal decomposition is not necessarily the origin of gelation [17, 25]. In fact, in
order to have a gel some agency should come into play to arrest PVA crystallization
and/or LL phase separation at an intermediate stage, creating circumstantial metastability [25]. In the case of PVA, the network structure may be easily created by
occurrence of LL phase separation, which arrests crystallization, and/or vitrification
that arrests both crystallization and LL phase separation (Fig. 5b). Since the glass
transition of dry PVA is equal to %85
C and is lowered to %37
C by the presence
of %10 wt% water, LL phase separation of aqueous solutions of PVA are likely to
occur below the Berghmans point [17, 25]. This means that PVA hydrogels formed
below the spinodal may correspond to metastable frozen-in configurations affected
by vitrification. Whatever the exact mechanism of gelation of PVA, the physical
junctions of the macroscopic network are not only PVA crystallites, but also
entanglements and hydrogen bonds.
Formation of physical gels from aqueous PVA solutions may also occur by
impairing the thermodynamic quality of the solvent as a result of the introduction of
nonsolvent additives [4, 5, 45]. However, in all cases, the so-obtained thermoreversible PVA gels possess low melting temperatures (T m < 30–40
C), low
mechanical strength, and they are not able to retain their size and dimensions for
a long time.
Attempts to make stronger physical hydrogels of PVA include induction of
crystallization through dehydration at a slow drying rate [1, 46] and/or fast drying
rate procedures such as annealing, under different conditions [1, 47]. The presence
of crystals in physical hydrogels of PVA generally results in better mechanical
properties than in PVA gels prepared through chemical or radiation-induced
techniques.
174
C. De Rosa et al.
The formation of physical gels from water solutions of PVA has been widely
studied. In fact, it is well known that aqueous PVA solutions undergo gelation
upon cooling. In these gels, the physical cross-links are generally fringed micellelike PVA crystallites. In particular, Komatsu et al. [43] found that the phase
diagram of this system shows an upper critical solution temperature (UCST), and
that the spinodal curve crosses the sol–gel transition curve. This phase diagram is
redrawn in Fig. 8.
It is apparent that gelation takes place both below and above the spinodal curve,
indicating that gels may be formed either accompanied by spinodal decomposition
or without liquid–liquid phase separation, respectively [43]. In both cases, crystalline gels are formed where small crystals of PVA in the monoclinic form [40, 41]
act as junctions [43, 44]. Statistical gels having a homogeneous structure are formed
in the temperature–concentration region above the spinodal; however, the supramolecular structure of the hydrogels that develop in the temperature–concentration
region below the spinodal is considerably different and more heterogeneous than
the structure of the statistical gels [17, 24, 25, 43]. As discussed in Sect. 2, the
development of an interconnected structure in the polymer-rich phase formed by
spinodal decomposition is not necessarily the origin of gelation [17, 25]. In fact, in
order to have a gel some agency should come into play to arrest PVA crystallization
and/or LL phase separation at an intermediate stage, creating circumstantial metastability [25]. In the case of PVA, the network structure may be easily created by
occurrence of LL phase separation, which arrests crystallization, and/or vitrification
that arrests both crystallization and LL phase separation (Fig. 5b). Since the glass
transition of dry PVA is equal to %85
C and is lowered to %37
C by the presence
of %10 wt% water, LL phase separation of aqueous solutions of PVA are likely to
occur below the Berghmans point [17, 25]. This means that PVA hydrogels formed
below the spinodal may correspond to metastable frozen-in configurations affected
by vitrification. Whatever the exact mechanism of gelation of PVA, the physical
junctions of the macroscopic network are not only PVA crystallites, but also
entanglements and hydrogen bonds.
Formation of physical gels from aqueous PVA solutions may also occur by
impairing the thermodynamic quality of the solvent as a result of the introduction of
nonsolvent additives [4, 5, 45]. However, in all cases, the so-obtained thermoreversible PVA gels possess low melting temperatures (T m < 30–40
C), low
mechanical strength, and they are not able to retain their size and dimensions for
a long time.
Attempts to make stronger physical hydrogels of PVA include induction of
crystallization through dehydration at a slow drying rate [1, 46] and/or fast drying
rate procedures such as annealing, under different conditions [1, 47]. The presence
of crystals in physical hydrogels of PVA generally results in better mechanical
properties than in PVA gels prepared through chemical or radiation-induced
techniques.
174
C. De Rosa et al.