higher than 2π/q min (%300 nm) [24, 37, 78], with q min ¼ 0.02 nm
À1 being the
lowest value of the scattering vector q that was achieved in the experimental set-up.
In this second hypothesis, the scattering intensity is expected to show a peak at
q < q min , but this low q region was not sampled [24, 37].
During thawing, the scattering cross-section decreases (Fig. 16b). This indicates
that, upon defrosting, the frozen solution evolves towards a system characterized by
alternation of dense and diluted regions that have origin in the regions occupied at
subzero temperatures by ice and the unfrozen liquid microphase, respectively
[77]. For PVA concentrations higher than a critical value, dense and diluted regions
are likely to form an interpenetrated network of two co-continuous phases, resulting
in a transparent gel in the nascent state (curve c of Fig. 16b) where the characteristic
size of the phase-separated domains is of the order of few tens of nanometers [77].
The nascent gels become opaque upon aging at room temperature for few hours
due to the increase in size of the regions that are alternatively dense and diluted
[77]. The coarsening of the heterogeneous structure imprinted by the cryotropic
treatment is driven by the tendency of the dense and dilute regions to minimize their
surface of contact, and is the hallmark that these gels are in a state far from
equilibrium. In fact, the fast cryogenic treatment causes the formation of transparent gels having the same composition as the initial homogeneous solution. On the
basis of the phase diagram by Komatsu et al. [43] (Fig. 8), solutions containing 10–
12 wt% PVA (corresponding to PVA volume fraction Φ in the range 0.086–0.12)
are in the one-phase region and should not give rise to a gel at room temperature.
However, the nascent gels obtained from these solutions by cryotropic treatment at
À13
C do not transform back into the initial homogeneous solution once they are
brought back to room temperature because a strong network scaffolding has already
been formed due to the presence of PVA crystallites. These gels are stable up to 50–
60
C [57] and can be aged for long time in sealed vials, maintaining their
properties. Moreover, the nascent gels obtained after a single freeze–thaw cycle
are already too strong to evolve towards complete elimination of the solvent
[24]. Nascent gels, instead, react via microsyneresis and the size of the dense and
diluted regions increases [24]. The coarsening of the heterogeneous structure of the
nascent and transparent gels, up to becoming opaque, also suggests that spinodal
decomposition occurs [79, 80]. This phenomenon has indeed been observed for
other gels [79, 80]. It generally occurs when a swollen gel is suddenly brought into
another state that may be located either in the two-phase or one-phase region of its
phase diagram [80]. Regardless of the region of the phase diagram in which the new
state is located, it has been shown that the system becomes opaque without any
appreciable volume change, suggesting occurrence of spinodal decomposition in
both cases. However, in contrast to the usual fluids, the domain growth is slow
because the elastic force of the gel suppresses the surface tension force, which is the
driving force of domain growth [79, 80].
188
C. De Rosa et al.
À1 being the
lowest value of the scattering vector q that was achieved in the experimental set-up.
In this second hypothesis, the scattering intensity is expected to show a peak at
q < q min , but this low q region was not sampled [24, 37].
During thawing, the scattering cross-section decreases (Fig. 16b). This indicates
that, upon defrosting, the frozen solution evolves towards a system characterized by
alternation of dense and diluted regions that have origin in the regions occupied at
subzero temperatures by ice and the unfrozen liquid microphase, respectively
[77]. For PVA concentrations higher than a critical value, dense and diluted regions
are likely to form an interpenetrated network of two co-continuous phases, resulting
in a transparent gel in the nascent state (curve c of Fig. 16b) where the characteristic
size of the phase-separated domains is of the order of few tens of nanometers [77].
The nascent gels become opaque upon aging at room temperature for few hours
due to the increase in size of the regions that are alternatively dense and diluted
[77]. The coarsening of the heterogeneous structure imprinted by the cryotropic
treatment is driven by the tendency of the dense and dilute regions to minimize their
surface of contact, and is the hallmark that these gels are in a state far from
equilibrium. In fact, the fast cryogenic treatment causes the formation of transparent gels having the same composition as the initial homogeneous solution. On the
basis of the phase diagram by Komatsu et al. [43] (Fig. 8), solutions containing 10–
12 wt% PVA (corresponding to PVA volume fraction Φ in the range 0.086–0.12)
are in the one-phase region and should not give rise to a gel at room temperature.
However, the nascent gels obtained from these solutions by cryotropic treatment at
À13
C do not transform back into the initial homogeneous solution once they are
brought back to room temperature because a strong network scaffolding has already
been formed due to the presence of PVA crystallites. These gels are stable up to 50–
60
C [57] and can be aged for long time in sealed vials, maintaining their
properties. Moreover, the nascent gels obtained after a single freeze–thaw cycle
are already too strong to evolve towards complete elimination of the solvent
[24]. Nascent gels, instead, react via microsyneresis and the size of the dense and
diluted regions increases [24]. The coarsening of the heterogeneous structure of the
nascent and transparent gels, up to becoming opaque, also suggests that spinodal
decomposition occurs [79, 80]. This phenomenon has indeed been observed for
other gels [79, 80]. It generally occurs when a swollen gel is suddenly brought into
another state that may be located either in the two-phase or one-phase region of its
phase diagram [80]. Regardless of the region of the phase diagram in which the new
state is located, it has been shown that the system becomes opaque without any
appreciable volume change, suggesting occurrence of spinodal decomposition in
both cases. However, in contrast to the usual fluids, the domain growth is slow
because the elastic force of the gel suppresses the surface tension force, which is the
driving force of domain growth [79, 80].
188
C. De Rosa et al.