carried out because of the very low crystallinity in these systems and the presence of
a broadly diffuse scattering due to the water [1, 49].
Watase and Nishinari [8] examined PVA hydrogels obtained by freeze–thaw
cycles (À20
C for 11 h, followed by 15
C for 7 h), starting from an aqueous
15 wt% PVA solution. PVA specimens used were of five different degrees of
hydrolysis, ranging from 96.0 up to 99.9 mol% hydrolyzed acetate groups. In this
study, the PVA gel samples were immersed in water for 4 days before the analysis.
The authors [8] reported X-ray diffraction patterns for PVA gels in the stretched
and unstretched states. The X-ray diffraction profiles exhibited a halo characteristic
of amorphous materials at 2θ ¼ 26–27
and a weak crystalline peak at about
2θ ¼ 19.5
, corresponding to a d spacing of 4.53 Å. This characteristic crystalline
peak was not observed for gels with a rather low degree of hydrolysis of PVA
(96.0 mol%) stretched at low deformation, but it appeared at draw ratios higher than
five times the initial length. The differential scanning calorimetry (DSC) thermograms of the PVA hydrogels showed an endothermic peak at about 60
C, attributed
to the disentanglements of flexible molecular chains. Another endothermic peak at
higher temperature was found for all samples except for the sample prepared with
PVA at the lowest degree of hydrolysis. This second peak was considered to be
due to the melting of crystalline regions. The authors concluded that repeated
freeze–thaw cycles as well as stretching increase the degree of crystallinity and
that the elasticity is strongly affected by the latter parameter [8]. In a following
study on PVA hydrogels obtained by freeze–thaw cycles, the same authors [50]
pointed out by rheology, DSC, and X-ray analysis how slight differences in the
degree of hydrolysis may significantly change the gel structure. They showed that
the presence of bulky acetate groups can inhibit the formation of PVA gels [50].
The structure and dynamics of PVA hydrogels obtained by freeze–thaw cycles
have been studied by
13 C NMR and
1 H pulse NMR methods [51–54]. Kobayashi
et al. [51, 52] used high-resolution solid-state
13 C NMR experiments to show the
role of intermolecular hydrogen bonds in the formation of PVA hydrogels through
the formation of interchains crosslinks. The hydrogen bonds can be studied by
observing the methine carbon lines in the immobile regions of gels; the lines can be
assigned to carbons involved in two, one, and no hydrogen bonds [55]. The
hydrogen bonds in the crosslinked region in PVA gels have the same NMR
characteristics as those observed in the solid neat PVA. From such evidence, it
was concluded that microcrystallites are formed in PVA gels.
As an example, the X-ray powder diffraction profiles of freeze–thaw PVA
hydrogel samples are reported in Fig. 9 after subtraction of a straight baseline,
approximating the background contribution. These gels were obtained by
subjecting a 11 wt% aqueous solution of PVA to a different number of consecutive
cycles consisting of a freezing step (20 h at À22
C) followed by a thawing step (4 h
at 25
C) [42]. The as-formed PVA hydrogels obtained by one to nine freeze–thaw
cycles are denoted as GEL-1 to GEL-9 samples. For comparison, the diffraction
profile of pure water is also shown in Fig. 9.
176
C. De Rosa et al.
a broadly diffuse scattering due to the water [1, 49].
Watase and Nishinari [8] examined PVA hydrogels obtained by freeze–thaw
cycles (À20
C for 11 h, followed by 15
C for 7 h), starting from an aqueous
15 wt% PVA solution. PVA specimens used were of five different degrees of
hydrolysis, ranging from 96.0 up to 99.9 mol% hydrolyzed acetate groups. In this
study, the PVA gel samples were immersed in water for 4 days before the analysis.
The authors [8] reported X-ray diffraction patterns for PVA gels in the stretched
and unstretched states. The X-ray diffraction profiles exhibited a halo characteristic
of amorphous materials at 2θ ¼ 26–27
and a weak crystalline peak at about
2θ ¼ 19.5
, corresponding to a d spacing of 4.53 Å. This characteristic crystalline
peak was not observed for gels with a rather low degree of hydrolysis of PVA
(96.0 mol%) stretched at low deformation, but it appeared at draw ratios higher than
five times the initial length. The differential scanning calorimetry (DSC) thermograms of the PVA hydrogels showed an endothermic peak at about 60
C, attributed
to the disentanglements of flexible molecular chains. Another endothermic peak at
higher temperature was found for all samples except for the sample prepared with
PVA at the lowest degree of hydrolysis. This second peak was considered to be
due to the melting of crystalline regions. The authors concluded that repeated
freeze–thaw cycles as well as stretching increase the degree of crystallinity and
that the elasticity is strongly affected by the latter parameter [8]. In a following
study on PVA hydrogels obtained by freeze–thaw cycles, the same authors [50]
pointed out by rheology, DSC, and X-ray analysis how slight differences in the
degree of hydrolysis may significantly change the gel structure. They showed that
the presence of bulky acetate groups can inhibit the formation of PVA gels [50].
The structure and dynamics of PVA hydrogels obtained by freeze–thaw cycles
have been studied by
13 C NMR and
1 H pulse NMR methods [51–54]. Kobayashi
et al. [51, 52] used high-resolution solid-state
13 C NMR experiments to show the
role of intermolecular hydrogen bonds in the formation of PVA hydrogels through
the formation of interchains crosslinks. The hydrogen bonds can be studied by
observing the methine carbon lines in the immobile regions of gels; the lines can be
assigned to carbons involved in two, one, and no hydrogen bonds [55]. The
hydrogen bonds in the crosslinked region in PVA gels have the same NMR
characteristics as those observed in the solid neat PVA. From such evidence, it
was concluded that microcrystallites are formed in PVA gels.
As an example, the X-ray powder diffraction profiles of freeze–thaw PVA
hydrogel samples are reported in Fig. 9 after subtraction of a straight baseline,
approximating the background contribution. These gels were obtained by
subjecting a 11 wt% aqueous solution of PVA to a different number of consecutive
cycles consisting of a freezing step (20 h at À22
C) followed by a thawing step (4 h
at 25
C) [42]. The as-formed PVA hydrogels obtained by one to nine freeze–thaw
cycles are denoted as GEL-1 to GEL-9 samples. For comparison, the diffraction
profile of pure water is also shown in Fig. 9.
176
C. De Rosa et al.