3 Poly(Vinyl Alcohol) Hydrogels
PVA is one of the most studied gel-forming polymers and is able to give rise to a
large variety of gels of widespread interest, using several kinds of solvents and
under a large variety of preparation/processing conditions [1]. Confining our
attention to PVA hydrogels, it has been shown that aqueous solutions of PVA can
form gels under several conditions and that the properties of the resultant systems
are largely dependent on the structure fixed by the preparative route.
In this section, after a brief discussion of the principal categories of PVA
physical hydrogels in general, the PVA hydrogels obtained by cryogenic treatments
are treated in detail.
3.1 Crystal Structure of PVA
PVA is an atactic polymer obtained by free radical polymerization of vinyl acetate
and successive hydrolysis. The hydrolysis of poly(vinyl acetate) (PVAc) does not
convert all the acetate groups into hydroxyl groups, but gives rise to PVA polymers
with a partial degree of hydrolysis that depends on the extent of the reaction.
The properties of PVA are strongly influenced by the degree of hydrolysis
[39]. For instance, the water solubility of PVA depends on the degree of hydrolysis
and the molecular weight [12]. The higher the degree of hydrolysis of PVA grades,
the lower the corresponding PVA solubility. Residual acetate groups in partially
hydrolyzed PVA improve water solubility by disrupting polymer–polymer interand intrachain hydrogen bonding between hydroxyl groups and, consequently,
promote polymer–solvent interactions [12]. In highly hydrolyzed PVA grades, the
water solubility decreases due to unhindered formation of polymer–polymer
interchain and intrachain hydrogen bonds between the pendant hydroxyl groups,
so that establishment of effective interactions of PVA chains with the solvent are
prevented.
In spite of the lack of stereoregularity, atactic PVA is a semicrystalline polymer
[40, 41]. The crystal structure of PVA was resolved in 1948 by Bunn [41]. It is
characterized by chains in a trans-planar conformation, packed in a monoclinic unit
cell with a ¼ 7.81 Å, b ¼ 2.52 Å (chain axis), c ¼ 5.51 Å, and β ¼ 91.7
(see
Fig. 6).
The arrangement of chains inside the crystals can be described as consisting of
double layers of chains running parallel to the bc-plane, and stacked along the aaxis. These double layers are defined by the hydrogen bonds established between
the hydroxyl groups belonging to first adjacent chains facing along a. Consecutive
double layers establish only weak van der Waals interactions. The degree of
stereoregularity of PVA significantly affects the crystallizability, and atactic PVA
is more easily crystallized than the isotactic and syndiotactic counterparts [1, 12].
172
C. De Rosa et al.
PVA is one of the most studied gel-forming polymers and is able to give rise to a
large variety of gels of widespread interest, using several kinds of solvents and
under a large variety of preparation/processing conditions [1]. Confining our
attention to PVA hydrogels, it has been shown that aqueous solutions of PVA can
form gels under several conditions and that the properties of the resultant systems
are largely dependent on the structure fixed by the preparative route.
In this section, after a brief discussion of the principal categories of PVA
physical hydrogels in general, the PVA hydrogels obtained by cryogenic treatments
are treated in detail.
3.1 Crystal Structure of PVA
PVA is an atactic polymer obtained by free radical polymerization of vinyl acetate
and successive hydrolysis. The hydrolysis of poly(vinyl acetate) (PVAc) does not
convert all the acetate groups into hydroxyl groups, but gives rise to PVA polymers
with a partial degree of hydrolysis that depends on the extent of the reaction.
The properties of PVA are strongly influenced by the degree of hydrolysis
[39]. For instance, the water solubility of PVA depends on the degree of hydrolysis
and the molecular weight [12]. The higher the degree of hydrolysis of PVA grades,
the lower the corresponding PVA solubility. Residual acetate groups in partially
hydrolyzed PVA improve water solubility by disrupting polymer–polymer interand intrachain hydrogen bonding between hydroxyl groups and, consequently,
promote polymer–solvent interactions [12]. In highly hydrolyzed PVA grades, the
water solubility decreases due to unhindered formation of polymer–polymer
interchain and intrachain hydrogen bonds between the pendant hydroxyl groups,
so that establishment of effective interactions of PVA chains with the solvent are
prevented.
In spite of the lack of stereoregularity, atactic PVA is a semicrystalline polymer
[40, 41]. The crystal structure of PVA was resolved in 1948 by Bunn [41]. It is
characterized by chains in a trans-planar conformation, packed in a monoclinic unit
cell with a ¼ 7.81 Å, b ¼ 2.52 Å (chain axis), c ¼ 5.51 Å, and β ¼ 91.7
(see
Fig. 6).
The arrangement of chains inside the crystals can be described as consisting of
double layers of chains running parallel to the bc-plane, and stacked along the aaxis. These double layers are defined by the hydrogen bonds established between
the hydroxyl groups belonging to first adjacent chains facing along a. Consecutive
double layers establish only weak van der Waals interactions. The degree of
stereoregularity of PVA significantly affects the crystallizability, and atactic PVA
is more easily crystallized than the isotactic and syndiotactic counterparts [1, 12].
172
C. De Rosa et al.