explanation is the reinforcement of additional existing crystals within the structure
with each additional FTC, up to a maximum level [19, 34].
2.4.3 Freeze Holding Time
The freezing holding time has a significant effect, with samples frozen up to 10 days
at À10
C giving the most mechanically strong PVA hydrogels [34]. On the other
hand, holding the sample at a lower temperature for varying amounts of time did not
seem to have an effect on the mechanical properties. Wan et al. showed that holding
times of 1 or 6 h at À20
C did not cause any change in the tensile properties of the
PVA hydrogel [7]. Nevertheless the lower temperature limit, at which the hydrogel
freezes, has an effect on the phase equilibrium of PVA, with storage of frozen
solutions at higher negative temperatures resulting in PVA-C that is to some extent
more rigid [21].
3 Properties of PVA and PVA Composites
3.1 Mechanical Properties
PVA cryogel (PVA-C) has caught the interest of researchers in the biomedical field
since its creation in the early 1980s [43]. Apart from its long-term biocompatibility
and nontoxicity [44], its mechanical properties, which can be tailored to mimic a
wide range of soft tissues [7, 45], are the main reason why PVA-C is an attractive
candidate material for many prosthetic devices such as heart valves, blood vessels,
and articular cartilages.
In terms of mechanical strength (compressive or tensile), PVA-C can be isotropic or anisotropic [10]. Its Young’s modulus is nonlinear and dependent on strain,
strain rate, and temperature [46]. It is viscoelastic and strongly hydrophilic. Its
strength is a function of concentration of the PVA solution, the mean molecular
weight of the polymer material, the number of FTCs it has gone through during its
formation, post-hydration processing, the type of buffer solution, and the solution
temperature.
To understand the mechanical properties of PVA-C, one has to understand the
microstructure of the cryogel, which is a direct result of its formation process.
Although the gelation mechanism is still under discussion, it is thought to be a
combination of mechanisms involving hydrogen bonding [47], crystallite formation, and liquid–liquid phase separation through spinodal decomposition [48].
Willcox et al. proposed a gelation scheme in support of the above hypothesis.
When a PVA solution is subjected to a number of FTCs, during the first freezing
cycle, ice crystallizations cause the remaining polymer solution to concentrate,
bringing the molecular chains closer together. This promotes PVA crystallite
290
W. Wan et al.
with each additional FTC, up to a maximum level [19, 34].
2.4.3 Freeze Holding Time
The freezing holding time has a significant effect, with samples frozen up to 10 days
at À10
C giving the most mechanically strong PVA hydrogels [34]. On the other
hand, holding the sample at a lower temperature for varying amounts of time did not
seem to have an effect on the mechanical properties. Wan et al. showed that holding
times of 1 or 6 h at À20
C did not cause any change in the tensile properties of the
PVA hydrogel [7]. Nevertheless the lower temperature limit, at which the hydrogel
freezes, has an effect on the phase equilibrium of PVA, with storage of frozen
solutions at higher negative temperatures resulting in PVA-C that is to some extent
more rigid [21].
3 Properties of PVA and PVA Composites
3.1 Mechanical Properties
PVA cryogel (PVA-C) has caught the interest of researchers in the biomedical field
since its creation in the early 1980s [43]. Apart from its long-term biocompatibility
and nontoxicity [44], its mechanical properties, which can be tailored to mimic a
wide range of soft tissues [7, 45], are the main reason why PVA-C is an attractive
candidate material for many prosthetic devices such as heart valves, blood vessels,
and articular cartilages.
In terms of mechanical strength (compressive or tensile), PVA-C can be isotropic or anisotropic [10]. Its Young’s modulus is nonlinear and dependent on strain,
strain rate, and temperature [46]. It is viscoelastic and strongly hydrophilic. Its
strength is a function of concentration of the PVA solution, the mean molecular
weight of the polymer material, the number of FTCs it has gone through during its
formation, post-hydration processing, the type of buffer solution, and the solution
temperature.
To understand the mechanical properties of PVA-C, one has to understand the
microstructure of the cryogel, which is a direct result of its formation process.
Although the gelation mechanism is still under discussion, it is thought to be a
combination of mechanisms involving hydrogen bonding [47], crystallite formation, and liquid–liquid phase separation through spinodal decomposition [48].
Willcox et al. proposed a gelation scheme in support of the above hypothesis.
When a PVA solution is subjected to a number of FTCs, during the first freezing
cycle, ice crystallizations cause the remaining polymer solution to concentrate,
bringing the molecular chains closer together. This promotes PVA crystallite
290
W. Wan et al.