uniformity and degree of crystallinity in the higher molecular weight PVA due to an
increase in chain length. This contributes to additional crystallization during swelling and increased mobility because of less physical crosslinking, as indicated by
higher overall volume swelling ratio.
Lozinsky et al. found that gels produced with a lower molecular weight were
more rigid than those of a higher molecular weight, up to a maximum. The unfrozen
liquid microphase (ULMP), specific to cryotropic gelation, is important in
explaining this phenomenon. The viscosity of the ULMP is highest in the system
with the highest molecular weight polymer because the polymers have the longest
chain length. This decreased mobility reduces intermolecular interactions. Therefore, although the rigidity normally increases as polymer molecular weight
increases, a build-up in ULMP viscosity limits this increase [20].
The ability to change the degree of crystallinity by adjusting molecular weight is
an important factor for biomedical applications. Degree of crystallinity has an
effect on the mechanical properties and diffusion properties. For example,
PVA-C can be tuned in this way to achieve mechanical properties that mimic tissue
ranging from cardiovascular tissue to skin [7].
2.2 Solution Concentration
Initial PVA solution concentration was studied as one of the first processing
parameters that can be altered to affect the structure and properties of PVA-C.
Trieu and Qutubuddin showed that many processing parameters have an effect on
the structure and mechanical properties of PVA cryogels, including initial PVA
concentration. They found that a higher initial PVA concentration produces a
structure with less porosity. This, in turn, lowers the equilibrium swelling, resulting
in an inverse relationship between equilibrium swelling and porosity [21].
Hassan and Peppas varied the PVA solution concentration and noted that the
higher concentration solutions resulted in more stable gels that have higher degrees
of crystallinity and lower secondary crystallization. Lower degrees of swelling in
higher concentration solutions indicate that more crosslinking occurs in higher
concentration solutions [19].
Lozinsky et al. demonstrated that an increase in PVA concentration results in an
increase in cryogel rigidity. This is due to the increased concentration of hydroxyl
groups present, creating an increase in intermolecular hydrogen bonding. This
factor was also determined to be more effective in controlling the properties than
the effect of ULMP viscosity previously described. Furthermore, as polymer
concentration is increased, porosity decreases and a more ordered structure
results [20].
An increase in the concentration of PVA has been shown to produce more
crystalline structures with greater stability. This, in turn, causes an increase in the
tensile strength and tear resistance [22]. Wan et al. found that an increase in the
PVA concentration caused the stiffness of PVA-C to increase significantly. With an
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
287
increase in chain length. This contributes to additional crystallization during swelling and increased mobility because of less physical crosslinking, as indicated by
higher overall volume swelling ratio.
Lozinsky et al. found that gels produced with a lower molecular weight were
more rigid than those of a higher molecular weight, up to a maximum. The unfrozen
liquid microphase (ULMP), specific to cryotropic gelation, is important in
explaining this phenomenon. The viscosity of the ULMP is highest in the system
with the highest molecular weight polymer because the polymers have the longest
chain length. This decreased mobility reduces intermolecular interactions. Therefore, although the rigidity normally increases as polymer molecular weight
increases, a build-up in ULMP viscosity limits this increase [20].
The ability to change the degree of crystallinity by adjusting molecular weight is
an important factor for biomedical applications. Degree of crystallinity has an
effect on the mechanical properties and diffusion properties. For example,
PVA-C can be tuned in this way to achieve mechanical properties that mimic tissue
ranging from cardiovascular tissue to skin [7].
2.2 Solution Concentration
Initial PVA solution concentration was studied as one of the first processing
parameters that can be altered to affect the structure and properties of PVA-C.
Trieu and Qutubuddin showed that many processing parameters have an effect on
the structure and mechanical properties of PVA cryogels, including initial PVA
concentration. They found that a higher initial PVA concentration produces a
structure with less porosity. This, in turn, lowers the equilibrium swelling, resulting
in an inverse relationship between equilibrium swelling and porosity [21].
Hassan and Peppas varied the PVA solution concentration and noted that the
higher concentration solutions resulted in more stable gels that have higher degrees
of crystallinity and lower secondary crystallization. Lower degrees of swelling in
higher concentration solutions indicate that more crosslinking occurs in higher
concentration solutions [19].
Lozinsky et al. demonstrated that an increase in PVA concentration results in an
increase in cryogel rigidity. This is due to the increased concentration of hydroxyl
groups present, creating an increase in intermolecular hydrogen bonding. This
factor was also determined to be more effective in controlling the properties than
the effect of ULMP viscosity previously described. Furthermore, as polymer
concentration is increased, porosity decreases and a more ordered structure
results [20].
An increase in the concentration of PVA has been shown to produce more
crystalline structures with greater stability. This, in turn, causes an increase in the
tensile strength and tear resistance [22]. Wan et al. found that an increase in the
PVA concentration caused the stiffness of PVA-C to increase significantly. With an
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
287