polymer concentration resulting in pores [8–10]. The solution is then thawed back
to room temperature leading to the formation of a solid gel—PVA-C. The micro-/
nanostructure of PVA-C has been examined through several techniques including
transmission electron microscopy (TEM) [9], small angle X-ray scattering (SAX)
[9] and small angle and ultrasmall angle neutron scattering (SANS and USANS)
[10, 11]. Observations conclude that the first freeze–thaw cycle (FTC) produces
polymer-rich regions due to the formation of ice crystals in the amorphous regions
[10, 12]. Subsequent FTCs further modify the structure of the polymer matrix.
Formation of the PVA-C takes place during the thawing stages [13]. Crystallization and phase separation are two important mechanisms that contribute to the
structure of PVA-C, with crystallization occurring in the first three FTCs and phase
separation through at least six cycles. Phase separation has a very important impact
on the mechanical properties of PVA, even apart from crystallization [14].
A range of parameters in the processing procedures for PVA cryogels can be
modified to alter the structure and properties of PVA-C and therefore its application. Polymer molecular weight and PVA solution concentration both have significant effects on structure [7, 12, 15–17]. The conditions of the FTCs, such as
freezing and thawing rate, number of FTCs, and upper and lower temperature
limits, all contribute to the determination of polymer matrix structure [7]. This
wide range of processing parameters provides alternatives for tuning the properties
of PVA cryogels. This is extremely beneficial because it makes the material useful
for a large number of applications, and precise adjustments can be made to tailor the
material for a specific application.
Significant work has been reported on how the Young’s modulus of PVA
cryogel is affected by processing parameters [7]. Work by Pazos et al. studied the
nonlinear elastic response of PVA cryogel under uniaxial tension. The authors
found that varying the number of cycles and the thawing rate could have similar
effects on the elastic modulus, but changing the thawing rate gave finer control.
Gels processed under specific conditions were found to mimic the uniaxial elastic
response of healthy porcine coronary arteries [18]. Studies by our group using
SANS were able to show that anisotropic mechanical properties can be achieved for
PVA cryogels through processing the gel under controlled applied stress. This is
extremely beneficial for biomedical devices such as coronary bypass grafts, where
the tissue being replaced possesses orientational-dependent mechanical
properties [10].
2.1 Molecular Weight
Molecular weight has a significant effect on PVA-C formation [19]. As molecular
weight increases, the number and size of the crystalline regions increases due to the
increase in length of the polymer chain. However, this effect is limited by the
decrease in free volume and mobility of the high molecular weight polymers.
Hassan and Peppas found that during swelling there was more instability in crystal
286
W. Wan et al.
to room temperature leading to the formation of a solid gel—PVA-C. The micro-/
nanostructure of PVA-C has been examined through several techniques including
transmission electron microscopy (TEM) [9], small angle X-ray scattering (SAX)
[9] and small angle and ultrasmall angle neutron scattering (SANS and USANS)
[10, 11]. Observations conclude that the first freeze–thaw cycle (FTC) produces
polymer-rich regions due to the formation of ice crystals in the amorphous regions
[10, 12]. Subsequent FTCs further modify the structure of the polymer matrix.
Formation of the PVA-C takes place during the thawing stages [13]. Crystallization and phase separation are two important mechanisms that contribute to the
structure of PVA-C, with crystallization occurring in the first three FTCs and phase
separation through at least six cycles. Phase separation has a very important impact
on the mechanical properties of PVA, even apart from crystallization [14].
A range of parameters in the processing procedures for PVA cryogels can be
modified to alter the structure and properties of PVA-C and therefore its application. Polymer molecular weight and PVA solution concentration both have significant effects on structure [7, 12, 15–17]. The conditions of the FTCs, such as
freezing and thawing rate, number of FTCs, and upper and lower temperature
limits, all contribute to the determination of polymer matrix structure [7]. This
wide range of processing parameters provides alternatives for tuning the properties
of PVA cryogels. This is extremely beneficial because it makes the material useful
for a large number of applications, and precise adjustments can be made to tailor the
material for a specific application.
Significant work has been reported on how the Young’s modulus of PVA
cryogel is affected by processing parameters [7]. Work by Pazos et al. studied the
nonlinear elastic response of PVA cryogel under uniaxial tension. The authors
found that varying the number of cycles and the thawing rate could have similar
effects on the elastic modulus, but changing the thawing rate gave finer control.
Gels processed under specific conditions were found to mimic the uniaxial elastic
response of healthy porcine coronary arteries [18]. Studies by our group using
SANS were able to show that anisotropic mechanical properties can be achieved for
PVA cryogels through processing the gel under controlled applied stress. This is
extremely beneficial for biomedical devices such as coronary bypass grafts, where
the tissue being replaced possesses orientational-dependent mechanical
properties [10].
2.1 Molecular Weight
Molecular weight has a significant effect on PVA-C formation [19]. As molecular
weight increases, the number and size of the crystalline regions increases due to the
increase in length of the polymer chain. However, this effect is limited by the
decrease in free volume and mobility of the high molecular weight polymers.
Hassan and Peppas found that during swelling there was more instability in crystal
286
W. Wan et al.