Young’s moduli were measured at 5 and 20 % strain. They found that the Young’s
modulus increased with an increase in strain, PVA-C concentration, and number of
FTCs. The Young’s modulus measured was between 0.001 and 2.117 MPa at all
combinations of concentration, FTCs, and strain level [54].
Duboeuf et al. reported unconfined compression tests on 10 % PVA-C samples
through 2–5 FTCs using a strain rate of 5 mm/min (~0.45 %/s) with strain up to 8 %
at room temperature (22 Æ 2
C). They found that the stress–strain curves were
linear and that the Young’s modulus increased with the number of FTCs. The
Young’s modulus, measured between 3 and 8 % strain, was found to range from
0.065 to 0.167 MPa from two to five cycles. The authors also measured the stability
of the Young’s modulus over a 7-month period. For cycles 2 and 4, the authors
found that the Young’s modulus did not change significantly. However for cycles
3 and 5, there was a slight increase [55].
Nishinari, Watase, and coworkers studied the effect of degree of polymerization
(DP) on the strength of PVA-C. They performed dynamic mechanical analysis on
PVA-C samples of seven DPs (ranging from 470 to 17,900 of cycle 1) and of
different PVA concentrations. The storage Young’s modulus E
0 and the mechanical
loss of the samples were measured using a 2-Hz excitation strain and at a temperature range of 2–85
C at a rate of 2
C/min. The authors found that the storage
Young’s modulus E
0 increased with concentration, and the same E
0 value could be
obtained at lower concentration by increasing the DP [56]. In addition to the results
outlined above, there were several other related studies that reported similar results
[20, 57].
Based on the findings of the above papers, all concluded that the unconfined
compressive Young’s modulus of PVA-C is dependent on the strain, strain rate, DP
of PVA, concentration of PVA solution, and the number of FTCs. The Young’s
modulus reported falls in the range 2–20 MPa, depending on the chosen composition, processing parameters, and the testing conditions used.
The compressive strength of PVA-C can be greatly enhanced by pre- and postgelation processes such as solvent dehydration and thermal annealing; however,
these treatments will significantly reduce the water content of the PVA-C and its
lubricating properties, which is undesirable for cartilage and other orthopedic
applications. Bodugoz-Senturk et al. introduced a method to counteract this effect
by adding poly(ethylene glycol) (PEG) and poly(acrylamide) (PAAm) to PVA,
creating PVA-PEG theta gels and PVA-AAm hydrogels [58]. Unfortunately,
instead of reporting improvements in compressive strength, they reported improvements in creep resistance.
3.1.2 Stress Relaxation and Creep
Stress relaxation characterizes how viscoelastic materials relieve stresses over time
under a constant strain. Creep characterizes how viscoelastic materials deform
slowly over time under constant stress. Both stress relaxation and creep are typically measured using the normalized value (stress or strain) versus time curve after
stress or strain is applied at time zero.
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
295
modulus increased with an increase in strain, PVA-C concentration, and number of
FTCs. The Young’s modulus measured was between 0.001 and 2.117 MPa at all
combinations of concentration, FTCs, and strain level [54].
Duboeuf et al. reported unconfined compression tests on 10 % PVA-C samples
through 2–5 FTCs using a strain rate of 5 mm/min (~0.45 %/s) with strain up to 8 %
at room temperature (22 Æ 2
C). They found that the stress–strain curves were
linear and that the Young’s modulus increased with the number of FTCs. The
Young’s modulus, measured between 3 and 8 % strain, was found to range from
0.065 to 0.167 MPa from two to five cycles. The authors also measured the stability
of the Young’s modulus over a 7-month period. For cycles 2 and 4, the authors
found that the Young’s modulus did not change significantly. However for cycles
3 and 5, there was a slight increase [55].
Nishinari, Watase, and coworkers studied the effect of degree of polymerization
(DP) on the strength of PVA-C. They performed dynamic mechanical analysis on
PVA-C samples of seven DPs (ranging from 470 to 17,900 of cycle 1) and of
different PVA concentrations. The storage Young’s modulus E
0 and the mechanical
loss of the samples were measured using a 2-Hz excitation strain and at a temperature range of 2–85
C at a rate of 2
C/min. The authors found that the storage
Young’s modulus E
0 increased with concentration, and the same E
0 value could be
obtained at lower concentration by increasing the DP [56]. In addition to the results
outlined above, there were several other related studies that reported similar results
[20, 57].
Based on the findings of the above papers, all concluded that the unconfined
compressive Young’s modulus of PVA-C is dependent on the strain, strain rate, DP
of PVA, concentration of PVA solution, and the number of FTCs. The Young’s
modulus reported falls in the range 2–20 MPa, depending on the chosen composition, processing parameters, and the testing conditions used.
The compressive strength of PVA-C can be greatly enhanced by pre- and postgelation processes such as solvent dehydration and thermal annealing; however,
these treatments will significantly reduce the water content of the PVA-C and its
lubricating properties, which is undesirable for cartilage and other orthopedic
applications. Bodugoz-Senturk et al. introduced a method to counteract this effect
by adding poly(ethylene glycol) (PEG) and poly(acrylamide) (PAAm) to PVA,
creating PVA-PEG theta gels and PVA-AAm hydrogels [58]. Unfortunately,
instead of reporting improvements in compressive strength, they reported improvements in creep resistance.
3.1.2 Stress Relaxation and Creep
Stress relaxation characterizes how viscoelastic materials relieve stresses over time
under a constant strain. Creep characterizes how viscoelastic materials deform
slowly over time under constant stress. Both stress relaxation and creep are typically measured using the normalized value (stress or strain) versus time curve after
stress or strain is applied at time zero.
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
295