Long-chained polymers of sodium alginate can form a network by (a) ionic crosslinks, , or by (b) covalent crosslinks, adipic acid dihydrazide (AAD).
(a) Schematic of a stress-relaxation test. A disk of a gel is immersed in a PBS solvent and placed between two impermeable rigid plates. A strain is suddenly applied to the gel. (b) Subsequently, the strain is held constant, while the stress is recorded as a function of time.
The stress-relaxation behavior of a gel with ionic crosslinks, and of a gel with covalent crosslinks. The two gels have a similar elastic modulus, but very different relaxation times.
Photographs of gels before and after deformation. The gels were subjected to the stress relaxation test. After the test, the load was removed to allow the gels to recover. (a) For a gel with covalent crosslinks, deformation fully recovers after the load is removed. (b) For a gel with ionic crosslinks, part of the deformation is permanent after the load is removed.
The weights of ionic and covalent gel disks before the stress-relaxation test, right after the test, and after storing in PBS for 24 h.
(a) For gels with ionic crosslinks, the stress-relaxation behavior is independent of the radius of the disk. (b) For gels with covalent crosslinks, the relaxation slows down as the radius of the disk increases. The radius of the disk, however, does not affect the levels of the stress at short and long times.
The effect of the radius of the disk on the stress-relaxation behavior of a gel with covalent crosslinks. When the of stress is plotted as a function of , the curves for the disks of different radii collapse into a single curve.
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