1.3 Properties and Structural Characterization of Hydrogels
1.3.2 Mechanical Properties
The mechanical properties of hydrogels were characterized using a universal testing machine (MTS CMT8502). The tests were conducted at room temperature with a constant rate of 50 mm/min. First, uniaxial tensile tests to failure were performed on samples with crosslinking densities of 1.32×10^-4, 2.63×10^-4, 3.95×10^-4, 5.27×10^-4, 6.59×10^-4, 9.22×10^-4, 1.05×10^-3, 1.98×10^-3, 2.63×10^-3, and 3.29×10^-3. Stress-strain curves were recorded to obtain the elastic modulus, tensile strength, elongation at break, and toughness (area under the stress-strain curve) to screen for the optimal formulation. Based on this optimal formulation, uniaxial tensile tests to failure were conducted on samples P-0.1, P-0.5, P-1, Y-0.1, Y-0.5, Y-1, Sr-0.1, Sr-0.5, Sr-1, I-0.1, I-0.5, and I-1 to study the effects of added inorganic salts on the elastic modulus, tensile strength, and elongation at break. Subsequently, load-unload cyclic tests were performed on samples P-1, Y-1, Sr-1, and I-1. The samples were stretched to a preset strain of 100% and then unloaded at the same rate, with 100 cycles to evaluate their fatigue resistance.
1.3.3 Swelling Kinetics
Hydrogel samples with a crosslinking density of 3.95×10^-4 (n=3) were frozen in liquid nitrogen for 10 minutes and then immediately placed in a freeze dryer for 12 hours. The mass of the freeze-dried samples was measured, and they were subsequently immersed in ultrapure water. At predetermined time points (0, 2, 4, 12, 24, 36, 48, 60 hours), the samples were removed, and surface moisture was gently blotted off with filter paper before immediate reweighing. When the mass of the sample showed no significant change over three consecutive measurements, it was considered to have reached swelling equilibrium. The swelling ratio (R) was calculated using the following formula:
