2.10. Gel Fraction and Crosslinking Density

The hydrogels were obtained after three-step washing (as described in Section 2.3) to remove residual monomers. The dried hydrogels were exposed to distilled water and left to swell until the system reached maximum equilibrium state. The maximally swollen hydrogels were initially weighed and left uncovered at room temperature for 3 days. The samples were further dried in an oven for 3 h under 70 °C before measuring the final weight. The solution fraction and the gel fraction of the hydrogels were calculated by their insoluble part after extraction as follows [83];

where GF represents gel fraction, Mf is the weight of the dried sample, and Mi represents the weight of the original hydrogel before drying.

The equilibrium volume fraction of the hydrogels were calculated using the following equation [84];

where Vp is equilibrium volume fraction and Q represents the swelling ratio.

Crosslinking density of the hydrogels was calculated by the Flory–Rehner equation, which is based on equilibrium swelling of three-dimensional polymers. The Flory–Rehner equation can be described as follows [84,85];

where v is crosslinking density, Vp is equilibrium volume fraction at swelling state, and X represents the solvent interaction Flory parameter.