Rheological characterization of a charged cationic hydrogel network across the gelation boundary

The in situ rheological behavior across the gelation threshold has been investigated for an affine network of a completely charged cationic monomer (3-acrylamidopropyl)-trimethylammonium chloride (APTMAC1) when it is crosslinked with a neutral crosslinker (N,N′-methylenebisacrylamide) to form a fully charged novel cationic hydrogel. The elastic moduli (G′) near the gel point (during the crosslinking or ‘curing’ process) show a power law dependence of the form G′(t)=εz, where ε=((t−tc)/tc) is the distance from the gel point (tc). The critical exponent, z, for the hydrogel series investigated is estimated to be 1.5, slightly lower than the predictions based on percolation theory (z∼1.7–1.9). From the equilibrium (after the curing process) rheological measurements of a series of samples, it is inferred that there is a critical crosslinker mole percent (Xc) with respect to the monomer concentration, required to form a well-defined three-dimensional network with a solid-like behavior. The value of this Xc is found to be between 0.5 and 1%. The theoretically predicted value of Xc using the percolation theory (for the percolation of crosslinks, G0(X)∝[|X−Xc|/Xc]z) and the exponent estimated from the in situ measurements (z=1.5), is Xc∼0.6, which is in good agreement with the experiments. The results may have applicability in translating from bulk systems to the nanoscale in hydrogel design.