Modulating physico-mechanical, swelling, and adsorption properties of fibrous nanoclay embedded anionically-modified semi-IPNs as new promising materials

Semi-IPN nanocomposite cryogels based on anionically modified polyacrylamide and a fibrous nanofiller sepiolite (SEP) have been designed. To achieve a homogeneous filler dispersion within semi-IPN matrix, synthesis is performed via in situ polymerization in the presence of anionic comonomer sodium acrylate (NaA). Interfacial interactions between SEP and semi-IPN matrix have been studied. Effect of incorporation of anionic NaA, linear polymer PEG and SEP on elasticity and swelling was investigated. Uniaxial compression tests were performed to measure effect of SEP content and polymerization temperature on elastic modulus. In comparison with SEP-free semi-IPNs, mechanical properties of nanocomposites have been improved with addition of SEP. In particular, for semi-IPN nanocomposite cryogels with 5.0% (w/v) of SEP, a 17.8-fold increase in compressive modulus was observed compared to semi-IPN without SEP. Real effective crosslinking density of semi-IPNs presented by a cubic polynomial as a function of SEP. While swelling decreased with an increase in SEP, it increased with an increase in polymerization temperature. Swelling of semi-IPNs was investigated in twelve different sets of solvent with different solubility parameters and chloroform/n-hexane was found as best solvent combination. Semi-IPN gels exhibited high sensitivity to pH and on-off switching behavior as reversible swelling and de-swelling was recorded at pH 2.1–11.2. Dynamic swelling was analyzed to evaluate diffusion mechanisms of water. Diffusional exponent n < 0.5 found to predict sorption mechanism of water at pH 11.2 indicated that swelling was mainly diffusion controlled. Saline sensitivity of semi-IPNs was investigated to understand effect of counterion on equilibrium swelling. Increasing salinity concentration led to a significant decline in swelling and response to counterions is reverse of classical Hofmeister series. In order of decreasing expansion ratio, anions followed order of l⁻ ≈ NO³⁻ > Br⁻ > CI⁻. Both methylene blue and SEP caused important modifications on microstructure of semi-IPN matrix, leading to changes on adsorption and diffusional behavior. Ho's pseudo second order model best described kinetics of adsorption process which renders nanocomposites promising for efficient removal of cationic dye, Methylene Blue.