Mechanistic insight into impact of clay on compressive elasticity of poly(N-alkyl acrylamide)/Laponite-based nanocomposite cryo-beads

Abstract

Relationship between formation pathway and Hertzian elasticity of poly(N-isopropylacrylamide-co‑sodium acrylate)/Laponite® XLG, P(NIPA-NaA)/LAP, based nanocomposite cryo-beads was evaluated. Millimeter-sized nanocomposite cryo-beads were prepared by combination of dropwise freezing into cryogenic liquid (DFLC) method with sedimentation cryogelation method. pH-sensitive 3D-templete was formed by conducting free-radical crosslinking cryopolymerization of NIPA and NaA in the presence of synthetic hectorite Laponite® XLG as inorganic crosslinker and N,N′-methylenebis(acrylamide) as chemical crosslinker. The effects of network template, ionic comonomer, inorganic clay incorporation, pH- and ionic strength of swelling medium on the swelling and elasticity were evaluated as a function of bead size. The deformability of swollen cryo-beads was investigated using Hertzian elasticity. The crosslink density and Hertz modulus of nanocomposite beads vary with the droplet size and the polymer spheres with low crosslink density tend to absorb more water. In DFLC method, the desired sphere size and swelling can be adjusted by selecting the needle size used for dripping. The swelling degree increased significantly as the size of nanocomposite cryo-beads increased. At a fixed swelling ratio, the cylindrical nanocomposite cryogels exhibited greater elastic moduli and crosslink density than that of spherical gels. Correlation of elastic modulus of cryo-beads with swelling and cross-linked polymer concentration predicted a higher scaling exponent than the classical estimate for swelling in good solvents. It was investigated how the presence of 30 mol% pH-sensitive anionic comonomer NaA in the structure of all nanocomposite gels affects their pH-dependent swelling behavior. At pH < 5.7, the gels exhibited a lower swelling ratio, whereas at pH > 6.6 they tended to swell and followed second-order swelling kinetics by Fickian diffusion. When pH of swelling medium was alternately changed, all gels exhibited a pH-switch and swelling rate was two times higher than shrinkage rate. By investigating the effect of KCl, NaCl and K2CO3 on the swelling of cryobeads, it was determined how the salt cation changes the electrostatic repulsion between network charges. Nanocomposite cryo-beads showed fast pH-responsiveness and immediately reabsorbed the water they lost due to pH change, therefore, they can be designed as drug release carriers by pH-switching modulation.