Mechanical and sorption properties of transparent nanocomposite hydrogels

Abstract

In this article a nanocomposite hydrogel system consisting of isocyanate-terminated, star-shaped poly(ethylene oxide) and spherical silica nanoparticles is presented that allows tuning of the mechanical and sorption properties by varying the particle size. With 9 nm silica particles, an almost threefold increase in both the hardness and reduced modulus is observed compared to the pure gel without losing moisture-adsorption capacity. Krenchel's efficiency factors indicate a very weak interfacial interaction between the polymer network and the small particles. The 21 nm particles give rise to a more than 25-fold increase in the hardness and a more than one hundredfold increase in the reduced modulus. However, the maximum water-uptake is reduced to half of that for the pure gel. Surface amino-functionalisation of the particles that allows covalent binding to the polymer is detrimental as it leads to inhomogeneous materials. Thermodynamic investigations reveal a transition of the water molecules into a more ordered, but only weakly hydrogen-bonded state upon adsorption rather than a condensed water phase. This might be a general phenomenon in hydrogels with consequences for synthetic materials as well as biological tissue.