Polymer/Montmorillonite–Iron Oxide Nanocomposite Hydrogels with Anisotropic Properties

Anisotropic hydrogels are valuable for their unique directional properties. Here, low magnetic fields were used to produce polymer nanocomposite hydrogels with aligned nanoparticles and anisotropic mechanical properties. Montmorillonite (MMT) was modified with magnetite (IO) via in situ coprecipitation, forming MMT–IO nanohybrids. Subsequently, both polyacrylamide/montmorillonite–iron oxide and poly(N-isopropylacrylamide)/MMT–IO (PNIPAM/MMT–IO) nanocomposite hydrogels were prepared through free radical polymerization under a magnetic field (∼20 mT), which was measured computationally and experimentally. The resulting polymer nanocomposite hydrogels displayed superior mechanical properties along the direction of the magnetic field, with tensile strengths up to 3.5 times the value of the pristine polymer hydrogels. The Young’s modulus of the oriented hydrogels approximately followed the Halpin–Tsai and Mori–Tanaka micromechanical models for composites. Confocal laser scanning microscopy confirmed nanohybrid alignment along the magnetic field. Additionally, PNIPAM/MMT–IO nanocomposites exhibited anisotropic volume transitions upon temperature changes, hence enabling the production of self-shaping structures. These anisotropic and smart nanocomposite hydrogels are promising for actuation and aquatic loading applications.