Mechanical Properties and Structure of Polymer−Clay Nanocomposite Gels with High Clay Content

The mechanical properties and structures of nanocomposite gels (NC gels), consisting of poly(N-isopropylacrylamide) (PNIPA) and inorganic clay (hectorite), prepared using a wide range of clay concentration (∼25 mol % against water) were investigated. All NC gels were uniform and transparent, almost independent of the clay content, Cclay. The tensile modulus (E) and the strength (σ) were controlled without sacrificing extensibility by changing Cclay. The E, σ, and fracture energy observed for as-prepared NC gels attained 1.1 MPa, 453 kPa, and 3300 times that of a conventional chemically cross-linked gel, respectively, and σ increased to 3.0 MPa for a once-elongated NC25 gel. From the tensile and compression properties, in addition to optical transparency, it was concluded that a unique organic/inorganic network structure was retained regardless of Cclay. The effects of Cclay on the tensile mechanical properties on the first and second cycles, the time-dependent recovery from the first large elongation and the optical anisotropy of NC gels, and also the disappearance of the glass transition and the formation of clay−polymer intercalation in the dried NC gel were revealed. Thus, it became clear that the properties and the structure changed dramatically for an NC gel with a critical clay content ( ≈ NC10) or above. The structural models for NC gels with low and high Cclay, exhibiting different clay orientation and residual strain, were depicted.