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, C_clay. The tensile modulus (E) and the strength (σ) were controlled without sacrificing extensibility by changing C_clay. 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 C_clay. The effects of C_clay 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 (C_clay^c ≈ NC10) or above. The structural models for NC gels with low and high C_clay, exhibiting different clay orientation and residual strain, were depicted.