The effect of water content on the ultimate properties of rubbery nanocomposite gels

<jats:title>Abstract</jats:title><jats:p>An investigation was conducted into the effects of water content (<jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-1.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-1"/>) on the ultimate tensile properties of nanocomposite hydrogels (NC gels) based on poly(<jats:italic>N</jats:italic>‐isopropylacrylamide)/clay networks. Rubbery NC gels with low clay contents (<NC10) exhibited unique changes in their stress–strain curves, depending on the <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-3.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-3"/>. At high <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-5.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-5"/>, where PNIPA chains are fully hydrated, NC gels retained their rubbery tensile properties, whereas they changed to exhibit plastic‐like deformations with decreasing <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-7.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-7"/>. Consequently, for a series of NC gels with different <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-9.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-9"/>, a failure envelope was obtained by connecting the rupture points in the stress–strain curves. Here, the counterclockwise movement was observed as either the <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-11.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-11"/> decreased or the strain rate increased. This seemed to be analogous to that of a conventional elastomer (e.g., SBR), although the mechanisms are different in the two cases. From the <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-13.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-13"/> and <jats:italic>C</jats:italic><jats:sub>clay</jats:sub> dependences of the ultimate properties, three critical values of <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-16.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-16"/> were defined, where <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-18.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-18"/> showed a maximum strain at break, a steep increase in initial modulus, and onset of brittle fracture. Compared with NC gels, OR gels (chemically crosslinked hydrogels) showed similar but very small changes in their stress–strain curves on altering <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-20.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-20"/>, whereas LR (viscous PNIPA solution) showed a monotonic decrease (increase) in <jats:italic>ε</jats:italic><jats:sub>b</jats:sub> (<jats:italic>E</jats:italic><jats:sub>i</jats:sub>) with decreasing <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-inf-24.gif" xlink:title="urn:x-wiley:08876266:media:POLB21829:tex2gif-inf-24"/>. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2328–2340, 2009</jats:p>