Characteristic role of crosslinker on thermally induced volumetric contraction–expansion processes in poly( N ‐isopropylacrylamide) networks and water systems

In this work, the degree of crosslinking on the volumetric contraction–expansion processes of hydrogels made of poly( N ‐isopropylacrylamide) (NIPA) (initial amount: C m ) with varied amount ( z ) of crosslinking agent methylene‐bis‐acrylamide (BIS) in reference to most commonly used NIPA gel that was synthesized with C m = 700 m M and z = 8.6 m M was investigated by applying our recently developed pycnometry. We focused on characteristic role of four polymeric NIPA residues directly bonded to a single BIS molecule by evaluating the total volume of gels per four NIPA residues directly bonded to a BIS molecule, plus associated water [ν sp (gel)(NIPA) bonded )( T )], and the corresponding number of water molecules per four NIPA residues [ N s (gel)(NIPA) bonded )( T )]. We elucidated how these quantities characteristically contribute to changes in the volumetric contraction–expansion processes of hydrogels. A comparison of these quantities with the corresponding quantities for (NIPA) unbonded residues clearly revealed a significant structural difference between (NIPA) bonded and (NIPA) unbonded . © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009