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

Abstract

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 ¼ 700 mM and z ¼ 8.6 mM 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 [m 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 .