Improvement of elasticity and strength of poly(N-isopropylacrylamide) hydrogels upon copolymerization with cationic surfmers

The mechanical properties of thermoresponsive homopolymer hydrogels based on N-isopropylacrylamide (NIPAAm) and copolymer gels based on NIPAAm and two different cationic surfactant monomers (surfmers) were investigated. The hydrogels were prepared without the aid of any chemical cross-linker using γ-ray induced polymerization in aqueous micellar solution. Two surfmers were used, the reactive unit either being attached to the polar head (H-surfmer) or the hydrophobic tail (T-surfmer) of the molecule. (11-(Acryloyloxy)undecyl) trimethylammonium bromide (AUTMAB) was used as the T-surfmer, and (2-(methacryloyl)ethyl)dodecyldimethylammonium bromide (MEDDAB) as the H-surfmer. The mechanical stability of the hydrogels was investigated using elongational flow measurements. It was found that the mechanical stability of the gels increases with increasing surfmer concentration in the gel. Hencky-strain at break ε _H ^max and true stress at break σ _H ^max are clearly higher for copolymer gels than for pure poly-NIPAAm (P-NIPAAm) gels. A hydrogel containing 1% (w/w) MEDDAB can be elongated up to a true stress being nearly six times as large, and a Hencky-strain more than twice as large as the P-NIPAAm homopolymer gel. The improved stability is explained by the presence of copolymerized micellar aggregates, which act as additional, stable cross-linking units. It is also shown that the mechanical stability of copolymer gels containing either the T- or H-type surfmer is different. A structure model correlating the mechanical properties with possible micelle structures derived from surface tension measurements is presented.