Thermal and rheological properties of hydrophobically modified responsive gels

Thermally responsive hydrogels have been prepared of N-isopropylacrylamide (NIPAAM) and a fluorinated hydrophobic comonomer, either hexafluoroisopropy l methacrylate (HFIPMA) or hexafluorobutyl methacrylate (HFBMA). Terpolymer gels containing an ionizable comonomer, methacrylic acid (MAA), were also synthesized. The swelling and mechanical properties of the gels were invest igated in pure water and in aqueous NaCl solutions. All gels show thermosen sitivity typical of polyNIPAAM, swelling at lower temperature and deswellin g above a certain transition temperature. Hydrogels of NIPAAM and HFIPMA ar e homogeneous and the corresponding gel modified with methacrylic acid show s the character of a polyelectrolyte gel. However, hydrogels of NIPAAM and HFBMA are heterogeneous and the corresponding acidic gel does not behave as is typic al of a polyelectrolyte gel. The acidic gel containing hexafluoro isopropyl-substituted comonomers has a high storage modulus G', showing tha t this gel favourably combines the properties of hydrophobically associatin g polymers and polyelectrolytes. Comparison of the macroscopic gels with ch emically similar latex particles showed that the thermal behavior of the pa rticles is mainly determined by the chemical composition but not by the res idual surfactant adsorbed onto the polymer particles.