Synthesis and viscoelastic behavior of water-soluble polymers modified with strong hydrophobic side chains

A new series of associating polymers were prepared by grafting highly hydrophobic side chains: poly(n-butyl acrylate), PNBA; poly(n-butyl methacrylate), PNBMA; and poly(N-(tert-butyl)acrylamide), PTBA of different sizes onto a poly(sodium acrylate), PAANa, backbone. Due to the strong hydrophobic character of the stickers, the dynamics of the associations is very slow as compared to more conventional water-soluble polymers modified with short alkyl chains and the physical associations mainly behave as chemical ones in the experimental conditions. As a consequence, all the copolymers readily self-assemble in aqueous solution forming clusters in very dilute conditions and then gels at higher concentrations. From dynamic measurements, it was shown that the copolymer solutions follow the same scaling relation η∗ ∼ ca, where a is a frequency dependent exponent. In these conditions, all the copolymer solutions exhibit a sol-gel transition which obeys the main rules of the percolation theory. For each copolymer, the critical gel concentration cg depends strongly on the hydrophobic character of the stickers and a single master curve can be drawn by plotting the complex viscosity vs. the reduced concentration, c/cg. Although the temperature dependence of the viscoelastic properties is very weak, due to the slow dynamics of the associations, it was clearly evidenced that the alkyl acrylamide derivative (PAANa-g-PTBA) exhibits a slight thermothickening behavior which contrasts with the thermothinning behavior of alkyl(meth)acrylate derivatives (PAANa-g-PNBA and PAANa-g-PNBMA). The opposite type of behavior is explained by the presence of the amide function which is known to play an important role in the LCST (lower critical solution temperature) phase diagram of N-alkyl derivatives.