Hydrophobically modified dimethylacrylamide synthesis and rheological behavior

The synthesis and rheological behavior of hydrophobically modified copolymers based upon N,N '-dimethylacrylamide (DMAM) containing dodecyl or octadecyl groups are described. The polymers were synthesized by free radical copolymerization in homogeneous solutions of toluene. This synthesis method ensured that the hydrophobic groups were incorporated individually into the copolymer, i.e., in a nonblocky fashion. This method contrasts with the more commonly produced hydrophobically modified polyacrylamides, synthesized by a micellar polymerization technique, resulting in multiblock structures. Associative behavior of the DMAM copolymers in water was investigated by viscosity measurements. Significant enhancement in viscosity was measured in the semidilute unentangled and entangled regimes. Viscosity enhancement was attributed to the formation of intermolecular hydrophobic aggregates, which act as transitory physical cross-links. While it is well established in the literature that blocky copolymers containing hydrophobic groups can significantly enhance solution viscosity, the same effect produced by hydrophobically modified acrylamide polymers based on randomly copolymerized hydrophobic and hydrophilic monomers is less well-known or understood. The results presented in this paper demonstrate that if long alkyl chains are used as stickers, then hydrophobic aggregation between neighboring chains can promote viscoelastic properties in the semidilute regime. The rheological behavior of these statistical copolymers can be described on the basis of recent theoretical models specially developed for solutions of associating polymers.