Hydrophobically modified poly(N,N-dimethylacryl-amide): Synthesis, aqueous solution behaviour, and rheological properties in aqueous mixtures with hydrophobically modified poly(sodium acrylate)

Copolymers of N,N-dimethylacrylamide with a low fraction of N-dodecylmethacrylamide or N-octadecylacrylamide were prepared by radical copolymerisation in methanol. The similar conversion rates of the comonomers during the polymerisation reaction suggest that the microstructure of these copolymers is close to that of the random one. The great tendency of the alkyl groups to associate in an intra or intermolecular way, is clearly revealed from their rheological behaviour in aqueous solution and it is further supported by 13C NMR and steady state fluorescence data. At low polymer concentration (< 0.01 g·cm–3) intramolecular association prevails and the viscosity of the aqueous solution is clearly lower compared to that of the non-hydrophobic poly(N,N-dimethylacrylamide). Compared to the hydrophobic derivatives of poly(sodium acrylate), the present copolymers exhibit a higher tendency to aggregate which is obviously due to the nonionic character of their backbone. The most interesting behaviour is observed by mixing solutions with a low viscosity of the hydrophobic derivatives of both poly(N,N-dimethylacrylamide) and poly(sodium acrylate). In some cases the viscosity of these mixtures is increased by several orders of magnitude. This behaviour is attributed to the formation of mixed aggregates involving alkyl groups from both kinds of copolymers.