Influence of the hydrophobe structure on composition, microstructure, and rheology in associating polyacrylamides prepared by micellar copolymerization

Acrylamide polymers modified with low amounts of alkyl- or alkylarylacrylamides (1-5 mol %) have been prepared by an aqueous micellar copolymerization technique. This method is known to lead to multiblock copolymers in which the number and length of the hydrophobic blocks vary with the initial number of hydrophobes per micelle. The incorporation behavior of different types of hydrophobes and their effects on the rheological copolymer properties have been investigated. Interestingly, the use of disubstituted acrylamides leads to an average copolymer composition independent of the degree of conversion, in contrast to what is observed with monosubstituted acrylamides. Solubility measurements of both types of hydrophobes indicate that the micellar dynamics is not responsible for this behavior, but rather the difference in polarity between the bulk phase and the micellar phase. This microenvironment effect modifies the reactivity ratios of those hydrophobes capable of forming hydrogen bonds, whereas the reactivity of the other hydrophobes remains unaffected. The rheological properties of the samples are discussed in terms of copolymer microstructure and type of hydrophobe used (bulkiness, degree of branching, and alkyl chain length). For example, at similar hydrophobe levels, double-chain hydrophobes considerably enhance the thickening efficiency with respect to single-chain hydrophobes.