Hydrophobically modified polyacrylates (hmPAAs) with long alkyl chains – Self-assembly in aqueous solution

Hydrophobically modified polyacrylates (hmPAAs) obtained by Atomic Transfer Radical Copolymerisation (ATRP) of mixtures of different alkyl acrylates and t-butyl acrylate and subsequent hydrolysis of the t-butyl acrylate to polyacrylic acid (PAA), were studied with respect to their aggregation behaviour in aqueous solution. The hydrophobic modifications were long alkyl chains that were varied systematically from dodecyl to stearyl, thereby expecting a strong propensity for hydrophobic aggregation. Their structure in aqueous solution was investigated comprehensively by small-angle neutron scattering (SANS) and complemented by dynamic light scattering (DLS) experiments. In these random copolymers the degree of amphiphilicity and hydrophobicity was varied by the percentage of alkyl chains and pH. Due to the length of the alkyl chain all these copolymers have a tendency for microphase separation, but this tendency and the structure of the formed hydrophobic domains depend in a systematic fashion on the precise molecular composition. The domain size scales with the length of the hydrophobic alkyl modification and, for chains of C16 and longer, there is apparently a tendency for elongation. Another interesting observation is that for 20 mol% hydrophobic modification rather self-contained and isolated hydrophobic micellar domains are formed. In contrast, the copolymers with 10 mol% alkyl modification contain interconnected hydrophobic domains as evidenced from the low-q scattering seen in SANS and confirmed by DLS observations. This tendency is further enhanced when fully charging the copolymers. In summary, the molecular design of such random amphiphilic copolymers allows in a nice way to control the aggregation state in solution.