Aggregation Behavior of Polystyrene/Poly(acrylic acid) Heteroarm Star Copolymers in 1,4‐Dioxane and Aqueous Media

Ionic heteroarm star copolymers bearing polystyrene (PS) and poly(acrylic acid) (PAA) arms (PS_nPAA_n) were prepared by quantitative hydrolysis of the poly(tert‐butyl acrylate) (PtBA) arms of the corresponding PS_nPtBA_n star copolymer. The aggregation properties of these copolymers were studied in various solvents. In 1,4‐dioxane PS₁₂PAA₁₂ (with nearly symmetrical PS and PAA arms) forms reverse micelles of low aggregation number (N_agg) and spherical morphology. In an 80 : 20 (v/v) 1,4‐dioxane/water mixture these micelles are transformed to regular micelles with an unexpectedly high N_agg and an elongated rod‐like structure. An abnormal behavior was observed in aqueous solutions of charged PS₂₄PANa₂₄ (with asymmetrical PS and PAA arms, W_PS = 19 wt.‐%) at low concentrations. A non‐equilibrium physical gel is formed, characterized by a very high viscosity and an elastic response upon oscillatory shearing.