Tuning the thermoresponsive properties of weak polyelectrolytes:: Aqueous solutions of star-shaped and linear Poly(N,N-dimethylaminoethyl methacrylate)

We investigated the thermoresponsive behavior of aqueous solutions of star-shaped and linear poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). The observed cloud points strongly decrease with increasing pH of the solution. This is explained by a weak charging of the star polymers with decreasing pH. A significant decrease of the cloud points with increasing molecular weight for high pH, i.e., for the almost uncharged state, was found to be virtually independent of the arm number and arm length. These findings are explained by classical Flory-Huggins theory. The increase of cloud points upon charging is captured by introduction of an effective degree of polymerization. Polymers with shorter arms show slightly higher cloud, points at low pH than polymers with longer arms. The intramolecular segment density also influences the observed apparent pK(b) values, leading to higher values for stars with higher arm numbers.