Effect of Temperature on Self-Assembly of Amphiphilic Block-Gradient Copolymers of Styrene and Acrylic Acid

Summary The effect of temperature on self-assembly of a novel type of amphiphilic ionic copolymers comprising a hydrophilic poly(acrylic acid) PAA block and an amphiphilic poly(acrylic acid)-grad-poly(styrene) PAA-grad-PS copolymer block has been studied by Small Angle Neutron and Dynamic Light Scattering (SANS, DLS). The polymers have been synthesized using direct Nitroxide-Mediated Polymerization (NMP) as described elsewhere and proved to be capable of formation of the equilibrium (dynamic) micelles responsive to variations in pH and ionic strength of the solution. We have demonstrated that an increase in temperature from 20 to 80 °C has almost no influence if the assembly occurs at pH∼6 in a wide range of salt concentration. Under these conditions stable micelle aggregates are formed. At pH∼6.9 virtually no aggregation occurs at any temperature in salt-free solution, whereas at 1 M salt concentration we observed an appearance of the correlation peak in the scattering curves and an increase in hydrodynamic radius in dilute solutions evidencing self-association of polymer chains. The magnitude of the correlation peak increases upon an increase in temperature. This behaviour may be attributed to the combined effect of temperature on the ionization constant and on the strength of hydrophobic interactions.