Structure of micelles of poly(n-butyl acrylate)-block-poly (acrylic acid) diblock copolymers in aqueous solution

We report the structure and dynamics of micelles of the amphiphilic diblock copolymers poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-PAA). These self-assembled nanostructures consist of a liquid hydrophobic core and a pH- and ionic strength- sensitive hydrophilic corona. In the first part of this series,(1) we reported the synthesis and micellization of these block copolymers in aqueous media without the need of any cosolvent. Here we present a detailed study on the structural and dynamic properties of these micelles in aqueous solutions under various conditions using static and dynamic light scattering (SLS, DLS), small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM). The block copolymers spontaneously dissolve in water, forming rather monodisperse micelles. Although the corona thickness depends on external stimuli, such as pH and salinity, the micelles do not significantly change their shape or aggregation number upon modifications of these parameters, in spite of the liquidlike nature of the hydrophobic block at room temperature. Moreover, the structure of the formed micelles depends on the preparation conditions: aggregates of micelles are initially formed when the polymers are dissolved in saline aqueous solutions even at pH 6.5, which disintegrate within weeks, resulting in isolated micelles with significantly larger size compared to micelles at the same ionic strength but initially prepared in the absence of added salt. The results are explained in terms of a kinetic control of the micellization process, which is dynamic in terms of unimer exchange but slow on the experimental time scale in adapting to external stimuli.