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.

Amphiphilic diblock copolymers, poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-PAA), with narrow molecular weight distribution (PDI ≤ 1.07) were prepared by atom transfer radical polymerization (ATRP) of n-butyl acrylate and tert-butyl acrylate (tBA), followed by selective acidolysis of the PtBA block. These polymers possess a soft PnBA hydrophobic block with a constant chain length of 90−100 monomer units and pH- and ionic strength-sensitive hydrophilic PAA block with DPPAA = 33−300 AA monomer units. They were expected to form stimuli-responsive micelles. The block copolymers with DPPAA ≥ 100 are directly soluble in water at pH > 4.7. Pyrene steady-state fluorescence spectroscopy and fluorescence correlation spectroscopy (FCS) studies indicate the existence of a very low critical micelle concentration (cmc ∼ 10-8 mol/L). The number-average hydrodynamic radii of the micelles, as determined by FCS, range from 28 to 55 nm, depending on the PAA block length. FCS data indicate that micellar sizes significantly decrease upon dilution for salt-free systems. This is attributed to a dynamic, but kinetically controlled, behavior of these self-assembled nanostructures. In saline solutions the micellar sizes remain constant above the “apparent” cmc (cmc*), which is attributed to slower dynamics of unimer exchange between micelles.

We report the structure and dynamics of micelles of the amphiphilic diblock copolymers poly(nbutyl 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.