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.
The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca 2+ -ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca 2+ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing the NaPA concentration at a constant Ca 2+ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca 2+ . Clearly, excessive Na + -ions displace the Ca 2+ -ions from the NaPA chains.
Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance a s can be smaller than the initial unperturbed particle diameter σ 0, due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕ eff above close packing (ϕ cp), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕ cp using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕ eff. We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕ eff ≫ ϕ cp, a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.
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