Counterion distribution in urchinlike charged copolymer micelles Monte Carlo simulation and small angle x-ray scattering

Fluctuation effects and monomer-counterion correlations in starlike polyelectrolyte systems

We performed molecular-dynamics simulation to investigate fluctuation effects of monomers and counterions in starlike polyelectrolyte systems. We further study correlations between monomers and counterions. These quantities are systematically studied by variation of two parameters: the functionality f and the degree of dissociation alpha. Fluctuations of monomers and counterions are quantified by comparing the corresponding form factors of the monomers and counterions with a mean-field approximation. Fluctuations being correlated at length scales of the order of the star radius turn out to be negligible with increasing arm number f of the stars. At small arm numbers where fluctuation effects play a significant role, the use of theoretical mean-field models is questionable for a proper description of the monomer- and counterion-form factors. Correlations between monomers and counterions are quantified by a cross-correlation function. At small degree of dissociation alpha this function vanishes and indicates weak correlations. Furthermore, we compare our results with theoretical mean-field models. We give an appropriate analysis of the theoretical form factors and test commonly used counterion density distributions with our form factor analysis.

Structure of strongly interacting polyelectrolyte diblock copolymer micelles

The structure of spherical micelles of the diblock poly(styrene-block-acrylic acid) [PS-b-PA] copolymer in water was investigated up to concentrations where the polyelectrolyte coronal layers have to shrink and/or interpenetrate in order to accommodate the micelles in the increasingly crowded volume. We obtained the partial structure factors pertaining to the core and corona density correlations with small angle neutron scattering and contrast matching in the water. The counterion structure factor was obtained with small angle x-ray scattering (SAXS) with a synchrotron radiation source. Furthermore, we have measured the flow curves and dynamic visco-elastic moduli. The functionality of the micelles is fixed with a 9 nm diameter PS core and a corona formed by around 100 PA arms. As shown by the SAXS intensities, the counterions are distributed in the coronal layer with the same density profile as the corona forming segments. Irrespective ionic strength and micelle charge, the corona shrinks with increasing packing fraction. At high charge and minimal screening conditions, the polyelectrolyte chains remain almost fully stretched and they interdigitate once the volume fraction exceeds the critical value 0.53+/-0.02. Interpenetration of the polyelectrolyte brushes also controls the fluid rheology: The viscosity increases by three orders of magnitude and the parallel frequency scaling behavior of the dynamic moduli suggests the formation of a physical gel. In excess salt, the coronal layers are less extended and they do not interpenetrate in the present concentration range.

Spherical polyelectrolyte block copolymer micelles: structural change in presence of monovalent salt

Spherical polyelectrolyte block copolymer micelles were investigated as a function of added NaCl salt concentration using Small-Angle Neutron Scattering (SANS) and Light Scattering (LS). The micelles are formed by the self-association of charged-neutral copolymers made of a long deuterated polyelectrolyte moiety (NaPSS(d))251 and a short hydrophobic moiety (PEP)52. In presence of salt, the core shape and the aggregation number of the micelles are not affected. The hydrodynamic radius of the micelle is found to be identical to the radius of the whole micelle deduced from neutron scattering and thus the hydrodynamic radius is a valid measure of the corona thickness. At the lowest salt concentrations investigated the thickness of the corona, R(s), remains essentially constant and a contraction is observed above an added-salt concentration c(s) of 2 x 10(-2) M where this crossover concentration corresponds to the average ionic strength of the free counterions in the corona. The contraction takes place while maintaining a rod-like behavior of the chains at short scale and obeys to: R(s) approximately c(s)(-0.18). The exponent 0.18 suggests an electrostatic persistence length proportional to the Debye screening length.

Thermodynamic behaviors of polyampholytes at low temperatures

The thermodynamic properties of 6-mer polyampholyte chains were simulated with the multicanonical Monte Carlo method in a three-dimensional off-lattice model. The results demonstrate that the number of the charged monomers on the chain and the sequence significantly affect the transition behaviors of polyampholyte chains. In the case of the random distribution of the charged monomers, two oppositely charged monomers act as two stickers during a collapse process, due to which the movement of monomers is suppressed and the chains can more directly reach their solid state. Interestingly, in a proper arrangement of the charged monomers, the chains may have a proteinlike nondegenerate state, i.e., the chain is designable. The transition of such polyampholyte chains appears to be of a two-state, all-or-none type.