Structure and interactions of polyelectrolyte block polymers in solution
Block polyelectrolytes consisting of a charged hydrophilic block and a hydrophobic block are of interest for a variety of industrial applications; including viscosity modifiers for personal care products and drilling fluids for oil field applications. Such systems also exhibit fascinating morphological behavior. The self-assembly can be controlled by varying the ionic strength, solvent, or degree of polymerization of the diblocks. However, a fundamental understanding of the effect of such parameters on the morphology and rheology on these systems is lacking. To utilize these materials in a wide variety of applications, such as templating and drug delivery, there needs to be a more thorough understanding on the effect of system parameters on the self-assembly and rheological properties.
In this work, a hydrophobically modified block polyelectrolyte of polystyrenepoly(acrylic acid)/poly (ethyl acrylate) (PS-PAA/EA), which forms attractive spherical micelles in water, was studied. The system shows interesting rheological behavior as the amount of ethyl acrylate along the polyelectrolyte backbone is changed. We examine the effect of the extent of hydrolysis on the self-assembly and micellar interactions on the self-assembly, micellar interactions, and gel structure via small-angle neutron scattering (SANS), ultra small-angle neutron scattering (USANS), and fluorescence microscopy. SANS spectra were fit using a variety of models to determine the micelle structure. We find that as more poly(ethyl acrylate) is converted to poly(acrylic acid) (i.e., as the corona becomes more charged and more hydrophilic), the micellar aggregation number decreases, consistent with charged polymer chains occupying a larger effective volume due to electrostatic effects. To determine the structure of the gels formed, USANS experiments were performed. The USANS data shows that PS-PAA/EA forms an attractive glass with micron-sized polydisperse aggregates. Fluorescence and confocal microscopy was also used to confirm the presence of large scale aggregates as suggested from the USANS data. Our data suggests USANS as a technique to distinguish between a repulsive glass, an attractive glass, and an attractive gel.
Also, this work investigates the effect of added surfactant on PS-PAA/EA. When a anionic surfactant is added to PS-PAA/EA gels, the rheology of the system shows that the interactions between micelles decreases rapidly. However, on the addition of a cationic surfactant; the strength of the gels initially increases, then decreases. SANS experiments show that there is no morphology change with added anionic surfactant. However, with added cationic surfactant, there is a morphological change in the PS-PAA/EA gels. A model system of PS-PAA with added surfactant was also studied via SANS. These systems formed elastic solids at much higher polymer concentrations than PS-PAA/EA.