Structure and Rheology of Polymer Mixtures at the Boundary Between Soft Solid and Viscous Liquid Behaviors - From Fundamental Study to Application

Abstract

This thesis presents results for two polymeric systems with liquid-to-solid transitions, i.e., oppositely charged polyelectrolytes in water with salt that can form a gel and polydimethylsiloxane (PDMS)-particle suspensions that exhibit yield stress in 3D printing applications. A protocol is developed for quantifying the compositions of both supernatant and coacervate phases using a combination of H NMR, C NMR, TGA, conductivity, and titration. With these methods, the phase diagrams of polyacrylic acid /poly (diallyl dimethylammonium chloride) (i.e., PAA/PDADMAC) in potassium chloride at different pH values are generated experimentally. We find that with increasing salt concentration, the coacervate phase volume initially increases and then quickly drops to zero at the critical salt concentration, and we quantify the concentrations of both polyelectrolytes in both phases for some compositions. At low pH, we also find a novel phase separation re-entry at high salt concentration, probably related to the solubility of PAA as a function of pH and salt, implying strong non-electrostatic driving forces for coacervation at low pH. The linear viscoelasticity of coacervates is reviewed, with a focus on time-temperature/salt/pH/hydration superpositions. A variety of polyelectrolyte pairs show successful time-salt superposition, with master curves similar to those for neutral polymers. However, in some cases, a solid-like, as opposed to a fluid-like, response is observed at low frequencies, especially at low salt concentrations. Some coacervates seem to fit the “sticky diffusion” theory reasonably well, wherein relaxation is controlled by the breakage rate of ion pairs; the dependence of the “sticker” lifetime on salt concentration has been explored but is not well understood as yet. Direct ink writing additive manufacturing with a static mixer and fine-tip nozzle is studied by printing PDMS mixed either with fumed silica or as a two-part commercial liquid silicon rubber (LSR) mixed with polyethylene glycol (PEG). We assess their printability by printing a hollow slump cone, whose print quality is correlated with rheological measurements, including 1) a shear rate up-ramp followed by a down-ramp in shear rate, 2) creep tests at a series of increasing stresses, and 3) oscillatory shear with increasing amplitude well into the nonlinear regime. The PDMS-fumed silica mixtures fail to print even at the highest fumed silica loading used (9 wt.%), while LSR-PEG with 4 or 6 wt.% PEG prints very well even with low Shore hardness LSR. These large differences in printability of two classes of PDMS materials correlate poorly with rheological behavior in many of the above tests. The exceptions are the apparent yield stress during a down-ramp in shear rate following a previous up-ramp to the maximum shear rate of 1000 s-1, which is similar to the highest shear rate in the print nozzle, the stress at the crossover of the apparent G’ and G’’ curves in a strain amplitude sweep, and the stress at which irreversible flow becomes dominant in a stress sweep. These results indicate that the printability of the materials considered here depends strongly on both their yield stress and their ability to rebuild structure and yield stress quickly after experiencing the high shear rates characterizing their emergence from a narrow nozzle tip.