Inkjet jettability and physical characterization of water-ethanol solutions of low molecular weight sodium polyacrylate and poly-diallyl dimethyl ammonium chloride (polyDADMAC)

Polyelectrolytes are water-soluble polymers having repeat units carrying electrolyte groups. As polyionic molecules having like charge units, they are self-repelling with a rod-like conformation in solution. Inkjet applications of polyelectrolytes include particle dispersing, surface modification, and multilayer structures. This work investigates the physical properties of low molecular weight sodium polyacrylate (NaPA) and poly-diallyl dimethyl ammonium chloride (polyDADMAC) polyelectrolyte solutions in the water-ethanol mixture in relation to their behavior in inkjet deposition. In rotational rheometry measurements, the solutions are found to behave in a Newtonian fashion once the effects of experimental artifacts are taken into account. The range of NaPA concentrations that could be studied was limited to 1 wt./wt. % by the poor solubility of NaPA in the presence of ethanol, and at these concentrations, the addition of NaPA to the solvent did not have a significant effect on the jetting behavior. PolyDADMAC had good solubility, and concentrations up to 10 wt./wt. % were studied and jetted successfully. While an increase in polyelectrolyte concentration resulted in a slow increase in ink viscosity, this was not found to have a significant effect on the required jetting voltage or maximum stable jetting frequency, though drop detachment and satellite droplet formation times were found to increase. As a practical limitation of polyDADMAC inks, solvent evaporation was found to lead to idle nozzles becoming non-jetting, with the allowed idle time decreasing rapidly as ink polyDADMAC concentration increased. This non-jetting behavior is likely due to residence time at the nozzle exit leading to the local surface tension and/or viscosity increase, differing from the bulk ink properties.