Aqueous dispersions of polyurethane anionomers: Effects of countercation

The influence of counterions on the properties of aqueous dispersions of polyurethane (PU) anionomer have been investigated. The PU anionomers were prepared by polyaddition reaction of 4,4′‐methylenebis(phenylisocyanate) with poly(propylene glycol) (PPG), dimethylolpropionic acid (DMPA), and 1,4‐butanediol, followed by neutralization of pendant COOH groups by various bases. Stable dispersions are obtained for PU anionomers containing 0.8–1.1 wt % COOH groups (17.8–24.4 mmol/100 g polymer) that have been neutralized by ammonia, trimethylamine, triethylamine, LiOH, NaOH, and KOH, respectively. It is found that average particle size is determined mainly by the zeta potential of the dispersions. The number‐average diameter of the dispersions containing metal cations (Li‐, Na‐, and K‐0.9) is between 60–73 nm (zetal potential −65 to −70 mV), while that of the dispersions containing ammonium cations (AMM‐, TMA‐, and TEA‐0.9) is between 104–251 nm (zeta potential −47 to −64 mV). The surface tension of the dispersions decreases with concentration rapidly and levels off at concentrations higher than 20 mmol/L. The decreased decomposition temperature of the PU anionomers, compared to unionized PUs, can be attributed to the disordering of hard domains after ionization or decomposition reaction catalyzed by alkali metal carboxylates. The thermal decomposition proceeds mainly via cyclic elimination.