Viscoelastic properties of hydrogel microcapsule membranes

The viscoelastic properties of poly(L-lysine-alt-terephthalic acid) (PPL) microcapsules were studied as a function of medium pH. An abrupt increase in the apparent relative viscosity of PPL microcapsule suspensions was observed in the pH range between 4.0 and 7.0 due to the increased total particle volume concentration caused by a sudden increase in microcapsule size. Above pH 7.0, the relative viscosity decreased, being indicative of a deformability of the microcapsules. The adiabatic compressibility of PPL microcapsule membranes was the lowest at pH 4.0 and increased remarkably when the pH value departed from 4.0. On the contrary, the membrane density was the highest at pH 4.0 and decreased as medium pH shifted to either side of this value, implying that the microcapsules behave as compact particles at low pH and loose particles at high pH. The amount of hydrated water also showed a similar change when the pH of the medium was altered.