Neutralization and Salt Effect on the Structure and Mechanical Properties of Polyacrylic Acid Gels under Equivolume Conditions

Abstract

The effects of neutralization and salt on the structure and mechanical properties of polyacrylic acid (PAA) gels under equivolume conditions were investigated by small-angle X-ray scattering (SAXS) measurements and tensile tests. We attained the equivolume condition by immersing a piece of PAA gel sample in an ion reservoir containing linear PAA, NaOH, and NaCl at prescribed concentrations (post-ion-tuning). The volume fraction of the linear polymer was set to be the same as that of the gel so as to satisfy the iso-osmotic pressure at the reference state. Various types of reservoirs were prepared by adding NaOH and/or NaCl with different concentrations to the reference reservoir, followed by immersing a PAA gel piece. In the SAXS measurements, a scattering peak appeared, and the scattering intensity at q = 0 decreased by neutralization, while the addition of salt increased the scattering intensity. On the other hand, Young’s modulus measured with the tensile test decreased with neutralization; however, it scarcely changed with the addition of salt. The newly developed equivolume post-ion-tuning technique may serve as a new standard scheme to study polyelectrolyte gels.

Effects of concentration of hydrophobic component and swelling in saline solutions on mechanical properties of a stretchable hydrogel

“…This behavior was due to the osmotic pressure caused by the difference in ion concentration in and outside the gel samples. 49 The percentage of swelling and deswelling in those solutions for the samples with different PPGDA concentrations is shown in Fig. 5.…”

“…Previous studies have shown that NaCl can reduce water intake. 49,51,52 The mechanical properties of the gels after swelling and deswelling were measured using tensile testing. The stress vs. strain data are plotted in Fig.…”