Dual polyelectrolyte-ionomer behavior of poly(acrylic acid) in methanol: 1. Salt-free solutions

Abstract

The behavior of poly(acrylic acid) (PAA) partly neutralized with tetramethylammonium hydroxide in methanol was studied by the viscometry, 1 H NMR spectroscopy, light scattering, and conductivity methods. It was shown that, when the degree of PAA neutralization increases from 0.1 to 0.2, a transition from a polyelectrolyte to ionomer state occurs. This transition is caused by the formation of ion pairs due to the binding of counterions with the charged groups of the polymer chain and is accompanied by a macromolecule collapse. Collapsed macromolecules aggregate. At degrees of neutralization above 0.4, the restored polyelectrolyte state is accompanied by polymer swelling. The data obtained are qualitatively explained.

Dual polyelectrolyte-ionomer behavior of poly(acrylic acid) in methanol: 2. Salt solutions

“…Let us discuss the case, when the salt is absent (curve 1 ). At low degrees of neutralization, α , the system is in the polyelectrolyte state [9]. A rise in the viscosity with α is caused by an increase in the swelling osmotic pressure of free counterions, as well as by an increase in the repulsion between the similarly charged units of macromolecules that leads to the swelling of macromolecular coils [9].…”

“…This transition is caused by the fact that, when the α 1 value is achieved, the gain in free energy due to the formation of ion pairs increases because of the formation of multiplets and begins to exceed the translational entropy of counterions [8,9,11,12]. Upon a further increase in α , the viscosity first does not change, and, at α > α 2 , it begins to increase again (Fig.…”

“…1). This repeated increase in the viscosity can be explained by the fact that, in the case of large counterions, such as tetramethylammonium ion, the formation of multiplets at larger α values becomes hindered sterically, and the system loses an additional gain in energy [9]. Therefore, when the α 2 value is achieved, the free state of counterions becomes more advantageous than their bound state.…”

“…Tetramethylammonium chloride was prepared by adding an equimolar amount of hydrochloric acid to a TMAOH solution followed by the lyophilization. Procedures for the sample preparation and the measurements of viscosity and conductivity are described in [9].…”

“…The ratio between these two forms of counterions is governed by the balance between the counterion translational entropy, which facilitates the dissociation of ion pairs, and the formation energy of ion pairs between the charged groups of a macromolecular chain and the counterions [3][4][5][6][7][8][9][10]. The addition of a low-molecular-weight salt decreases the gain in the translational entropy of polymer counterions; hence, it can be expected that the addition of low-molecular-weight salt will promote the transition from the polyelectrolyte to the ionomer state.…”