Thermodynamics of multiple stimuli-responsive polyelectrolytes with complexing ability towards the copper(II) ion

Abstract

The thermodynamics of protonation of carboxylate groups in polymers containing amido and isopropyl moieties was studied in 0.1 M NaCl at different temperatures. Compared with polymers with the l-leucine moiety, poly(N-acryloyl-l-valine) showed a wider decreasing linearity of the basicity constant (log K) in relation to the degree of protonation (α), with a minimum at α = 0.65. Under these conditions, a sharp endothermic peak revealed the presence of hydrophobic interactions as the macromolecule assumed a compact coil conformation and water molecules were released with an increase in entropy. The graft polymers on the surface of a polyurethane film displayed larger potentiometric hysteresis loops with respect to the cellulose support, as the nature of the polymer was similar to that of the hydrophobic substrate. The reversible configuration of the graft polymer chains instantly responded to changes in pH and temperature, modifying the rate of water filtration through the membrane pores. As the temperature approached 31°C an abrupt increase in water permeation was observed even though the graft polymer was in the uncharged and coiled state. The polyelectrolyte formed a hydroxo-complex species, involving two monomer units (L⁻) of the polymer, with the Cu(II) ion. Electron paramagnetic resonance (EPR) spectra, supported by conductometric data, revealed Cu(OH)₂L₂²⁻ stoichiometry in solution. The potentiometric data showed a good fit when processed by the Superfit program and a decreasing pattern of log β in relation to pH.