Thermodynamic Behavior of Polyelectrolytes with the Lower Critical Solution Temperature (LCST) Phenomenon

Abstract

A series of vinyl polymers with L‐valine and L‐leucine residues, and related copolymers with N‐isopropylacrylamide, were studied in aqueous solution at different temperatures (25, 30 and 35°C) and at two ionic strengths (0.01 M and 0.1 M NaCl). The protonation behavior revealed great differences between the polymers that were attributed to the size of the hydrophobic lateral group. Macromolecular shrinkage, occurring above a critical degree of protonation β, was related to hydrophobic forces outweighing the electrostatic repulsions between COO – groups. Low salt concentrations increased the electrostatic potential while high temperatures increased the hydrophobic interaction at lower β. The release of fewer water molecules structured around the polymer chain, responsible for the lower critical solution temperature phenomenon, revealed lower entropy changes at higher temperatures. The reversible configuration of graft polymer chains instantly responded to changes in pH and temperature, modifying the water filtration rates through the pores of cellulose membranes.

Stimuli-responsive poly(ampholyte)s containing L-histidine residues: synthesis and protonation thermodynamics of methacrylic polymers in the free and in the cross-linked gel forms

“…This reduces the electrostatic contribution of the charges and shows as the monomers are randomly distributed with a predominance of block-like NIPAAm units. Similar results were previously reported for vinyl related copolymers containing L-valine residues [30,31]. In the latter case, the decreasing trend of the logK for the acrylates was ascribed to the increased distance between the charges, while methacrylate compounds showed a closer homopolymer polyelectrolyte behaviour.…”

“…As a result the M w and M p molar mass values are substantially correct, while the polydispersity index M w /M n (see Table 2) and in general the DMM shape (see Figure 4) are only approximate. In the meantime, the new band of the NIPAAm Amide I increased and slightly shifted to greater wavenumbers, in the same way as happened for the 1459 cm -1 band of the isopropyl group [31,32].…”