ABSTRACT: Vinyl polymers carrying L-phenylalanine residues were synthesized in the free and in the cross-linked hydrogel form, as homo- and copolymers with N-isopropylacrylamide (NIPAAm). The protonation reaction thermodynamics of the COO- group were studied in aqueous media, at different temperatures and at increased concentrations of sodium chloride, mainly by potentiometry and solution calorimetry. The soluble polymer, namely poly(N-acryloyl-L-phenylalanine), and its copolymers with low NIPAAm content displayed characteristic features. Their basicity constants (log K), as well as the enthalpy (-¢H°) changes in relation to R (degree of protonation) showed an abrupt drop at R ) 0.5. This was ascribed to the formation of hydrogen bonds between the protonated and the ionized neighboring COOgroups. The process was driven by the side-chain aromatic rings that improved hydrophobic interactions. The entropy (¢S°) changes sharply increased as a result of the increased macromolecular conformational freedom and the release of water molecules surrounding the hydrophilic groups of the polymer. The corresponding cross-linked polymers formed hydrogels that were responsive to pH, temperature, and ionic strength. The two hydrogels, P9 (homopolymer with 9 mol% cross-links) and CP2 (copolymer with 90 mol% of NIPAAm and 2 mol% cross-links), were characterized for their pH- and temperature-responsive behavior by equilibrium and oscillatory swelling studies. They demonstrated a strong pH-dependent volume phase-transition and an unusual sodium chloride phase-transition phenomenon. Moreover, the hydrogel CP2 exhibited a temperature-dependent volume phase-transition (LCST, lower critical solution temperature) behavior in aqueous solution, where the LCST decreased by lowering the pH. It was nontoxic against the RAW264 cell line.
Scheda prodotto non validato
Scheda prodotto in fase di analisi da parte dello staff di validazione
|Titolo:||Stimuli-responsive polymers based on L-phenylalanine residues: the protonation thermodynamics of free polymers and cross-linked hydrogels|
|Citazione:||Casolaro, M., Paccagnini, E., R., M., & Y., I. (2005). Stimuli-responsive polymers based on L-phenylalanine residues: the protonation thermodynamics of free polymers and cross-linked hydrogels. MACROMOLECULES, 38, 2460-2468.|
|Appare nelle tipologie:||1.1 Articolo in rivista|