Salt Addition Effects on the Poly(organophosphazenes) Thermothickening

摘要

Thermothickening behaviors of aqueous polyphosphazene solutions were modified by series of salts. To compare positive ion effect, cation-chloride series LiCI, NaCI, and KCI were compared while for those of negative ion, anion-sodium series NaCI, NaBr and Nal were used. These ions have different water binding ability depending on the charge density where small radius high charge density ion shows stronger binding to water sequenced as CI~- >^sBr~- >^sI~- and Li~+ > Na~+ > K~+. We observed different ion actions on the different structured polyphosphazenes. Two different length of α-amino-ω-methoxy-poly(ethylene glycols) denoted as AMPEG 550 and AMPEG 750 and three different amino acid ethyl esters named L-isoleucine ethyl ester (lleOEt), Leucine ethyl ester (LeuOEt) and Valine ethyl ester (ValOEt) were substituted onto phosphazene backbone as hydrophilic and hydrophobic substituents respectively. In aqueous solutions, all the synthesized poly(organophosphazenes) in this study exhibited lower critical solution temperature (LCST) phase behavior where there is viscosity increase along with temperature. Hydrophobicity of amino acid esters and length of polyethylene glycols strongly affected on ion actions which modify the thermothickening behaviors of aqueous polyphosphazene solutions. High charge density anions strongly bind to water which precipitate polymers more effectively, which leads to phase transition temperature decrease. Cation complexed polyphosphazenes are precipitated in a sequence which is determined by water binding ability of the attached cations.