Thermo-responsive association of chitosan-graft-poly(N-isopropylacrylamide) in aqueous solutions

A comb-like dual hydrophilic graft chitosan copolymer, chitosan-graft-poly(N-isopropylacrylamide) (CS-g-PNIPAM), was synthesized by means of atom transfer radical polymerization (ATRP) and click chemistry. The thermo-responsive association behavior of the copolymer in dilute aqueous solutions has been investigated by laser light scattering (LLS), zeta potential, and transmission electron microscopy (TEM). The core-shell structured micelles with the hydrophobic PNIAPM as a core and the hydrophilic CS as a shell were formed at low pH (<4) and high temperature (>32 degrees C), whereas the obtained micelles became large aggregates and precipitated in alkaline solutions (pH > 7). Additionally, the low critical solution temperature (LCST) phase transitions of dense copolymer solutions in the presence of salts (NaCl and NaI) and a cationic surfactant (dodecyltrimethylammonium bromide, DTAB) respectively were studied by micro differential scanning calorimetry (DSC) and UV turbidimetry. It was found that both salts (NaCl and NaI) and DATB could shift the LCST to lower or higher temperatures from a pure copolymer solution, depending on the beneath mechanisms.