Upper or lower critical solution temperature, or both? Studies on cationic copolymers of N-isopropylacrylamide

Abstract

The solution properties of statistical copolymers of N-isopropyl acrylamide (NIPAm) and cationic (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA) have been studied. The phase behavior of the copolymers in aqueous solutions is strongly affected by the addition of lithium bis(trifluoromethane)sulfonimide (LiNTf₂), NaCl, or both. Hydrophobic NTf₂ counter ions bind to the AMPTMA repeating units. By adjusting the balance between hydrophobic and electrostatic interactions the transition temperature of the copolymers may be tuned over a wide temperature range. It was observed that a homopolymer PAMPTMA undergoes an UCST-type phase separation in an aqueous solution in the presence of both NaCl and LiNTf₂. When AMPTMA and NIPAm are present in the copolymer in nearly equal amounts both LCST and UCST can coexist. It was observed that the effect of LiNTf₂ is similar to that of the salts in the kosmotropic end of the Hofmeister series for PNIPAm.