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

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.