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

The soln. properties of statistical copolymers of N-iso-Pr acrylamide (NIPAm) and cationic (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA) have been studied. The phase behavior of the copolymers in aq. solns. is strongly affected by the addn. 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 temp. of the copolymers may be tuned over a wide temp. range. It was obsd. that a homopolymer PAMPTMA undergoes an UCST-type phase sepn. in an aq. soln. in the presence of both NaCl and LiNTf2. When AMPTMA and NIPAm are present in the copolymer in nearly equal amts. both LCST and UCST can coexist. It was obsd. that the effect of LiNTf2 is similar to that of the salts in the kosmotropic end of the Hofmeister series for PNIPAm.