Tacticity effect on the upper critical solution temperature behavior of Poly(N-isopropylacrylamide) in an imidazolium ionic liquid.

Abstract Isotactic poly (N -isopropylacrylamides) (PNIPAMs) with a wide range of tacticity exhibit upper critical solution temperature (USCT) type phase transition in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]). The phase transition temperature becomes higher with increasing isotacticity, since more amide groups are aligned along the same side of backbones to form stronger intramolecular hydrogen bonding. Phase diagrams of highly isotactic PNIPAMs show prominent UCST peaks at low concentrations, similar with the typical polymer solution, which implies the dissolution of isotactic PNIPAMs is dominated by the entropy-driven effect. The increase in phase transition temperature with molecular weight is also from the entropic reason. The phenomena above reveal that the phase transition behavior of PNIPAMs in ionic liquids is highly influenced by tacticity effect. Graphical abstract Left: The phase separation temperature as a function of tacticity for 5% PNIPAM in [BMIM][TFSI] solutionsRight: The phase diagram for PNIPAM with 78–79% meso-diad content in [BMIM][TFSI] solutionsCodes: m represents PNIPAMs with molecular weight of 34900–43600 g/mol, while HMWm represents PNIPAMs with molecular weight of 60200–85700 g/mol. Image 1 Highlights • A UCST phase transition is observed in PNIPAM/ionic liquid mixture. • The transition temperature increases with increasing isotacticity. • The phase diagram exhibits a prominent UCST peak for isotactic PNIPAM. • The transition temperature increases with increasing molecular weight. [ABSTRACT FROM AUTHOR]