Thermo-Responsive Poly(N-Isopropylacrylamide) and its Critical Solution Temperature Type Behavior in Presence of  Hydrophilic Ionic Liquids
Creators: name: Nayak, Purnendu K
Publisher: name: University of Massachusetts Amherst
Year: 2015

description: Thermo-responsive homopolymer poly(N-isopropylacrylamide), is a widely studied and used polymer. Our recent observations on thermal behavior of aqueous solutions of this polymer requires a short overview of existing results in order to understand the formation of different phases, both stable and unstable with the addition of hydrophilic Ionic liquids (ILs) 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF<sub>4</sub>]), 1-Butyl-3-methylimidazolium acetate ([BMIM][OAc]) and 1-Butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) to the system. PNIPAM is soluble in cold water due to its inter- and intramolecular hydrogen bonding but phase separates upon heating at T > 32 , which is its lower critical solution temperature (LCST). PNIPAM exists in an expanded coil like conformation in water below its LCST which gives a transparent homogenous solution but at T > LCST it undergoes hydrophobic collapse marked by cloudiness of solution and conformational change from coil to globule state. All aqueous PNIPAM solutions undergo phase separation or cloud point transition at T > 32 , regardless of the molar mass of the polymer.
Room temperature Ionic liquids (ILs) are unique designer fluids because of the novel physico-chemical properties arising from their structure, which have tremendous implications in the field of IL as solvents or co-solvents for polymeric solutes. During recent years a number of different imidazolium based ILs have also been tested for solubilization and stabilization of proteins as well as polymers due to hydrogen bond formation of the IL ions. Recent studies have shown that certain imidazolium based ILs can decrease the LCST of PNIPAM aqueous solution by hydrophobic collapse/aggregation of the PNIPAM chains, as well as some can induce an upper critical solution temperature (UCST) behavior of PNIPAM in neat IL solution. Even so, experimental studies of such phase transition/ instability of thermoresponsive polymer-IL systems has been a challenging task. In this research we have explored the critical solution temperature (CST) type phase behavior of multicomponent systems i.e. PNIPAM in solution media of water, neat IL and aqueous solutions of IL. The overall fundamental challenge is to understand how the interactions among the components control both structure and dynamics of PNIPAM network in solution. For example the disruption of hydrogen bonding or desolvation interactions between blocks of a PNIPAM molecule and solvent molecules in aqueous mixtures that lead to a LCST type transition at higher temperatures.
Interestingly, it was found in our case that PNIPAM shows both LCST and UCST-type phase transition in some aqueous solutions of hydrophilic IL [BMIM][BF<sub>4</sub>]. It was found for the first time that this IL can influence the LCST type behavior of PNIPAM in aqueous solutions based on our visual and experimental cloud point (CP) observations. In our experiments the effect of the ILs [BMIM][BF<sub>4</sub>] and [BMIM][OAc] is qualitatively similar to influence of Kosmotropic salts on the LCST of aqueous PNIPAM solutions as predicted by the Hofmeister series.