Microstructure of copolymeric micelles modulated by ionic liquids: investigating the role of the anion and cation

Abstract

Here we report comprehensive analysis of the influence of ionic liquids (ILs) on the physicochemical properties of an ethylene oxide–propylene oxide (EO–PO) star shaped block copolymer, Tetronic®1304 (total mol. wt = 10 500 and %PEO = 40%) hereafter referred to as T1304, by employing cloud point (CP), viscosity, dynamic light scattering (DLS), small-angle neutron scattering (SANS), high sensitivity differential scanning calorimetry (HSDSC) and fluorescence measurements and ¹H NMR spectroscopy. 1-Alkyl-3-methylimidazolium based ILs varying in alkyl chain length (C₄–C₁₀) and anion, namely chloride, tetrafluoroborate, trifluoromethanesulfonate and hexafluorophosphate, were used. At lower concentrations, the ILs with different anions showed identical effects on the T1304 micelles but a significant change was noticed at higher concentrations. ILs with longer alkyl chains formed smaller mixed micelles while those with shorter chains remained in the bulk. Moreover, an increase in the alkyl chain length significantly increased the CP while the apparent hydrodynamic diameter (D_h) of the micelles decreased. In line with this, the variation in alkyl chain length has no significant effect on the CMT up to C₈mimCl but decreases efficiently for C₁₀mimCl. On the other hand, the variation in counter-ions seldom influenced the CP/D_h/CMT of the T1304 solutions at lower concentrations of the IL, but showed sufficient change at higher concentrations. The presence of NaCl suppressed the effect of the IL on the T1304 micelles. The microstructural changes as observed for the T1304 micelles in the presence of different ILs are explained using SANS data. Fluorescence studies with coumarin-481 as a probe showed shifts in the emission spectra and enhancement in the fluorescence decay, suggesting incorporation of the ILs in the micelles. The penetration of ILs with longer alkyl chains in the T1304 micelles was further confirmed using ¹H NMR spectroscopy. The present study provides valuable information on tuning the solution behaviour of T1304 with ILs that may prove to be beneficial for different industrial applications.