Ultrasound- and Thermo-Responsive Ionic Liquid Polymers

Abstract

Poly(sodium 2-acrylamido-2-methylpropanesulfonate) (PAMPSNa) was prepared via reversible addition-fragmentation chain transfer (RAFT) radical polymerization. An ionic liquid polymer (PAMPSP₄₄₄₈) was then prepared by exchanging the pendant counter cation from sodium (Na⁺) to tributyl-n-octylphosphonium (P₄₄₄₈⁺). We studied the ultrasound- and thermo-responsive behaviors of PAMPSP₄₄₄₈ in water. When the aqueous PAMPSP₄₄₄₈ solution was heated from 5 to 50 °C, the solution was always transparent with 100% transmittance. Unimers and interpolymer aggregates coexisted in water in the temperature range 5⁻50 °C. Generally, hydrogen bonding interactions are broken as the temperature increases due to increased molecular motion. Above 25 °C, the size of the interpolymer aggregates decreased, because hydrophobic interactions inside them were strengthened by dehydration accompanying cleavage of hydrogen bonds between water molecules and the pendant amide or sulfonate groups in PAMPSP₄₄₄₈. Above 25 °C, sonication of the aqueous solution induced an increase in the collision frequency of the aggregates. This promoted hydrophobic interactions between the aggregates to form larger aggregates, and the aqueous solution became turbid. When the temperature was decreased below 8 °C, hydrogen bonds reformed between water molecules and the pendant amide or sulfonate groups, allowing PAMPSP₄₄₄₈ to redissolve in water to form a transparent solution. The solution could be repeatedly controlled between turbidity and transparency by sonication and cooling, respectively.

Keywords: electrostatic repulsion; hydrophobic interaction; ionic liquid polymer; lower critical solution temperature (LCST); reversible addition-fragmentation chain transfer (RAFT) polymerization; thermo-responsive; ultrasound.

Figure 1

Chemical structures of (a) PAMPSP₄₄₄₈ and (b) PAMPSP₄₄₄₄.

Figure 2

¹H NMR spectra for (a) PAMPSNa in D₂O and (b) PAMPSP₄₄₄₈ in CDCl₃.

Figure 3

Percent transmittance (%T) at 800 nm for PAMPSP₄₄₄₈ in (a) pure water and (b) 0.1 M aqueous NaCl as a function of temperature at C_p = 5 g/L. The temperature was increased from 0 to 50 °C (heating; —) and decreased from 50 to 0 °C (cooling, ---) at 1 °C/min with stirring. PAMPSP₄₄₄₈ in pure water was sonicated for 10 min at 50 °C and then cooled.

Figure 4

(a) Digital photograph and (b) phase-contrast microscope images of the turbid aqueous PAMPSP₄₄₄₈ solution at C_p = 5 g/L after sonication.

Figure 5

(a) Light scattering intensity (LSI) for aqueous PAMPSP₄₄₄₈ solution as a function of temperature. (b) Hydrodynamic radius (R_h) distributions for unsonicated transparent aqueous PAMPSP₄₄₄₈ solutions at 5 and 50 °C.

Figure 6

TEM images of aqueous PAMPSP₄₄₄₈ solutions under (a) transparent and (b) turbid conditions.

Figure 7

Fluorescence spectra of pyrene excited at 334 nm in the presence of PAMPSP₄₄₄₈ in water for transparent aqueous solutions at 5 (---) and 50 °C (---), and a turbid aqueous solution at 50 °C (—).

Figure 8

Conceptual illustration of the ultrasound- and thermo-responsive behavior of PAMPSP₄₄₄₈ in water.