Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Thermoresponsive behavior of poly[trialkyl-(4-vinylbenzyl)ammonium] based polyelectrolytes in aqueous salt solutions

A systematic method to induce thermoresponsive behavior for polycations with salts from the reversed Hofmeister series is introduced.

Enhanced Forward Osmosis Desalination with a Hybrid Ionic Liquid/Hydrogel Thermoresponsive Draw Agent System

Forward osmosis (FO) has emerged as a new technology for desalination and exhibits potentials for applications where reverse osmosis is incapable or uneconomical for treating streams with high salinity or fouling propensity. However, most of current draw agents in FO are salts and difficult to be recycled cost- and energy-effectively. In this work, we demonstrate a new and facile approach to efficiently recover water from the FO process with enhanced water purity by using a binary ion liquid/hydrogel system. The hybrid ion liquid/hydrogel draw solution system demonstrated in this work synergistically leverages the thermoresponsive properties of both the ionic liquid (IL) and hydrogel to improve the overall FO performance. Our findings corroborate that the hydrogel mitigates the water flux decline of the IL as the draw agent and provide a ready route to contiguously and effectively regenerate water from the FO process. Such a route allows for an efficient recovery of water from the draw solute/water mixture with enhanced water purity, compared with conventional thermal treating of lower critical solution temperature IL draw solute/water. Furthermore, hydrogels can be used in a continuous and readily recyclable process to recover water without heating the entire draw solute/water mixture. Our design principles open the door to use low-grade/waste heat or solar energy to regenerate draw agents and potentially reduce energy in the FO process considerably.

Rheopectic Gel Formation of Stimuli-Responsive Ionic Liquid/Water Mixtures

A new class of hydrophobic and polar ionic liquids was prepared by coupling hydrophobic tetraoctylphosphonium cation and polar phosphonate-derived anions. Mixtures of these ionic liquids and water showed lower critical solution temperature-type phase behaviour. Furthermore, these mixtures displayed thermoreversible, however, non-linear viscosity change despite their large content of water. The abrupt increase in the viscosity was explained by the occurrence of rheopectic gelation of the ionic liquid/water mixtures by external stimuli such as shear stress.

In-depth study of the phase separation behaviour of a thermoresponsive ionic liquid and a poly(ionic liquid) in concentrated aqueous solution

The temperature-induced phase transition behaviors of a thermoresponsive ionic liquid (tributylhexylphosphonium 3-sulfopropylmethacrylate, [P4,4,4,6][MC3S]) and its polymer (poly-tributylhexylphosphonium 3-sulfopropylmethacrylate, P[P4,4,4,6][MC3S]) have been investigated using DSC, optical microscopy, temperature-variable (1)H NMR, and FT-IR in combination with two-dimensional analysis methods, including perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2Dcos). We found that there exists a distribution gradient of water molecules in P[P4,4,4,6][MC3S] ranging from hydrophobic backbones to hydrophilic sulfonates. Linked together by covalent bonds, P[P4,4,4,6][MC3S] would form an "urchin-like" structure, which can improve its stability and strengthen the gradient distribution of water. Finally, 2Dcos was employed to elucidate the sequential order of chemical group motions during heating. It is concluded that both [P4,4,4,6][MC3S] and P[P4,4,4,6][MC3S] experience the anionic dominated phase transition process. Moreover, the driving force for the phase transitions is shown to be the dehydration of hydrophobic ester carbonyls.

Reversible water uptake/release by thermoresponsive polyelectrolyte hydrogels derived from ionic liquids

Thermoresponsive polyelectrolyte hydrogels show reversible water uptake/release.

Thermoresponsive polyelectrolytes derived from ionic liquids

In this review we summarise recent progress on the design, properties, and potential applications of ionic liquid-derived polyelectrolytes showing thermoresponsive phase behaviour after mixing with water or other organic solvents.