Designing functional stimuli responsive hydrogel for emerging environmental application

Abstract

The growing demand for safe drinking water and increasing water pollution has prompted extensive research into developing innovative and sustainable water technologies. This grand objective warrants global and multidisciplinary efforts which include designing water processes from a material. The aim of this research is to design and develop stimuli-responsive hydrogels that can enable an energy-efficient separation process for water treatment. Stimuli-responsive hydrogels are promising materials contributed by their tailorable physicochemical properties. The considerable amount of water absorbed by them can be subsequently released under external stimuli such as changes in pH, pressure, temperature, light, etc. This enables facile regeneration of the hydrogel functionality at the end of the process. Nevertheless, their widespread applications are often limited by the extent of triggered changes in physiochemical properties and rate of response to external stimuli. In this work, I have used two different approaches to specifically address these issues. First, to optimize the amplitude of physicochemical change using a semi-IPN hydrogel system, and second, to enhance the rate of response using microgel systems. Semi-IPN hydrogel, formed by dispersing a hydrophilic polymer within a crosslinked polymeric network, displays considerable changes in physiochemical properties without sacrificing the rate of response. These hydrogels can be modified to enable multiple functionalities by loading specific polymers, nanoparticles, and fillers during in situ polymerization of the hydrogel network. However, the stability of these hydrogel composites has often been overlooked as possible leaching of the dispersed component can severely affect the recyclability and performance of the material. In this study, thermally responsive poly(N-isopropylacrylamide) (PNIPAM) and poly(sodium acrylate) (PSA) semi-IPN hydrogels with a high swelling rate and water uptake capacity were synthesized to systematically study the effect of the PSA retention within the PNIPAM matrix. By controlling the synthesis conditions, i.e., polymerization temperature, PSA concentration, and molecular weight (Mw), a semi-IPN hydrogel with a swelling ratio of up to 260 g/g was obtained; it can release more than 70% of the absorbed water when heated above 50 °C. On increasing the crosslinking density from 0.5 to 2 mol%, the swelling rate was found to have doubled, following non-Fickian diffusion. This observation was elucidated by correlating to microscopic morphology. The application of such semi-IPN hydrogels as self-regenerating water drawing agents in a forward osmosis (FO) process was also demonstrated. PNIPAM/PSA semi-IPN hydrogel can also be used to selectively reject or adsorb specific molecules by surface modification i.e., forming a skin layer. However, poor adhesion and delamination of the skin layer are often observed due to a difference in swelling ratio resulting from the crosslinking density gradient. By utilising branched polyethyleneimine (PEI) as a monomer to react interfacially with trimesoyl chloride (TMC), a highly robust polyamide (PA) skin layer on the PNIPAM/PSA semi-IPN hydrogel surface is formed, resulting from the non-covalent interaction of PEI with PSA. By controlling the PEI Mw, concentration, pH of the aqueous monomer solution during interfacial polymerization, surface morphology, porosity, thickness, and permeability of the skin layer on the hydrogel surface can be controlled. The PA/hydrogel composite bead can undergo 100 cycles of swelling (15°C) and deswelling (50°C) without any delamination or significant difference in rejection of methylene blue. The composite beads were easily regenerated by swelling and deswelling the hydrogel in DI water based on the volume phase transition temperature (VPTT), whereby the weakly adsorbed molecules on the surface could be washed down along with the released water. Besides incorporating PSA in the PNIPAM hydrogel matrix, the hydrogel network properties were also modified by incorporating carbon dots (CDs) during in-situ polymerization. The incorporation of CDs not only resulted in morphological change of the hydrogel but also enhanced the mechanical strength and salt loading capacity. We then studied the interaction of water molecules with the polymer and different bonding states of water in the modified PNIPAM network after the incorporation of CD and GO. The PNIPAM/CDs hydrogel composite was then used as support to disperse LiCl within the network to study the moisture absorption behaviour. The composite can absorb moisture at room temperature without showing deliquescence at 80% RH. The absorbed moisture can then be directly released under solar irradiation, however, relatively slow sorption kinetics was observed.

As swelling and deswelling of hydrogels is diffusion limited, microgels of several hundred nanometre in diameter were synthesized and studied in terms of their fast kinetics, rate of response, and increased accessibility to functional groups. Stimuli-responsive microgels based on copolymerization of PNIPAM with either sodium acrylate (SA) or 1-vinyl imidazole (VIM), can result in negative and positively charged microgels, respectively. Surfactant-free emulsion polymerization was used for the synthesis. Although the thermoresponsive behaviour of microgels is well reported in the literature, we designed a binary microgel system that exhibits thermoreversible heteroaggregation under specific mixing conditions combining positive and negatively charged microgels. The results show the interplay of both electrostatic and steric interactions whereby a well-dispersed microgel system at room temperature can be phase-separated from the solution by heating to 60 °C in less than 2 minutes, which can then be redispersed by gentle stirring at room temperature without any pH adjustment or ultrasonication. The high number of available surface functional groups when coupled with fast swelling and deswelling kinetics can also be explored as adsorbents. Lastly, a multi-stimuli responsive poly(2-(dimethylamino)ethyl methacrylate) microgel (PDMAEMA) microgel was synthesized that is both temperature and pH-responsive. The high charge density of the protonated amine functional groups resulted in the selective removal of perfluorooctanoic acid (PFOA) with a high absorption capacity from industrial wastewater containing organic solvents. A pH shift from 3 to 11 caused PFOA release, while a temperature shift from 25 to 50 °C made it easier to remove and reuse suspended microgels. In conclusion, the different types of thermally responsive hydrogels and microgels designed and developed in this study have immense potential for sustainable water treatment contributed by the facile regeneration.