Influence of imidazolium based green solvents on volume phase transition temperature of crosslinked poly(N-isopropylacrylamide-co-acrylic acid) hydrogel

Abstract

The volume phase transition temperature (VPTT) of crosslinked poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) hydrogel in water in the presence of five imidazolium based ionic liquids (ILs) was studied. The VPTT of PNIPAM-co-AA hydrogel can be modulated to different extents by the addition of different amounts of ILs. The modulations in VPTT values can be attributed to the IL-induced alterations in hydrophobic, hydrophilic and hydrogen bonding interactions of PNIPAM-co-AA hydrogel with the neighboring solvent and molecular chains. The influence of ILs having a common cation, 1-butyl-3-methylimidazolium cation ([Bmim]) and different anions, such as iodide (I-), tetrafluoroborate (BF4-), chloride (Cl-), acetate (CH3COO-) and hydrogen sulfate (HSO4-), on the phase transition of PNIPAM-co-AA hydrogel was monitored by the aid of differential scanning calorimetry (DSC), dynamic light scattering (DLS) and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, the interfacial properties between aqueous IL and polymer surface were scrutinized with the help of contact angle (CA) measurements. The overall specific ranking of ILs in preserving the hydration layer around the PNIPAM-co-AA hydrogel in water was [Bmim][I]>[Bmim][BF4]>[Bmim][Cl]>[Bmim][Ac]>[Bmim][HSO4]. The trend of these ILs followed the well-known Hofmeister series. Interestingly, the PNIPAM-co-AA hydrogel in water shows abnormal salting-out property in the presence of [Bmim][BF4] at higher concentration and this abnormal behavior can be explained based on the lack of sufficient binding sites on the macromolecule for higher number of [Bmim][BF4] at a higher concentration.

Volume phase transition mechanism of poly[oligo(ethylene glycol)methacrylate] based thermo-responsive microgels with poly(ionic liquid) cross-linkers

“…Besides, since water molecules have preferential interactions with hydrophilic ILs, the hydration of polymers in aqueous solution may be destabilized 32 Therefore, the introduction of hydrophilic IL additives may weaken the thermo-responsive behavior of polymers and depress the LCST. [32][33][34][35] Since IL monomers are chemically polymerized into polymer networks in the present study, the strong interaction between the PIL moiety and water molecule exactly promotes the swelling behavior of microgels and increases the transition temperature. Considering the influence of the crosslinking extent on the thermal property of LCST-type gels, it is observed that gel systems with higher crosslinking degrees lead to higher LCST with a broader transition process, due to the more constrained environment of molecular motions.…”

“…In fact, when IL additives are involved in LCST-type polymer based systems, the temperature induced phase transition behaviors can be altered by the complicated interactions between polymer, ILs and water, which have been investigated in detail. [31][32][33][34][35] However, as far as we know, there are few studies that have focused on the thermal phase behavior of POEGMA based systems involved with IL or PIL, and the impact of PIL moieties as polymerized constituents on the dynamic volume phase transition mechanism of thermo-responsive gel systems has seldom been discussed.…”

Overcharging and reentrant condensation of thermoresponsive ionic microgels

“…It is well-known that the volume phase transition is determined by the hydrophobic interactions within the PNiPAM molecule. Indeed, many studies have shown that the VPTT is modified by the addition of inorganic salts, 18 surfactants, 16,19 ionic liquids, 20,21 alcohols, 22 and urea. 17,23 Besides that, the VPTT and swelling/deswelling behavior are also modified by the introduction of charged groups (e.g., carboxyl, sulfonic, and amino group) into the PNiPAM microgel network.…”

Exploring the Behavior of Bovine Serum Albumin in Response to Changes in the Chemical Composition of Responsive Polymers: Experimental and Simulation Studies

“…Synthetic polymers, particularly stimuli responsive polymers, have received extensive attention and opened a broad field of potential applications in bioimaging, drug delivery, tissue engineering and bioactive surfaces [1,2,3,4,5], due to their extraordinary response to changes in temperature, pH, ionic strength and enzymes [6,7,8,9,10,11,12,13,14]. Among hundreds of polymers, poly( N -isopropylacrylamide) (PNIPAM) and its derivatives have widely been applied in protein science [15,16,17,18].…”