Reentrant Swelling Behavior of Poly(N-isopropylacrylamide) Gel

Abstract

The equilibrium swelling curves of poly(N-isopropylacrylamide) gel are measured in four binary mixed solvent systems of water and organic alcohols. The organic alcohols chosen here are methanol, ethanol, 1-propanol, and 2-propanol, respectively. The re-entrant equilibrium swelling curves are obtained in these four binary mixed solvent systems of water and alcohols. The gel collapses into compact state when the concentration of alcohol is increased from water. The gel, then, swells again if the concentration of alcohol is increased further. The collapse transition of the gel occurs at the mole fraction of organic alcohol of X., = 0.04 except for the system of methanol and water in which the collapse transition of the gel occurs at X., = 0.14. We find that the re-swelling processes of the gel, in these binary mixed solvent systems, strongly depend on the species of the organic alcohols used.

Modeling Poly(N-isopropylacrylamide) Hydrogels in Water/Alcohol Mixtures with PC-SAFT

“…Swelling of PNIPAAm in aqueous solutions of methanol and ethanol at 20 °C depending on the alcohol mole fraction in the solution. Spheres (methanol) and squares (ethanol) represent experimental literature data of the degree of swelling (DoS) of capillary microgels. The lines show the respective modeling results of PNIPAAm weight fraction in the gel phase (Solid: ethanol.…”

Re-Entrant Conformation Transition in Hydrogels

“…The re-entrant volume phase transition was observed in several hydrophobically modified hydrogels in water-organic solvent (cosolvent), or water-linear polymer mixtures. These include, (a) cationic PAAm hydrogel in acetone-water mixtures [44], (b) PNIPAM hydrogels in aqueous solutions of methanol [35,36], ethanol [35,36], propanol [36] and low molecular weight poly(ethylene glycol)s (PEGs) [45][46][47][48], (c) poly(N,N-dimethylacrylamide) (PDMAA) hydrogels in aqueous solutions of acetone, dioxane, THF, and t-butanol [49], (d) poly(methacrylic acid) hydrogels in aqueous solutions of PEG of molecular weight 6000 g/mol [50], and (e) TBA/AAm copolymer hydrogels based on N-t-butylacrylamide (TBA) and acrylamide (AAm) in aqueous solutions of DMSO, ethanol, and acetone [51,52]. From [9] with permission from the American Chemical Society.…”

“…Moreover, replacing DMSO with ethanol as a cosolvent shifts the re-entrant transition to a lower cosolvent volume fraction φ, which is attributed to the stronger hydrogen-bonding between ethanol and water [52]. Miki et al investigated the re-entrant transition behavior of nonionic PNIPAM hydrogels in aqueous solutions of methanol, ethanol, 1-propanol, and 2-propanol [36]. In methanol-water mixtures, the deswelling transition occurs at a methanol mole fraction x c of 0.14 (φ = 0.27) while for all other water-cosolvent mixtures, this happens at x c = 0.04 (φ = 0.12-0.15).…”

“…By taking the charge density as an adjustable parameter, it was shown that the extent of volume changes during phase transition increases with increasing charge density while nonionic hydrogels undergo smooth swelling and deswelling transitions [ 9 ]. Several mechanisms have been proposed for the re-entrant behavior of hydrogels basing on the competitive attractive interactions between hydrophobically modified hydrophilic polymer, water, and cosolvent components, and selective enrichment of the cosolvent inside the hydrogel [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ]. The re-entrant volume phase transition was observed in several hydrophobically modified hydrogels in water–organic solvent (cosolvent), or water–linear polymer mixtures.…”

“…This condition facilitates the movement of the cosolvent molecules through the PDMAA network leading to a large volume change in PDMAA gels. Note that, although PDMAA hydrogels is in the swollen state over the entire range of DMSO, methanol, and ethanol concentrations, PNIPAM and TBA/AAm hydrogels undergo re-entrant transitions in these solvent mixtures [ 35 , 36 , 51 , 52 ]. This is attributed to the higher hydrophobicity of PNIPAM and TBA/AAm networks in comparison to the PDMAA network producing stronger polymer–cosolvent and polymer–polymer hydrophobic interactions facilitating the re-entrant phenomenon.…”