Anion effects on the phase transition ofN‐isopropylacrylamide hydrogels

Abstract

NMR spectra were collected for poly(N‐isopropylacrylamide) (PNIPAAm) hydrogel using high‐resolution magic angle spinning (HRMAS) after gel pieces were hydrated in the presence of D2O, NaF, NaCl, and NaI aqueous solutions. Changes in the peak height intensity of the spectra provide quantitative insight into the phase transition process. The thermodynamic values of the phase transition were calculated using a van't Hoff analysis of the NMR data. Unlike the trend observed for decreases in the (LCST), changes in the enthalpy and entropy did not clearly display a linear dependence with respect to salt concentration. Rather, it was observed that increases in salt concentration did not affect the enthalpy and entropy to the extent as the initial change observed between no salt and 100 mM solutions. Finally, the effect of salts on the hysteresis of the rehydrating process was observed. Hysteresis occurs due to the need for hydrophobic interactions to break down before water is able to infiltrate the polymer matrix. NaF stabilizes hydrophobic interactions while NaI destabilize hydrophobic interactions, causing them to break down at higher temperatures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Cation Effects on the Phase Transition ofN‐isopropylacrylamide Hydrogels

“…This agrees with the Hofmeister series and also gives the ability to predict LCST values from the equations in Figure , developed from our quantitative NMR data. Compared with our previous work, the trends in LCST show that cations do not impact the PNIPAM phase transition as greatly as anions. Altering cations at a given concentration results in smaller ranges of LCST values than varying anions.…”

“…Poly( N ‐isopropylacrylamide) (PNIPAM) hydrogels with N , N′ ‐methylene‐bis‐acrylamide (MBA) crosslinker were analyzed using variable temperature high‐resolution magic angle spinning (HRMAS) NMR spectroscopy. This method has been described previously . The samples were heated or cooled in 0.2 °C increments.…”

“…If the effect of Na + = Mg 2+ = Ca 2+ , the LCST of 100 × 10 −3 mol MgCl 2 and 100 × 10 –3 mol CaCl 2 should be much lower, ≈28 °C because they contain 200 × 10 –3 mol of Cl − ions. Instead, the LCST is ≈29 °C because the Mg 2+ and Ca 2+ allow for NIPAM hydration to remain stable and counteract destabilization from the chloride anions . In this manner, Mg 2+ and Ca 2+ ions are functioning as kosmotropic ions to stabilize the hydrated NIPMAM polymer network.…”

“…When studying the phase transition of PNIPAM hydrogel, temperature‐dependent peak intensity data can be used to calculate equilibrium constants which, in turn, can be used to calculate thermodynamic parameters of the phase transition . In previous work, PNIPAM gel swollen in a series of salts with varying anion species was studied with HRMAS‐NMR . While the effect of various alkali metal cations was studied by Annaka et al, the effect of divalent cations has yet to be investigated.…”

Mechanics of inhomogeneous large deformation of photo-thermal sensitive hydrogels

“…The phase transition temperature is dependent on several factors, including the crosslink density of the polymer network (Harmon et al, 2003), the chemical potential of the solvent (Pastoor and Rice, 2012), and the intensity of light irradiation. In the present study, we investigate the effects of varying light intensity on the free swelling state of poly(N-isopropylacrylamide) (PNIPAm) using the parameters listed in Table 1.…”