Effect of Magnesium Salts with Chaotropic Anions on the Swelling Behavior of PNIPMAM Thin Films

Abstract

Poly(N-isopropylmethacrylamide) (PNIPMAM) is a stimuli-responsive polymer, which in thin film geometry exhibits a volume-phase transition upon temperature increase in water vapor. The swelling behavior of PNIPMAM thin films containing magnesium salts in water vapor is investigated in view of their potential application as nanodevices. Both the extent and the kinetics of the swelling ratio as well as the water content are probed with in situ time-of-flight neutron reflectometry. Additionally, in situ Fourier-transform infrared (FTIR) spectroscopy provides information about the local solvation of the specific functional groups, while two-dimensional FTIR correlation analysis further elucidates the temporal sequence of solvation events. The addition of Mg(ClO4)2 or Mg(NO3)2 enhances the sensitivity of the polymer and therefore the responsiveness of switches and sensors based on PNIPMAM thin films. It is found that Mg(NO3)2 leads to a higher relative water uptake and therefore achieves the highest thickness gain in the swollen state.

Electrically Powered Dissipative Hydrogel Networks Reveal Transient Stiffness Properties for Out-of-Equilibrium Operations

“…By controlling polymer–solvent interactions through the formation of temporary cationic charges on a polymer backbone, Eelkema and co-workers have developed a strategy to program hydrogel swelling without affecting the net cross-linking density of the bulk network . On the other hand, hydrogels, as membranes or coatings of devices interfaced with biological systems, play a central role in a wide range of biomedical applications. , Considerable effort has been devoted to the development of hydrogel films that employ light, temperature, chemical agents, , or electricity as stimuli to trigger hydrogel networks and switch their stiffness properties between two equilibrium states. Among stiffness-switching hydrogels, those utilizing redox-responsive cross-linkers based on supramolecular complexes, such as ferrocene/cyclodextrin host–guest complexes, , exhibit rapid expansion/contraction transitions and can be precisely controlled electrochemically, resulting in high spatiotemporal resolution and essentially no chemical waste.…”

KCl-Modulated Hydration and Subsequent Thermoresponsive Behavior of Poly(sulfobetaine)-Based Diblock Copolymer Thin Films

“…Characteristically, the CP′ PSBP in an aqueous solution increases initially with sodium salt concentration up to a maximum value, while further addition of salt decreases the CP′ PSBP strongly, often below the freezing point of water. , So far, the salt effects on LCST PNIPMAM of the homopolymer PNIPMAM are not clear yet . Still, a few studies on the modulation of the transition temperatures of diblock copolymers (DBCs) containing PNIPMAM by added salt were reported, in which a long PSBP block was combined with a short PNIPMAM block. , It was found, that LCST PNIPMAM decreases slightly with increasing polymer concentration and does not show significant sensitivity toward the added NaBr.…”

“…58 In addition, the micellar size and shape both changes by varying the PNIPMAM block length. 60,61 Recently, we reported the swelling behavior of PSBP-b-PNIPMAM thin films as a function of temperature and salt (NaBr) content, where the polymer consisted of a long zwitterionic PSBP 245 and a comparably short non-ionic PNIPMAM 105 block. 62 For hydrated films, the addition of NaBr notably shifted the volume phase transition temperature (VPTT) of PSB to lower temperatures upon heating, whereas the VPTT of PNIPMAM remained virtually unaffected.…”

“…Thereby, three or even four different solubility regimes of the DBC were realized by the addition of electrolytes to tune the CP′ PSBP and the cloud point of PNIPMAM (CP PNIPMAM ), respectively . In addition, the micellar size and shape both changes by varying the PNIPMAM block length. , …”

“…58,59 So far, the salt effects on LCST PNIPMAM of the homopolymer PNIPMAM are not clear yet. 60 Still, a few studies on the modulation of the transition temperatures of diblock copolymers (DBCs) containing PNIPMAM by added salt were reported, in which a long PSBP block was combined with a short PNIPMAM block. 59,61 It was found, that LCST PNIPMAM decreases slightly with increasing polymer concentration and does not show significant sensitivity toward the added NaBr.…”