Overcoming confinement limited swelling in hydrogel thin films using supramolecular interactions

Area of Science:

• Materials Science
• Polymer Science
• Surface Science

Background:

• Thermoresponsive hydrogels exhibit a lower critical solution temperature (LCST), undergoing significant volume changes with temperature.
• Understanding thin film behavior is crucial for applications in soft coatings and responsive materials.
• Poly(N-isopropylacrylamide-stat-2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate) (NIPAAm-stat-FOSA) is a promising class of thermoresponsive polymers.

Purpose of the Study:

• To elucidate the thin film behavior of NIPAAm-stat-FOSA hydrogels.
• To investigate the influence of film thickness on LCST and temperature-dependent swelling.
• To understand the mechanisms governing swelling behavior in confined geometries.

Main Methods:

• Quartz Crystal Microbalance with Dissipation (QCM-D) monitoring.
• Spectroscopic Ellipsometry (SE) for thickness and swelling measurements.
• Analysis of hydrogel films with thicknesses ranging from 10 nm to 121 nm.

Main Results:

• LCST slightly increased (>3 °C) in thin films compared to bulk, with no statistically significant thickness dependence.
• Volumetric swelling in thin films was similar to bulk hydrogels, contrary to 1D Flory-Rehner predictions.
• Swelling ratio increased with decreasing film thickness, particularly below the LCST.
• Hydrophobic crosslink rearrangement under stress was identified as a mechanism for enhanced swelling in thin films.
• Significant hysteresis in swelling was observed upon temperature cycling.

Conclusions:

• NIPAAm-stat-FOSA hydrogel thin films exhibit unique swelling behavior attributed to adaptable hydrophobic crosslinks.
• The materials show limited thickness dependence in LCST and swelling, making them suitable for soft coatings.
• Facile processing via solution casting without additional crosslinking chemistry enhances their applicability.