Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties

Area of Science:

• Polymer Science
• Materials Science
• Surface Chemistry

Background:

• Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermoresponsive polymer.
• Surface-attached polymer networks offer unique properties compared to bulk materials.
• Controlled synthesis of polymer films is crucial for understanding structure-property relationships.

Purpose of the Study:

• To investigate the temperature-responsive behavior of surface-attached PNIPAM network films.
• To explore the influence of film thickness and surface attachment on PNIPAM hydrogel swelling.
• To characterize the morphology and density profiles of the hydrogel films.

Main Methods:

• Synthesis of PNIPAM hydrogel films via thiol-ene click chemistry.
• Fabrication of films with controlled thickness (nanometers to micrometers) and cross-link density.
• Characterization using ellipsometry, neutron reflectivity, and atomic force microscopy.

Main Results:

• Sharp, high-amplitude swelling-collapse transitions observed for submicrometric films.
• Above the lower critical solution temperature (LCST), films collapse but retain water.
• Swelling below LCST is thickness-dependent, with ultrathin films ( < 150 nm) showing reduced swelling due to surface attachment.
• Thicker films (> 150 nm) exhibit uniform swelling regardless of thickness.

Conclusions:

• Surface-attached PNIPAM hydrogels demonstrate tunable thermoresponsive behavior.
• Film thickness and surface interactions play a critical role in swelling dynamics, especially for thin films.
• The observed morphology suggests the presence of pendant chains at the free surface, influencing film properties.