Roughness and salt annealing in a polyelectrolyte multilayer.

Area of Science:

• Materials Science
• Surface Chemistry
• Polymer Science

Background:

• Polyelectrolyte multilayers (PEMs) are versatile thin films with applications in coatings, drug delivery, and separation technologies.
• Understanding and controlling the surface roughness of PEMs is crucial for optimizing their performance in various applications.
• Previous studies have explored factors influencing PEM growth, but the precise mechanisms governing surface roughness development remain an area of active research.

Purpose of the Study:

• To investigate the impact of deposition conditions on the surface roughness of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) polyelectrolyte multilayers.
• To elucidate the relationship between film thickness, termination layer, and observed surface roughness.
• To explore the effect of post-deposition annealing on PEM surface morphology and stability.

Main Methods:

• Fabrication of PDADMAC/PSS polyelectrolyte multilayers under varying deposition conditions.
• Surface roughness characterization using techniques such as atomic force microscopy (AFM) for dry films and in situ imaging under electrolyte.
• Annealing experiments in salt solutions to assess the impact on film morphology and stability.

Main Results:

• Significant surface roughness develops in thicker PDADMAC/PSS films, particularly those terminated with PSS.
• In situ imaging reveals roughness in PDADMAC-terminated films, which is less apparent in dry states.
• Annealing in salt solutions substantially reduces roughness without causing polyelectrolyte loss, even above the glass transition temperature.
• Roughness wavelength correlates with film thickness, supporting a mechanism involving anisotropic swelling similar to hydrogels.
• Roughness is preserved by the glassy PSS layer and integrated during film growth.

Conclusions:

• The surface roughness of PDADMAC/PSS PEMs is strongly dependent on film thickness and termination layer.
• Anisotropic swelling due to water and polyelectrolyte uptake is a likely mechanism driving roughness development.
• Annealing offers a viable strategy to mitigate surface roughness in PEMs without compromising film integrity.
• The findings provide insights into controlling PEM morphology for tailored applications.