Complex Salt Dependence of Polymer Diffusion in Polyelectrolyte Multilayers.

Area of Science:

• Polymer science
• Materials science
• Physical chemistry

Background:

• Polyelectrolyte multilayers (PEMs) are promising for various applications.
• Understanding polymer chain dynamics within PEMs is crucial but poorly understood.
• Previous studies lacked microscopic insights into polymer diffusion in PEMs.

Purpose of the Study:

• To investigate the single-molecule transport of poly-l-lysine (PLL) within PEMs.
• To analyze the effect of varying NaCl concentrations on PLL diffusion dynamics.
• To correlate microscopic diffusion with macroscopic PEM properties.

Main Methods:

• Total internal reflection fluorescence microscopy (TIRFM) for single-molecule observation.
• Utilizing fluorescently labeled poly-l-lysine (PLL) within a PLL/poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) PEM.
• Statistical analysis of PLL trajectories across a range of NaCl concentrations (0-2 M).

Main Results:

• PLL exhibited nonergodic, subdiffusive, and temporally anticorrelated motion under all conditions.
• Microscopic diffusion was 2-3 orders of magnitude faster than macroscopic measurements.
• PLL diffusion varied nonmonotonically with salt concentration, mirroring PEM swelling and viscoelastic trends.

Conclusions:

• Microscopic polymer dynamics in PEMs are significantly faster than previously thought.
• Salt concentration profoundly impacts PLL diffusion through complex, salt-dependent viscoelastic changes.
• These changes involve a balance between intermolecular binding and polymer chain conformation.