Elastic properties and moisture response of polydopamine films and multilayers

Polydopamine (PDA) has the rare ability to attach to surfaces traditionally resistant to adhesion, making it a promising candidate for use as a coating. Membranes approximately 20 nm thick PDA were synthesized using cyclical voltammetry electropolymerization and used to produce four examined samples, comprising subsequently higher numbers of PDA stacked layers (from 1 to 4). The stacking of the membranes resulted in a few layered membranes with thicknesses of approximately 20-60 nm. Brillouin light scattering spectroscopy was utilized to investigate the samples' Young moduli and residual stress, determining their mechanical properties through the dispersion of GHz acoustic phonons. The samples exhibited substantial magnitudes of Young modulus, higher than for classical polymers, with the higher values retained in multilayer samples.
The freestanding PDA membranes' responses to varying concentrations of water vapor in the air wereobserved under an optical microscope for all samples. The experiments showed a reversible wrinkling/flattening of the membranes at different relative humidity values. A similar response to laser light, attributed to heat-induced water desorption, was investigated by a home-built setup that allowed for stroboscopic imaging of the membrane morphology with temporal resolution. The process was characterized by relaxation times, attributed to parts of the experiment when the laser light was turned on (flat) and off (wrinkled). The obtained actuation times indicate that the ultrathin membranes react to red laser light in millisecond timeframes, with the multilayer samples having longer relaxation times that remained within the same order of magnitude.

Acknowledgments
Z. K., A. K., M. P., and B. G. acknowledge the National Science Centre of Poland (NCN) for the OPUS grant UMO-2021/41/B/ST5/03038.

References
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