Polydopamine layer formation at the liquid – gas interface

The surface properties of a polydopamine layer at the air-water interface were studied by dilatational surface rheology, ellipsometry and Brewster angle microscopy (BAM). A significant increase of the dynamic surface elasticity was discovered when the concentration changed from 0.75 g/l to 2 g/l with the maximum value of about 60 mN/m at a concentration of 1 g/l. The obtained results indicate that the surface film consists of separate domains and the high surface elasticity is a consequence of the interactions between relatively rigid domains of the polymer film. This conclusion was confirmed by Brewster angle microscopy, which demonstrated different steps of the polydopamine film growth. Separate domains appeared at the first step while one can observe a continuous film close to equilibrium. An increase of the initial concentration led to faster polymerization and to the formation of a thicker film. The dynamic surface elasticity decreased in the concentration range from 2 g/l to 5 g/l when the thickness of the polymer film reached about 80 nm. In this case the film could be destroyed in the course of deformation. The cracks in the film resulted in a decrease of the dynamic surface elasticity.