Irreversible adsorption: new insights on molecular mobility, thermodynamics and interfacial interactions*

Irreversibly adsorbed polymer layers represent an intriguing class of novel nanomaterials with unexpected properties, strongly deviating from what observed in unbounded polymer melts. These extremely thin films are obtained via a small number of successive steps, easily reproducible: a polymer melt is placed in contact with an adsorbing substrate and nonadsorbed chains are washed away in a good solvent. Importantly, tuning the thickness of the adsorbed layer, an operational parameter equivalent to the number of chains adsorbed on a unit surface, allows modifying the interfacial free volume content and, hence, the performance of polymer coatings without affecting the interfacial chemistry.
Here, we discuss on the physics behind the formation of irreversibly adsorbed layers onto solid substrate. By analyzing the outcome of experiments and simulations, we show how changes in thermal energy and interaction potential affect the equilibrium and the nonequilibrium components of the kinetics. We verify that the monomer pinning mechanism is independent of surface coverage, while the progressive limitation of free sites significantly limits the adsorption rate. Importantly, in neat disagreement with current ideas on surface science, the equilibrium adsorbed amount – and, hence, interfacial interaction potential – is affected by nanoconfimenent (1).
Thinner films show a neat drop in adsorbed amount, implying a reduction in interfacial interactions between polymer and substrate upon confinement at the nanoscale level. We demonstrate that this phenomenon is totally ascribable to the smaller amplitude of the dispersive forces in thinner films, that can be parametrized via the Hamaker constant, and discuss on the impact of our new findings on previous experiments on multilayers systems.

(1) D. N. Simavilla et al. ACS Cent. Sci., 2018, 4 (6), pp 755–759