Kinetics of irreversible adsorption: thermodynamics versus molecular mobility

Abstract

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 layers (thickness < few tens of nanometers) are obtained via a small number of successive steps, easily reproducible in a laboratory environment: a polymer melt is placed in contact with an adsorbing substrate and nonadsorbed chains are washed away by soaking the sample 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 performance of polymer coatings without affecting the interfacial chemistry [1]. Here, we discuss on the physics behind the formation of irreversibly adsorbed layers onto solid substrate [2,3], highlighting the differences between measurements performed via dielectric spectroscopy and those via ellipsometry or atomic force microscopy. 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 identify a universal linear relation between the growth rates at short and long adsorption times, suggesting that the monomer pinning mechanism is independent of surface coverage, while the progressive limitation of free sites significantly limits the adsorption rate. We show that the equilibrium adsorbed amount is given by thermodynamics and depends on the interface interaction only (i.e. it is temperature independent in experiments). Importantly, in neat disagreement with current ideas on surface science, the equilibrium adsorbed amount – and, hence, interfacial interaction potential – is affected by nanoconfimenent [4].