Irreversible Adsorption Governs the Equilibration of Thin Polymer Films

Phys. Rev. Lett. 119, 097801

Confined but not forever

Molecules move faster as they get closer to adhesive surfaces, but this effect is not permanent. Since more than 20 years, several researchers have been studying the behavior of certain polymers, biomolecules, and liquid crystals at the nanoscale near an absorbing medium. In this case we would expect slower movement rates, but the experiments showed the opposite: molecules move faster as they get closer to an adhesive surface. We explain this odd movement via a

‘nanoconfinement effect’: the molecules that are in direct contact with the adhesive surface do move slower, or even not at all, but this in turn increases the movement rate of the next molecules, as they have more free space around them.

In this paper, we show that this effect is only temporary: movement ra

te gradually slows down as new molecules adhere to the surface and fill in the spaces left. After a while, molecules move as if they were far from the adhesive surface. Importantly, the time necessary to return to normal molecular movement rate is longer than what would be predicted by any current theory of polymer physics. As a result, we propose that the amount of available space at the interface between polymer and sticky wall is an important parameter to control the performance of nanomaterials.

Characterization of Adsorbed Polymer Layers: Preparation, Determination of the Adsorbed Amount and Investigation of the Kinetics of Irreversible Adsorption

David Nieto Simavilla et al  

Macromolecular Chemistry and Physics, in press

Understanding adsorbed layers

The increasing number of devices containing soft materials components of nanometric size –as for example for applications in flexible electronics, biomedical and tissue engineering – has focused the interest of researches towards thin polymer films and their interaction with solid substrates.
A remarkable finding is that, the properties of polymer/substrate interfaces differ greatly from those predicted from the bulk properties of each material. Here, we deal with those deviations from bulk behavior induced by the formation of layer of polymer molecules, as thin as a few nanometers, which irreversibly stick on a solid substrate. Understanding the physics behind the formation of these “irreversibly adsorbed layers” is necessary to control a large number of material properties: from the friction among molecules (viscosity) to the way such thin coatings expand with temperature, and also to the amount of water these membranes can uptake, or the way molecules can selforganize and form crystals. This Trend paper provides an introduction to: 1) The preparation of systems where irreversible adsorption can take place; 2) The modeling describing the kinetics of formation of the sticky layer; 3) The most common methods to determine their thickness and 4) A brief perspective on future applications and research outlooks derived from the extraordinary properties of these promising nanomaterials.