ACS Central Science (2018) in the NEWS
Size Matters in Nanoconfinement
Can we predict if two materials will stick together? Theory tells us that the more contacts are formed among molecules of the two materials placed one next to the other, the stronger will be their adhesion. Unfortunately, counting the number of contacts between two materials is a difficult task. To overcome this obstacle, it is possible to virtually reconstruct the interface between two materials and, using approximate equations, to estimate the force keeping them together. This procedure, currently used at both industrial and academic level, seems to work very well when the materials used are in the macroscale.
The outcome is not as good upon reduction of material dimension to the nanoscale (1 nm is one billionth of a meter, one single of your hairs can contain from 20000 to 200000 nm!).The origin of this discrepancy is because of a set of forces, known as van der Waals (vdW) forces, that depend on the dimension of the objects involves. These forces are extremely important, also in everyday life. vdW forces are for example those that permit the transparent packaging foil to stick to food and protect it from air, or also those allowing geckos to walk on flat walls as if they had dipped their tiny paws into glue.
Differently than covalent forces, responsible for strong chemical bonds, vdW forces take place over much larger distances, up to few hundreds of nanometers. Due to such a large length scale, vdW forces are affected by nanoconfinement, that is, their intensity changes if the size of the objects fit in boxes of nanometric dimension.
In this paper we show that it is possible to estimate how nanoconfinement affects the number of contacts formed by two materials placed in intimate contact. They considered wafers of silicon, as those largely used in microelectronics, coated by thin polymer layers of different thickness. The currently used approximate methods predict that the interaction between the two materials does not depend on the thickness of the polymer layer. On the contrary our team showed that size does matter. Molecules at the interface of thinner films form less contacts with the silicon wafer, because the vdW forces are weaker.
The method used permitted to verify a striking correlation between the intensity of the vdW forces and the number of contacts. This result shows that the current way we think at interfaces is not valid. In addition to the huge impact at the level of fundamental science, our results could be exploited on a large number of applications. Since almost a decade, several research groups have shown that properties of many thin coatings – such as flow, the ability to retain or be repel water, the velocity of formation of crystals – depend on the number of contacts between the film and its supporting substrate. Till now, to modify this number it was necessary to change the type of molecules at the interface, often involving
complex chemical reactions.
Our findings of Simavilla show that it is possible to tailor the performance of nanomaterials by simply changing their dimensions. Or even without! Placing a different material on top of the polymer layer in contact with the substrate, affects in a controllable way the vdW forces at the interface between polymer of given thickness and the substrate. This method, hence, allows controlling the polymer layer without touching it, as by using a remote control.
Phys. Rev. Lett. (2017) in the NEWS
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 (2017)
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.