Marion Grzelka, Iurii Antoniuk, Éric Drockenmuller, Alexis Chennevière, Liliane Léger, and Frédéric Restagno

The role of the polymer volume fraction, ϕ, on steady-state slippage and interfacial friction is investigated for semi-dilute polystyrene solutions in diethyl phthalate in contact with two solid surfaces. Significant slippage is evidenced for all samples, with slip lengths b obeying a power law dependence. Navier’s interfacial friction coefficient, k, is deduced from the slip length measurements and from independent measurements of the solution viscosity η. The observed scaling of k versus ϕ clearly excludes a molecular mechanism of friction based on the existence of a depletion layer. Instead, we show that the data of η(ϕ) and k(ϕ) are understood when taking into account the dependence of the solvent friction on ϕ. Two models based on the friction of blobs or of monomers on the solid surface well describe our data. Both point out that Navier’s interfacial friction is a semi-local phenomenon.

https://doi.org/10.1021/acs.macromol.0c02804

Alessio Zaccone and Laurence Noirez, J. Phys. Chem. Lett.12 (2021) 1, 650–657.

Liquids confined to sub-millimeter scales have remained poorly understood. One of the most striking effects is the large elasticity revealed using good wetting conditions, which grows upon further decreasing the confinement length, L. These systems display a low-frequency shear modulus in the order of 1–10³ Pa, contrary to our everyday experience of liquids as bodies with a zero low-frequency shear modulus.

While early experimental evidence of this effect was met with skepticism and abandoned, further experimental results and, most recently, a new atomistic theoretical framework have confirmed that liquids indeed possess a finite low-frequency shear modulus G′, which scales with the inverse cubic power of confinement length L. We show that this law is universal and valid for a wide range of materials (liquid water, glycerol, ionic liquids, non-entangled polymer liquids, isotropic liquids crystals). Open questions and potential applications in microfluidics mechanochemistry, energy, and other fields are highlighted.

https://doi.org/10.1021/acs.jpclett.0c02953.

Andrew M. Jimenez, Dan Zhao, Kyle Misquitta, Jacques Jestin and Sanat K. Kumar

Understanding the structure and dynamics of the bound polymer layer (BL) that forms on favorably interacting nanoparticles (NPs) is critical to revealing the mechanisms responsible for material property enhancements in polymer nanocomposites (PNCs). Here we use small angle neutron scattering to probe the temporal persistence of this BL in the canonical case of poly(2-vinylpyridine) (P2VP) mixed with silica NPs at two representative temperatures. We have observed almost no long-term reorganization at 150 °C (∼T_g,P2VP + 50 °C), but a notable reduction in the BL thickness at 175 °C. We believe that this apparently strong temperature dependence arises from the polyvalency of the binding of a single P2VP chain to a NP. Thus, while the adsorption–desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a process that occurs over a seemingly narrow temperature range.

https://doi.org/10.1021/acsmacrolett.8b00877

The development and implementation of micro- and bio-electronic devices often need the deposition of layers of organic substances on conductive or semiconducting surfaces. In that aim, surface chemical reactions (or grafting) are very effective. The miniaturization of electronic components requires the realization of a very localized grafting, at the micron and even sub-micron scale.

To graft locally a molecule onto a surface, the macroscopic surface of a substrate is usually exposed to very small quantities of solution. The localization can be also assisted by a light beam or a catalyst. Most of these techniques require a multiple step processes and their implementation is usually tedious and expensive. In this context, we show that it is possible through an original approach to make a local grafting in one single step, requiring no masking technique and by light and low cost technologies.

We have shown recently [1-4] that it was already possible to decorate, in one step and locally, by an organic film (electro-grafting of vinyl monomers), the surface of a sample with different zones of conductivity (Au / Si, Si / Si-doped). The spatial selectivity of the deposit is obtained simply in that case by playing on the imposed bias voltage, to promote the electronic transfer (and thus the grafting of the polymer) to one surface to the exclusion of another. The predefined pattern is perfectly respected, with a lateral resolution limited only by the thickness of the film (a few nm to few hundred nm).

At a time when we question the fossil fuel reserves of our planet and the consequences of the greenhouse effect on global warming, hydrogen is seen as the future energy vector for transportation. Research within CEA cover all stages of this chain: production, storage, transport, distribution and use. In that field, hydrogen produced from primary energy, solar, nuclear, wind, chemical ... is stored in the tank of a vehicle and a fuel cell, allowing the clean conversion (without CO₂ emission) of chemical energy into electrical energy, combined with an electric engine can replace the gasoline engine of our cars.

Among the various types of fuel cells suitable for transport applications, the most interesting are of PEMFC type (Proton Exchange Membrane Fuel Cell). These fuel cells contain, in particular, a polymer playing the role of the solid electrolyte. The Dupont De Nemours company sells a sulfonated perfluorocarbon membrane named Nafion^®. However, this membrane presents some drawbacks as a mediocre autonomy (< 5000h operating), a mechanical weakness or the inability to function in anhydrous media… The "Irradiated Polymer" research group within LSI is trying to address these problems by proposing a new type of membrane.