Probing the adsorption/desorption of amphiphilic polymers at the air-water interface during large interfacial deformations.

Hydrophobically modified polymers are good candidates for the stabilization of liquid interfaces thanks to the high anchoring energy of the hydrophobic parts. In this article we probe the interfacial anchoring of a series of home-made hydrophobically modified polymers with controlled degree of grafting by studying their behavior upon large area dilations and compressions. By comparing the measured interfacial tension to the one that we expect in the case of a constant number of adsorbed monomers, we are able to deduce whether desorption or adsorption occurs during area variations. We find that the polymer chains with the longest hydrophobic grafts desorb at larger compressions compared to the polymers with the shortest grafts, because of their larger desorption energy. Furthermore, for a given graft length, we observe more desorption for polymers with the highest grafting densities. We attribute this counter intuitive result to the fact that at high grafting densities, the length of the polymer loops is shorter, and hence the elastic penalty upon compression is larger for these layers, leading to a faster desorption. Comparing the elastic penalty to thermal energy, kBT, enables deducing a critical grafting density above which desorption of grafts is expected upon compression, which is consistent with our experimental results. In the case of large area dilations, the experiments reveal that the number of adsorbed anchors remains constant in the case of chains with a low grafting density while chains with the highest degree of grafting seem to show some degree of adsorption during the dilatation. Therefore, in these highly grafted chains there may be unadsorbed grafts remaining in the vicinity of the interface, which may adsorb quickly at the interface upon dilatation.

Influence of outer-layer finite-size effects on the dewetting dynamics of a thin polymer film embedded in an immiscible matrix.

In capillary-driven fluid dynamics, simple departures from equilibrium offer the chance to quantitatively model the resulting relaxations. These dynamics in turn provide insight on both practical and fundamental aspects of thin-film hydrodynamics. In this work, we describe a model trilayer dewetting experiment elucidating the effect of solid, no-slip confining boundaries on the bursting of a liquid film in a viscous environment. This experiment was inspired by an industrial polymer processing technique, multilayer coextrusion, in which thousands of alternating layers are stacked atop one another. When pushed to the nanoscale limit, the individual layers are found to break up on time scales shorter than the processing time. To gain insight on this dynamic problem, we here directly observe the growth rate of holes in the middle layer of the trilayer films described above, wherein the distance between the inner film and solid boundary can be orders of magnitude larger than its thickness. Under otherwise identical experimental conditions, thinner films break up faster than thicker ones. This observation is found to agree with a scaling model that balances capillary driving power and viscous dissipation with a no-slip boundary condition at the solid substrate/viscous environment boundary. In particular, even for the thinnest middle-layers, no finite-size effect related to the middle film is needed to explain the data. The dynamics of hole growth is captured by a single master curve over four orders of magnitude in the dimensionless hole radius and time, and is found to agree well with predictions including analytical expressions for the dissipation.

Adsorption dynamics of hydrophobically modified polymers at an air-water interface.

Using surface-tension measurements, we study the brush-limited adsorption dynamics of a range of amphiphilic polymers, PAAH-[Formula: see text]-[Formula: see text] composed of a poly(acrylic acid) backbone, PAAH, grafted with a fraction [Formula: see text] of alkyl moieties, containing either n = 8 or n = 12 carbon atoms, at pH conditions where the PAAH backbone is not charged. At short times, the surface tension decreases more sharply as the degree of grafting increases, while, at long times, the adsorption dynamics becomes logarithmic in time and is slower as the degree of grafting increases. This logarithmic behavior at long times indicates the building of a free-energy barrier which grows over time. To account for the observed surface tension evolution with the degree of grafting we propose a scenario, where the free-energy barrier results from both the deformation of the incoming polymer coils and the deformation of the adsorbed brush. Our model involves only two fitting parameters, the monomer size and the area needed for one molecule during adsorption and is in agreement with the experimental data. We obtain a reasonable value for the monomer size and find an area per adsorbed polymer chain of the order of 1 nm2, showing that the polymer chains are strongly stretched as they adsorb.

Molecular dynamics simulation of the capillary leveling of viscoelastic polymer films.

Surface tension-driven flow techniques have recently emerged as an efficient means of shedding light into the rheology of thin polymer films. Motivated by experimental and theoretical approaches in films bearing a varying surface topography, we present results on the capillary relaxation of a square pattern at the free surface of a viscoelastic polymer film, using molecular dynamics simulations of a coarse-grained polymer model. Height profiles are monitored as a function of time after heating the system above its glass-transition temperature and their time dependence is fitted to the theory of capillary leveling. Results show that the viscosity is not constant, but time dependent. In addition to providing a complementary insight about the local inner mechanisms, our simulations of the capillary-leveling process therefore probe the viscoelasticity of the polymer and not only its viscosity, in contrast to most experimental approaches.

Thermoelectricity and thermodiffusion in charged colloids.

The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle's entropy of transfer as high as 82 meV K(-1). Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.

A direct quantitative measure of surface mobility in a glassy polymer.

Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.