Kinetics of Irreversible Chain Adsorption

Abstract

Recent experimental evidence showed a strong correlation between the behavior of polymers under confinement and the presence of a layer irreversibly adsorbed onto the supporting substrate, hinting at the possibility to tailor the properties of ultrathin films by controlling the adsorption kinetics. At the state of the art, however, the study of physisorption of polymer melts is mainly limited to theory and simulations. To overcome this gap, we present the results of an extensive investigation of the kinetics of irreversible adsorption of entangled melts of polystyrene onto silicon oxide. We show that the process of chain pinning proceeds via a first order reaction mechanism, which slows down at large surface coverage, and the adsorbed amount scales with the predictions of reflected random walk. We propose an analytical form of the time evolution of the thickness of the adsorbed layer with two well-defined regimes: linear at short times and logarithmic at longer times, separated by a temperature independent crossover thickness and a molecular weight independent crossover time, in line with simulations and theory.

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

“…The number of available monomers is established by the monomer density and the number of chains making a contact with a flat wall per unit surface ( N −1/2 ) . The number of monomers adsorbed per units of surface at any given time is thus proportional to N 1/2 , while the chain adsorption rate is molecular weight independent …”

“…After drying, the residual adsorbed layer is finally ready for investigation. Nowadays a large number of different experimental techniques, ranging from ellipsometry to atomic force microscopy, X‐rays scattering, fluorescence and dielectric spectroscopy, have been employed to analyze the intriguing properties of these extremely thin films.…”

Unexpected Molecular Weight Effect in Polymer Nanocomposites

“…This annealing corresponds to 4 × 10 14 τ α , which by far exceeds the longest equilibration time for polystyrene thin films of a similar MW [44]. However, such strong annealing did not produce any detectable changes in the dielectric spectra or their temperature dependence (Fig.S4).…”

“…We see two possible reasons for this unexpected behavior: (1) PNCs require long annealing time to reach equilibrium [43,44]; or (2) the polymer chains experience a strong frustration in chain packing in the interfacial region due to the formation of large loops at higher MW. Recent studies of supported thin polymer films revealed a strong MW dependence of the equilibrium desorption times.…”

“…12 τ α for chains with MW~325 kg/mol [43,44]. To verify the desorption time effect, we annealed the PVAc253K-20% at T = 160 o C (τ α~3 ns) for an additional 15 days (t ann~1 .3×10 6 s).…”