Interphase of a Polymer at a Solid Interface

Abstract

Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymer−solid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film. ■ INTRODUCTIONPolymer−solid interfaces are at the center of intensive research due to their importance in polymer coatings, hybrid materials, lubrication, adhesion, etc. The polymer properties close to this interface are of paramount importance for the performance of these composite systems. From the viewpoint of fundamental science, the confinement imposed by a solid substrate on the polymer should affect the polymer's glass transition temperature, and this has led to a number of studies.Various properties of polymers confined in thin films, be it supported or free-standing, differ from their bulk properties. In the case of supported thin films, an interphase between the substrate and the bulk phase of the polymer is postulated, and the width of this interphase layer has been the focus of many studies. Confinement effects on polystyrene thin films and their implications for the polymer glassy dynamics were studied by molecular dynamics (MD) simulation. 1 For freestanding polymer films, the glass transition temperature in the boundary layer near the gas phase was found to be lower than in the bulk. 2 On a completely smooth, structureless, solid substrate modeled by a truncated 9−3 Lennard-Jones potential three layers with different density and glass transition behavior substrate, middle, surfacehave been shown in MD simulations. 3 MD simulations of polystyrene on gold, 4,5 polybutadiene on graphite, 6 and polyethylene on graphite 7 have shown the existence of a higher-density interface layer of a width of about 2 nm. The dynamical properties of polymers near an interface have also been probed using MD simulations. 4,6,8 Experimental work about polymers at the interface claim the existence of e.g. a "dead layer" or "Guiselin brushes" 9 of a few nanometers at the interface as permanently attached polymer chains where even washing with a good solvent cannot remove this layer again. Using gold nanoparticles as markers in a X-ray photon correlation (XPCS) experiment, 10 an irreversibly adsorbed layer with a surface reduced viscosity (compared to bulk viscosity) layer altering the dynamics up to a distance of approximately 20 nm from the substrate was postulated as interpretation of their results. With the recently developed technique resonance enhanced dynamic light scattering (REDLS), experiments on polybutadiene on gold 11 showed a slowing-down in dynamics, and hence an increase in viscosity goi...

Unexpected Molecular Weight Effect in Polymer Nanocomposites

“…Even larger change in density of the interfacial layer is expected assuming l int~2 .5 nm (Fig.3a), consistent with SAXS data (Table S2). These results are consistent with recent PALS measurements that revealed an increase of density in a supported oligomeric polymer thin film [45] and an increase in free volume in high MW PNCs [16]. The reduction of the density in the PVAc/SiO 2 PNC is also confirmed by the analysis of the specific volume data published in Ref.…”

Free Volume Profiles at Polymer–Solid Interfaces Probed by Focused Slow Positron Beam

“…This value for the interphase width (∼12 nm) is significantly higher than that determined from molecular dynamics (MD) simulations. 29 Part of this deviation might be explained by the diffusional broadening of the positron implantation profile, although the typical diffusion distance is generally assumed to be around 2 nm. 10,11 Density is mass per volume, and the volume is sum of the volume occupied by the atoms, the interstitial volume, and the "free volume".…”

Interphases in Polymer Solid-Contacts and Nanocomposites Probed by Positron Annihilation Lifetime Spectroscopy

“…Thus, measurements were performed, where the analysis of the interphase region is performed with a focused beam from the substrate side through a hole in the substrate. Therein, a more detailed analysis is possible and underway but beyond the scope of the present paper and will be published separately (43) Similar experiments have also been performed with Polystyrene on Au samples and will be discussed separately in connection with corresponding MD simulations (44).…”