Structure of the Interfacial Region in Adsorbed Poly(vinyl acetate) on Silica

Decoupling energetic modifications to diffusion from free volume in polymer/nanoparticle composites

Diffusion coefficients of small molecules in a model composite of spherical nanoparticles and polymer with attractive interfacial interactions are reduced from that in the pure polymer, to a degree far below the level expected from geometric tortuosity arguments. We determine whether such dramatic reductions are due to modifications to the matrix polymer free volume near the nanoparticle surface, or alternatively are due to energetic attractions between the diffusants and nanoparticle surface. We performed ethyl acetate sorption experiments within the vicinity of the polymer glass transition (T_g ≤ T ≤ T_g + 25 K) for a model polymer/nanoparticle composite, silica-filled poly(methyl acrylate). By application of the Vrentas-Duda free volume theory of diffusion we have decoupled the energetic effects from those related to free-volume and segmental dynamics. While the latter is unaffected by addition of nanoparticles, the energy needed for the ethyl acetate diffusant to overcome neighboring attractive forces doubles after adding 40 vol% nanoparticles with a diameter of 14 nm. This is qualitatively consistent with hydrogen bonding interactions between the silica surface and ethyl acetate slowing its rate of diffusion. On the other hand for benzene, which does not hydrogen bond to the silica surface, diffusion coefficients that can be explained by tortuosity effects were obtained. This work provides quantitative evidence that the diffusant-filler energetic interactions and geometric blocking effects can be fully responsible for the substantially reduced diffusivity commonly observed in polymer/nanoparticle composite systems.

Effect of Polymer/Solid and Polymer/Vapor Instantaneous Interfaces on the Interfacial Structure and Dynamics of Polymer Melt Systems

Polymers are used in a wide range of applications that involve chemical and physical processes taking place at surfaces or interfaces which influence the interaction between the polymer material and the substance that comes into contact with it. Polymer surfaces are usually modified either chemically or physically for specific applications such as facilitating wetting, reducing friction, and enhancing adhesion. The variety and complexity of surface and interfacial processes requires a molecular-level understanding of the structural and dynamical properties of the surface/interface layer to help in the design of materials with desired functional properties. Using molecular dynamics (MD) simulations, we investigate the structure and dynamics at the surface of polymer films. We find that the density profiles of the films as a function of distance relative to an instantaneous surface have a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. However, the interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface.