Tracer diffusion properties of core–shell latex films studied by photoinduced grating relaxation

This article reports the application of the Photo‐Induced Grating Relaxation technique (also known as Forced Rayleigh Scattering) to investigate the dynamics of films prepared from structured core–shell latex particles via the transport property of the photochromic tracer molecule Aberchrome 540®. The core–shell particles were prepared with a fluoropolymer core (immiscible and impenetrable to the tracer) and a poly(butyl methacrylate) shell. The incompletely dried films (with residual water) manifest their spatial heterogeneity via non‐Fickian behavior (spatial scale‐ dependent apparent diffusion coefficient). The diffusion data was interpreted using the two‐state diffusion model, previously developed to describe the tracer diffusion in latex films without any core–shell structure. In contrast to dry latex films made from homogeneous particles, where one observes Fickian diffusion indicative of a homogeneous polymer film, we find that the lattice of fluoropolymer cores leads to a length scale dependent diffusion coefficient for the tracer. This effect can be interpreted as microscopic evidence for a strain hardening effect due to the presence of a hardened layer of matrix polymer (= shell) surrounding the core, which act as nanofillers. This strain hardening effect could be quantified within the two‐state diffusion model in terms of tracer diffusion coefficients and root mean squared displacements. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2823–2834, 2007