Length Scale Dependent Probe Diffusion in Drying Acrylate Latex Films

Abstract

Applying forced Rayleigh scattering to monitor the drying behavior of poly(n-butylmethacrylate-co-acrylic acid) dispersions (T > Tg + 5 K) via the diffusion of a hydrophobic dye, we find a characteristic length scale dependence of the tracer diffusion coefficient Dapp(Λ) (Λ ) 0.17-10 µm), which allows one to quantitatively describe the transition from a wet, inhomogeneous to a dry, homogeneous polymer latex film within a two-state diffusion model. Dapp(Λ) showed an enhancement of up to 2 orders of magnitude when increasing the length scale. These findings can be quantitatively rationalized within the two-state model assuming Fickian diffusion proceeding slowly in the latex cores and fast diffusion in a heavily plasticized, interfacial phase. From the water content dependence of the model parameters we conclude that the drying process proceeds first by exclusive water loss from the interfacial phase, while the properties of the particle cores remain unchanged. When the Λ dependence of D app(Λ) disappears, water withdraws also from the cores and a homogeneous polymer film forms. Our approach allows one to quantitatively follow property changes in the different compartments of a drying latex dispersion.

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

“…In the last decade, the application of noninvasive techniques, such as small angle neutron scattering, nonradiative energy transfer,1 and atomic force microscopy2, 3 to study latex film formation has provided valuable information on the structural changes and polymer dynamics in nascent films on microscopic length scales. For a better understanding of film formation and the concomitant ability to control and tailor film properties for specific applications, there is, however, a need for further experimental techniques capable of probing the transport properties of nascent films 7. This is true especially in the case of structured emulsions such as the core–shell type.…”

“…This is true especially in the case of structured emulsions such as the core–shell type. The application of tracer techniques7–9 allow, by a judicious choice of probe molecules, to essentially monitor the transport properties of one specific compartment of a nascent film (viz. polymer phase, water phase, interface) and can, thus, provide complementary information, e.g., on water loss and surfactant mobility or polymer diffusion.…”

“…polymer phase, water phase, interface) and can, thus, provide complementary information, e.g., on water loss and surfactant mobility or polymer diffusion. The photoinduced grating relaxation technique, better known as Forced Rayleigh Scattering (FRS) in polymer science (the notation used throughout this paper), which relies on probing the dynamics of a tracer molecule, has shown to be a powerful tool to investigate the local mobility distributions and microstructure in phase separated systems such as block copolymers,10 drying latex films, etc.,7 yielding information on diffusion and spatial inhomogeneities in polymer systems10–18 and drying latex dispersions 7–9. The transport properties in heterogeneous systems are particularly interesting, since the supramolecular structure imposes spatial constraints on a diffusing moiety, which in special cases can lead to anisotropic behavior 19, 20.…”

“…Thus, a fluorinated polymer formed of a poly(tetrafluoroethylene)‐copolymer was the obvious choice for the core as such particles are at available from industrial sources. For the shell, we used poly(butyl methacrylate) (PBMA), as homopolymer latexes of this material have become a convenient model system for studies of latex film formation1, 4 reported in our previous studies 7–9. Thus, the system allows for easy comparison with results on homopolymer latex film formation.…”

“…Thus, the system allows for easy comparison with results on homopolymer latex film formation. Our main interest was to see whether the two‐state diffusion model applied previously7 could be extended to a more complex geometry and whether the geometry (e.g., the shell thickness) is reflected in the model parameters. In addition, we wanted to see whether a strain hardening of the film forming shell material due to the presence of hard impenetrable core occurs and can be detected on molecular length scales.…”