An Applied Approach to Film Formation The Glass Transition Temperature Evolution of Plasticized Latex Films
Voyutskii was the first to suggest that autohesion or polymer diffusion was necessary to develop mechanical properties.7 He referred to autohesion as a mechanism that leads to the healing of weak particle boundaries. Indirect evidence for this type of molecular re-arrangement or polymer diffusion has been obtained. Studies by electron microscopy show that after latex particles compact and densify, the evolving cells continue to change, leading eventually to a continuous, smooth film.2,8 Surface-force microscopes such as atomic force microscopy, first applied to latex films in 1992 showed that a film surface flattens with aging.9
During the 1980s and '90s, direct measurement of the diffusion of polymer chains in latex films became possible through the application of two spectroscopic techniques: small angle neutron scattering (SANS) and direct non-radiative energy transfer (DET). Hahn et al.10 and Sperling et al.11 monitored the diffusion of deuterated poly(butyl methacrylate) and deuterated polystyrene, respectively, using SANS. More recently, Winnik and coworkers at the University of Toronto12 developed the DET method for diffusion measurements of latex polymers. These techniques allow one to correlate the diffusion of polymer molecules across the intercellular boundaries in a latex film with the development of the mechanical strength of the film. In fact, estimates for the amount of interparticle volume fraction of mixing, i.e., film formation, were obtained.