Flattening of Latex Film Surface and Polymer Chain Diffusion

The extent of flattening of poly(butyl methacrylate) (PBMA) latex film surfaces was determined using the atomic force microscopy (AFM) technique and compared with the extent of polymer chain migration between adjacent particles in the film, which was determined by the nonradiative energy transfer (NRET) method. The film surface flattening was faster than the total chain migration between adjacent particles. This result indicates that the total chain migration between adjacent particles is not necessary for a complete flattening of latex film surfaces. In addition, we found that polymer chain movement inside the individual particles is necessary for latex film flattening to occur. AFM was used to compare the rate of film flattening for cross-linked and non−cross-linked PBMA and poly(methyl methacrylate) (PMMA) particles. The cross-linking of the polymer chain in the particles inhibits flattening or decreases its rate considerably. The internal viscosity of the particles is then so high that the interfacial surface tension between the polymer and air, which is the driving force for film flattening, becomes very weak compared with the resistance to deformation. Finally, AFM was also used to test the internal structure of core−shell latex particles made of a soft core (PBMA) and a hard shell (PMMA). As long as the annealing temperature is less than the Tg of the shell polymer there is no flattening of the film surface, even for temperatures at which the core of the particle is liquid like. Film flattening occurs only when the annealing temperature is greater than the shell polymer Tg. The rate of film flattening is then faster than the rate found for pure PMMA particles because of the presence of the PBMA core, which decreases the internal particle viscosity compared with the internal viscosity in pure PMMA particles.