Influence of intraparticle cross-linking on polymer diffusion in latex films prepared from secondary dispersions

Abstract

In this work, we synthesized surfactant-free latex dispersions of nanoparticles (NPs) based upon emulsification of a preformed proprietary BASF polymer (MnGPC = 5000 g/mol, Ð = 3, denoted as Pol), in which the —COOH groups were partially neutralized by ammonia. To obtain useful coatings with polymers of such low molecular weight (MW), we approached this problem through the reaction of carboxylated polymer dispersions with neopentyl glycol diglycidyl ether (NGDE, 20 to 60 mol% based on carboxyl groups) to increase the MW and to introduce gel content. We employed AFM to monitor the influence of cross-link density on the surface roughness and found the presence of cross-links increases the resistance of particles to deformation. Fluorescence resonance energy transfer (FRET) measurements were carried out to determine the extent of polymer mixing upon film formation. Our most important finding is that the gel content substantially limited the extent of molecular mixing at long annealing times or in corresponding solvent-cast films. In parallel, we carried out a series of similar reactions on the secondary dispersion particles with a monoepoxide, consisting of a mixture of dodecyl and tetradecyl glycidyl ether. This reaction maintained the size and size distribution of the latex particles. These polymers had no gel content, but the reaction with the epoxide lowered the Tg of the polymer. FRET studies showed a pronounced influence of Tg on the rates of polymer diffusion in these films.