In a previous publication [Macromolecules 2019, 52, 5245− 5254], we described the synthesis of surfactant-free latex dispersions of nanoparticles (NPs) based on emulsification of a preformed proprietary BASF polymer (M n GPC = 5000 g/mol, D̵ = 3), in which the −COOH groups were partially neutralized by using ammonia. The NPs in these dispersions were then partially cross-linked with neopentyl glycol diglycidyl ether (NGDE) to increase the molecular weight, followed by reaction with monoglycidyl ether to reduce the acid number and lower the glass transition temperature (T g ). In the work reported here, we used fluorescence resonance energy transfer (FRET) measurements to examine polymer diffusion rates in the films formed from these dispersions. We compared films formed from the uncross-linked NPs, with ones containing the NPs partially cross-linked with NGDE but not reacted with the monoepoxide. In this way, both the cross-linked and noncross-linked polymers had similar T g values. We also examined films formed from a similar polymer with M n GPC = 4000 g/mol, D̵ = 3. Because of the high T g of these polymers (ca. 65 °C), films were formed on heated substrates, and this led to skin formation at the film surface. We used FRET measurements to monitor the extent of polymer diffusion at both the film−air (F−A) and film−substrate (F−S) interfaces. We found that the onset of polymer diffusion occurred more rapidly within the skin at the F−A interface at elevated temperatures, but this was quickly surpassed by polymer diffusion at the bottom of the film because of the hydroplasticization effect. The presence of the skin layer retarded water evaporation and extended the time needed for the efficiency of energy transfer to reach its plateau value. We also found that the extent of chain diffusion in the partially cross-linked (XL) films was reduced compared to the non-XL samples because of limited interdiffusion of the polymer that formed the gel content. Dynamic mechanical analysis was employed to investigate the viscoelastic behavior of the samples using time−temperature superposition to generate master curves. We calculated apparent activation energies in the temperature range of the FRET experiments that were consistent with the strong dependence of polymer diffusion rates on the difference between the annealing temperature and glass transition temperature.

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