We recently described a new approach to surfactant-free latex dispersions prepared by emulsification of a partially neutralized linear acrylic acid-containing styrene-acrylic copolymer without the use of an organic solvent [Polym. Chem. 2017, 8, 2931–2941]. These secondary dispersions consisted of a polymer with a molecular weight (M n = 5000 g mol–1, D̵ = 3) too low to form a useful coating. In the work reported here, we built up the molecular weight of the polymer and introduced partial gel content via a reaction in the dispersed state with a bisepoxide, followed by a reaction with a monoepoxide to reduce the acid number. We monitored the reaction with bisepoxide with T 2-filtered 1H NMR to separate signals of highly mobile small molecules from those of less mobile polymer-bound species. Fluorescence resonance energy transfer measurements were carried out both to monitor polymer exchange among nanoparticles (NPs) as the dispersions in water were annealed and to measure the extent of polymer mixing upon film formation. Our most important finding was that the reaction with the monoepoxide reduced polymer T g and, to a limited extent, promoted polymer interdiffusion during film formation. We also found that the presence of cross-links in the NPs limited the extent of polymer mixing that can be achieved when the polymers were dissolved in a good solvent and cast as a film. The combination of cross-linking and molecular weight build-up strongly reduced the propensity for molecular exchange in the dispersed state.

We report experiments based upon fluorescence resonance energy transfer (FRET) measurements designed to examine mixing at the molecular level of the components of a waterborne 2K polyurethane (WB2KPU) formulation. The system consists of an acrylic polyol latex (M n = 4200 g/mol, D̵ = 2, T g ≈ 15 °C) with a uniform hydrodynamic diameter (d h ) ≈ 120 nm plus a water-dispersible polyisocyanate (hmPIC, Basonat HW1000 from BASF). We prepared components labeled with phenanthrene (Phen) as the donor dye or with a dimethylaminobenzophenone (Nben) as the acceptor dye. Dynamic light scattering was used to monitor the size and size distribution of the components in the dispersed phase in solution. This signal was dominated by the polyol nanoparticles, which were much larger than the tiny droplets formed by the hmPIC in water. Experiments were carried out at a mole ratio of NCO/polyol-OH of 1.3. We found that the particle size and narrow size distribution remained unchanged up to 22 h after mixing the polyol with the PIC. FRET experiments were carried out on samples in the dispersed state as well as on films formed from these dispersions. Films formed from a 1:1 mixture of (polyol-Phen + polyol-Nben) showed relatively little energy transfer (Φ ET = 0.19) even after several hours aging at ambient temperature, indicating that little polymer diffusion occurred in these low-molecular-weight latex films. In contrast, films formed from mixtures of (polyol-Phen + polyol-Nben + hmPIC) showed more extensive energy transfer (ET) (Φ ET = 0.51), indicating essentially complete mixing at the molecular level of the polymer molecules in the presence of hmPIC. The key conclusion is that hmPIC is a reactive plasticizer that promotes diffusion in this system on a much faster scale than the cross-linking reaction. This result is confirmed by experiments that examined mixtures of (hmPIC-Phen + polyol-Nben), which also showed essentially molecular scale mixing between these two different components. In this later system, aging at room temperature led to a small decrease in Φ ET over time that was more prominent for films aged at high humidity (75%) than at lower humidity (45%). This result suggests that hydrolysis of NCO groups in the film, leading to polyurea formation, promotes local phase separation accompanied by a net increase in the average separation of Phen and Nben groups in the film.

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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While the interaction of NCO with carboxylic acids has been documented, a comprehensive knowledge of its effect on the film formation in a two-component waterborne polyurethane (2K-WPU) system is lacking. In this work, we synthesized a series of carboxylfunctional acrylic polyols (M n ≈ 5000 g/mol, Đ ≈ 2.7, T g ≈ 15 °C) of uniform diameter (120 nm) with varying amounts of methacrylic acid introduced into the latex. For 2K-WPU dispersions with a molar NCO/ OH ratio of 1.3:1, we monitored particle size by dynamic light scattering and NCO consumption by FTIR. In the dispersion, we found that the particle size remained nearly constant, and the NCO was almost completely consumed after 2 days. Incorporating COOH in the polyol latex accelerated the consumption of NCO. We synthesized polyol samples labeled with donor and acceptor dyes for fluorescence resonance energy transfer (FRET) studies of polymer diffusion and molecular mixing. In films formed from the polyol latex in the absence of hmPIC, the COOH content of the polyols had no significant effect on the rate or extent of polymer diffusion. In contrast, in films formed from the 2K-WPU dispersions with a stoichiometric NCO/OH ratio (1.3:1), the extent of ET rapidly increased to the maximum value, showing that the full extent of molecular mixing of the two components can be accomplished in films. Thus, the hmPIC acts as a reactive plasticizer. In parallel experiments in which the hmPIC was labeled with the donor dye, we found evidence for partial mixing of the PIC and polyol in the dispersed state. We also showed that the rate of mixing of the hmPIC and the polyol in the films cast from these dispersions was accelerated by −COOH groups in the polyol.