The adaptation of a cylindrical geometry fracture toughness test specimen to testing of polymeric dental biomaterials is demonstrated. The specimen configuration facilitates the fabrication of small specimens and simplifies the experimental study of environmental effects on properties of dental biomaterials. The test method is used to measure critical stress intensity factor, KIC for poly (methyl methacrylate) (PMMA), particulate composite restorative materials, and dental polymers. Values obtained are in agreement with data reported for other specimen configurations.

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.

We examine the nature of the chemical reactions taking place in a waterborne two‐component polyurethane formulation consisting of an acrylic polyol latex and a hydrophilically modified polyisocyanate (hmPIC) based on the trimer of hexamethylene diisocyanate. The hmPIC was diluted with 30 wt.% of an organic solvent to reduce its viscosity and formed small (~20 nm) droplets when dispersed in water. In mixtures of the polyol and hmPIC, we monitored the rate of NCO group disappearance in the dispersed state by FTIR and showed that it varied with the choice of organic solvent. We developed a method based upon 19F NMR to distinguish the reaction of the NCO groups with OH groups from the polyol from its reaction with water. FTIR measurements on films formed from these dispersions showed how the disappearance of NCO groups depended on relative humidity. In a more semi‐quantitative way, these measurements indicated the relative extent of urethane versus urea formation in these model coatings.

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.

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