Waterborne two-component polyurethanes (WB 2K-PUs) represent an environmentally friendly alternative to solvent-borne coatings, with reduced use of volatile organic compounds (VOCs). These coatings still cannot achieve the outstanding performance of solvent-borne 2K-PU formulations. In many WB 2K-PU formulations, a key step involves coalescence between polyol latex particles and nanoparticles formed from a water-dispersible polyisocyanate (PIC). Interactions between the PIC and the polyol take place both in the aqueous medium and after application of the coating to a substrate. A deeper understanding of this mechanism should guide improvements in formulations to improve WB 2K-PU coating performance. Our long-term goal is to investigate the film formation mechanism of a series of different polyol nanoparticles with aqueous dispersions of PICs based on a hexamethylene diisocyanate (HDI) trimer; as a first step in this direction, we examined the nature of the nanoparticles formed by dispersing an oligo-ethylene glycol (OEG)-modified HDI trimer in water. Typical techniques for nanoparticle size characterization (dynamic light scattering (DLS) and capillary hydrodynamic fractionation chromatography, (CHDF)) led to misleading information. These techniques indicated the presence of uniform nanoparticles with a diameter of ca. 100 nm that persisted for days. In contrast, DOSY 1 H NMR gave a diameter of 20 nm. Nanoparticle tracking analysis (NTA) confirmed the presence of the larger particles, but showed that they represented only 18 wt % of the dispersion. Over time, the isocyanate groups in the particles reacted with water, and the nanoparticles evolved from liquid droplets to mechanically robust polyurea particles. At this stage, imaging by electron microscopy showed a predominance of 20 nm particles. The kinetics of this chemical transformation was monitored by Fourier transform infrared (FT-IR) spectroscopy. The reaction of the isocyanate groups followed pseudo-first-order kinetics with a half-life of 11 to 14 h.
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.
This paper aims to identify emerging evidence for endolymphatic sac surgery (ESS) in the treatment of Meniere's disease since the landmark study by Thomsen et al, published in 1998Thomsen et al, published in (conducted from 1981Thomsen et al, published in to 1989. Using the MEDLINE database (PubMed), a systematic review of the literature published from January 1990 to June 2014 was performed. We included all English-language, peer-reviewed randomised controlled trials (RCTs) and controlled studies. Single-arm cohort studies were included if the sample size was ≥ 90 with a response rate > 60%. Altogether, 11 studies fulfilled our inclusion criteria; one was an RCT, two were controlled trials and eight were single-arm cohort studies. There currently exists a low level of evidence for the use of ESS in the treatment of Meniere's disease. Further studies, in particular RCTs and/or controlled studies, are required to fully evaluate this modality. However, there are difficulties in designing a valid placebo and achieving adequate blinding of observers and investigators.
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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