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High-solid-content polystyrene and polyvinyl acetate dispersions of polymer particles with a 50 nm to 500 nm mean particle diameter and 12–55% (w/w) solid content have been produced via emulsion polymerization and characterized regarding their optical and physical properties. Both systems have been analyzed with common particle-size-measuring techniques like dynamic light scattering (DLS) and static light scattering (SLS) and compared to inline particle size distribution (PSD) measurements via photon density wave (PDW) spectroscopy in undiluted samples. It is shown that particle size measurements of undiluted polystyrene dispersions are in good agreement between analysis methods. However, for polyvinyl acetate particles, size determination is challenging due to bound water in the produced polymer. For the first time, water-swelling factors were determined via an iterative approach of PDW spectroscopy error (Χ2) minimization. It is shown that water-swollen particles can be analyzed in high-solid-content solutions and their physical properties can be assumed to determine the refractive index, density, and volume fraction in dispersion. It was found that assumed water swelling improved the reduced scattering coefficient fit by PDW spectroscopy by up to ten times and particle size determination was refined and enabled. Particle size analysis of the water-swollen particles agreed well with offline-based state-of-the-art techniques.

The scale-up process of the high solid content (up to 67 wt%) emulsion polymerisation of vinyl acetate and Versa®10 from 1 L over 10 L to 100 L was investigated. An emulsion copolymerisation of vinyl acetate and neodecanoic acid vinyl ester in a molar ratio of 9:1 was carried out in a starved-fed semi-batch operation. As a radical source, a redox initiator system consisting of L-ascorbic acid, tert-butyl hydroperoxide and ammonium iron (III) sulphate was used. The process parameters, such as the required stirring speed and heat dissipation, were determined and adjusted beforehand via reaction calorimetry to ensure a successful scale-up without safety issues. In addition, the emulsion polymerisation was monitored inline by Raman (qualitative monomer accumulation), as well as Photon Density Wave spectroscopy (particle size and scattering coefficient) and temperature measurements. The data provided by Raman spectroscopy and temperature measurements revealed mixing difficulties due to an insufficient stirring rate, while the inline measurement with Photon Density Wave spectroscopy offered an insight into the development of the product properties. It proved to be reliable and precise throughout the entire scale-up process, especially compared to conventional offline methods, such as dynamic light scattering or sedimentation analysis by means of a disc centrifuge, both of which encountered issues when using higher polymer contents.

The high solids semicontinuous emulsion polymerization of polyvinyl acetate using poly (vinyl alcohol-co-vinyl acetate) as protective colloid is investigated by optical spectroscopy. The suitability of Photon Density Wave (PDW) spectroscopy as inline Process Analytical Technology (PAT) for emulsion polymerization processes at high solid contents (>40% (w/w)) is studied and evaluated. Inline data on absorption and scattering in the dispersion is obtained in real-time. The radical polymerization of vinyl acetate to polyvinyl acetate using ascorbic acid and sodium persulfate as redox initiator system and poly (vinyl alcohol-co-vinyl acetate) as protective colloid is investigated. Starved–feed radical emulsion polymerization yielded particle sizes in the nanometer size regime. PDW spectroscopy is used to monitor the progress of polymerization by studying the absorption and scattering properties during the synthesis of dispersions with increasing monomer amount and correspondingly decreasing feed rate of protective colloid. Results are compared to particle sizes determined with offline dynamic light scattering (DLS) and static light scattering (SLS) during the synthesis.

Photon Density Wave (PDW) spectroscopy is used as process analytical technology (PAT) in three batch sizes, 1 L, 10 L and 100 L, of polyvinyl acetate—neodecanoic acid vinyl ester (Versa® 10) copolymerization. The effects on particle formation and growth are comparably analyzed. The data show comparability across scales up to a polymer volume fraction of around 0.15. Deviations beyond this suggest differences in particle growth dynamics. A detailed analysis of the dispersion dynamics and mixing properties provides an enhanced understanding compared to previous studies. Furthermore, the PDW spectroscopy data suggest inhomogeneity due to insufficient mixing at the beginning of the syntheses, despite very low feed-rates of the monomer mixture. PDW spectroscopy is thus capable of monitoring deviations in syntheses at different reaction volumes in real-time. These findings underline the potential of PDW spectroscopy not only for monitoring synthesis but also for enabling inhomogeneity analysis as a new application area. The integration of offline conversion and particle size measurements emphasizes the critical role of mixing efficiency in achieving optimal polymer dispersion properties and final product quality.

Turbidity measurements are frequently implemented for the monitoring of heterogeneous chemical, physical, or biotechnological processes. However, for quantitative measurements, turbidity probes need calibration, as is requested and regulated by the ISO 7027:1999. Accordingly, a formazine suspension has to be produced. Despite this regulatory demand, no scientific publication on the stability and reproducibility of this polymerization process is available. In addition, no characterization of the optical properties of this calibration material with other optical methods had been achieved so far. Thus, in this contribution, process conditions such as temperature and concentration have been systematically investigated by turbidity probe measurements and Photon Density Wave (PDW) spectroscopy, revealing an influence on the temporal formazine formation onset. In contrast, different reaction temperatures do not lead to different scattering properties for the final formazine suspensions, but give an access to the activation energy for this condensation reaction. Based on PDW spectroscopy data, the synthesis of formazine is reproducible. However, very strong influences of the ambient conditions on the measurements of the turbidity probe have been observed, limiting its applicability. The restrictions of the turbidity probe with respect to scatterer concentration are examined on the basis of formazine and polystyrene suspensions. Compared to PDW spectroscopy data, signal saturation is observed at already low reduced scattering coefficients.

Polyhydroxyalkanoates (PHAs) are biodegradable plastic-like materials with versatile properties. Plant oils are excellent carbon sources for a cost-effective PHA production, due to their high carbon content, large availability, and comparatively low prices. Additionally, efficient process development and control is required for competitive PHA production, which can be facilitated by on-line or in-line monitoring devices. To this end, we have evaluated photon density wave (PDW) spectroscopy as a new process analytical technology for Ralstonia eutropha (Cupriavidus necator) H16 plant oil cultivations producing polyhydroxybutyrate (PHB) as an intracellular polymer. PDW spectroscopy was used for in-line recording of the reduced scattering coefficient µs’ and the absorption coefficient µa at 638 nm. A correlation of µs’ with the cell dry weight (CDW) and µa with the residual cell dry weight (RCDW) was observed during growth, PHB accumulation, and PHB degradation phases in batch and pulse feed cultivations. The correlation was used to predict CDW, RCDW, and PHB formation in a high-cell-density fed-batch cultivation with a productivity of 1.65 gPHB·L−1·h−1 and a final biomass of 106 g·L−1 containing 73 wt% PHB. The new method applied in this study allows in-line monitoring of CDW, RCDW, and PHA formation.

The coil-to-globule transition of poly( N -isopropylacrylamide) (PNIPAM) microgel particles suspended in water has been investigated in situ as a function of heating and cooling rate with four optical process analytical technologies (PAT), sensitive to structural changes of the polymer. Photon Density Wave (PDW) spectroscopy, Focused Beam Reflectance Measurements (FBRM), turbidity measurements, and Particle Vision Microscope (PVM) measurements are found to be powerful tools for the monitoring of the temperature-dependent transition of such thermo-responsive polymers. These in-line technologies allow for monitoring of either the reduced scattering coefficient and the absorption coefficient, the chord length distribution, the reflected intensities, or the relative backscatter index via in-process imaging, respectively. Varying heating and cooling rates result in rate-dependent lower critical solution temperatures (LCST), with different impact of cooling and heating. Particularly, the data obtained by PDW spectroscopy can be used to estimate the thermodynamic transition temperature of PNIPAM for infinitesimal heating or cooling rates. In addition, an inverse hysteresis and a reversible building of micrometer-sized agglomerates are observed for the PNIPAM transition process.

In turbid biogenic liquid material, like blood or milk, quantitative optical analysis is often strongly hindered by multiple light scattering resulting from cells, particles, or droplets. Here, optical attenuation is caused by losses due to absorption as well as scattering of light. Fiber-based Photon Density Wave (PDW) spectroscopy is a very promising method for the precise measurement of the optical properties of such materials. They are expressed as absorption and reduced scattering coefficients ( μ a and μ s ′, respectively) and are linked to the chemical composition and physical properties of the sample. As a process analytical technology, PDW spectroscopy can sense chemical and/or physical processes within such turbid biogenic liquids, providing new scientific insight and process understanding. Here, for the first time, several bioprocesses are analyzed by PDW spectroscopy and the resulting optical coefficients are discussed with respect to established mechanistic models of the chosen processes. As model systems, enzymatic casein coagulation in milk, temperature-induced starch hydrolysis in beer mash, and oxy- as well as deoxygenation of human donor blood were investigated by PDW spectroscopy. The findings indicate that also for very complex biomaterials (i.e., not well-defined model materials like monodisperse polymer dispersions), obtained optical coefficients allow for the assessment of a structure/process relationship and thus for a new analytical access to biogenic liquid material. This is of special relevance as PDW spectroscopy data are obtained without any dilution or calibration, as often found in conventional spectroscopic approaches. Graphical Abstract Green Photon Density Wave created at the end of an optical fiber in beer mash, as new analytical tool for the in-line monitoring of (bio)chemical processes

The effects of photosynthetic photon flux density (PPFD) fluctuations in sunlight have already been investigated; however, the spectral photon flux density distribution (SPD) has hardly been considered. Here, sunlight SPD fluctuations recorded for 200 min in October in Tokyo, Japan were artificially reproduced using an LED-artificial sunlight source system. The net photosynthetic rate ( P n ) of cucumber leaves under reproduced sunlight was measured and compared with the P n estimated from a steady-state PPFD– P n curve for the same leaves. The measured and estimated P n agreed except when the PPFD was low, where the measured P n was lower than the estimated P n . The ratio of measured P n to estimated P n was 0.94–0.95 for PPFD ranges of 300–700 μmol m –2 s –1 , while the value was 0.98–0.99 for 900–1,300 μmol m –2 s –1 , and the overall ratio was 0.97. This 3% reduction in the measured P n compared with the P n estimated from a steady-state PPFD– P n curve was significantly smaller than the approximately 20–30% reduction reported in previous experimental and simulation studies. This result suggests that the loss of integral net photosynthetic gain under fluctuating sunlight can vary among days with different fluctuation patterns or may be non-significant when fluctuations in both PPFD and relative SPD of sunlight are taken into consideration.

Realizing nonlinear optical response in the low photon density limit in solid-state systems has been a long-standing challenge. Semiconductor microcavities in the strong coupling regime hosting exciton-polaritons have emerged as attractive candidates in this context. However, the weak interaction between these quasiparticles has been a hurdle in this quest. Dipolar excitons provide an attractive strategy to overcome this limitation but are often hindered by their weak oscillator strength. The interlayer dipolar excitons in naturally occurring homobilayer MoS 2 alleviates this issue owing to their formation via hybridization of interlayer charge transfer exciton with intralayer B exciton. Here we demonstrate the formation of dipolar exciton polaritons in bilayer MoS 2 resulting in unprecedented nonlinear interaction strengths. A ten-fold increase in nonlinearity is observed for the interlayer dipolar excitons compared to the conventional A excitons. These highly nonlinear dipolar polaritons will likely be a frontrunner in the quest for solid-state quantum nonlinear devices. Dipolar excitons enable large nonlinear interaction but are usually hampered by their weak oscillator strength. Here, the authors demonstrate the strong light-matter coupling of interlayer dipolar excitons having unusually large oscillator strength in bilayer MoS 2 resulting in highly nonlinear dipolar polaritons.