Werner Pauer

Ultrasmall Biocompatible Nanocomposites: A New Approach Using Seeded Emulsion Polymerization for the Encapsulation of Nanocrystals

We report a novel approach of seeded emulsion polymerization in which nanocrystals are used as seeds. Ultrasmall biocompatible polymer-coated nanocrystal with sizes between 15 and 110 nm could be prepared in a process that avoids any treatment with high shear forces or ultrasonication. The number of nanocrystals per seed, the size of the seeds, and the shell thickness can be independently adjusted. Single encapsulated nanocrystals in ultrasmall nanobeads as well as clusters of nanocrystals can be obtained. Polysorbat-80 was used as surfactant. It consists of poly(ethylene glycol) (PEG) chains, giving the particles outstanding biofunctional characteristics such as a minimization of unspecific interactions.

Modeling of the Suspension Polymerization Process Using a Particle Population Balance

On the basis of a population balance and the kinetic mechanism of free-radical suspension polymerization, a mathematical model of the suspension polymerization process is proposed. The population balance model which describes a mechanism involving the particle size distribution (PSD) in disperse systems leads to an integrodifferential equation. The basic numerical approach of this work is to use the finite-difference-differential technique with the logarithmic scale for particle size. The problem then was reduced to obtaining the numerical solution of a set of nonlinear ordinary differential equations. The numerical solutions were compared to experimental data, such as the reaction conversion which includes the gel effect and the particle size distributions, to yield the model parameters by Marquardts method. The regression demonstrates reasonably good accuracy.

Macroporous uniform azide- and alkyne-functional polymer microspheres with tuneable surface area: synthesis, in-depth characterization and click-modification

A series of uniform, macroporous poly(styrene-co-divinylbenzene) microspheres with diameters ranging from 6.6 ± 0.6 to 8.6 ± 0.2 μm was prepared in a multistep procedure involving precipitation polymerization synthesis of polystyrene seed particles, swelling of seed particles with plasticiser and porogen, and polymerization of styrene–divinylbenzene (S–DVB) inside the seed particles. Particles prepared with varying DVB feed ratios had comparable diameters (as evidenced by scanning electron microscopy) with specific surface areas increasing with DVB content from 11 to 467 m2 g−1 (measured by nitrogen adsorption). Residual double bonds were converted into azide functionality (through HBr addition and bromo-azide substitution) or alkyne functionality (Br2 addition followed by double elimination) which allowed for CuAAC-click chemistry conjugation with reagents carrying the respective complimentary alkyne or azide functional groups including the fluorescent dye derivatives 7-nitro-4-(prop-2-ynylamino)benzofuran (NBD-alkyne) and Rhodamine B hexylazide synthesised for this purpose. Efficiency of chemical transformations was determined using a combination of CHN and IC elemental analyses, solid state NMR spectroscopy, FT-IR spectroscopy, Raman spectroscopy, and confocal scanning fluorescence microscopy. Although the respective second steps in each modification route (substitution and elimination) suffered from lower yields (∼35%), porous particles with azide loadings of up to 0.71 mmol g−1 and alkyne loadings of up to 0.78 mmol g−1 were prepared. Confocal laser scanning microscopy on Rhodamine B-labelled microspheres indicated functionalization throughout the particles featuring a core–shell structure with higher functionalization in the outer layer of particles. Results are expected to contribute to the development of advanced, well-defined, macroporous particles with high, chemically accessible surface areas.

Synthesis and in-depth characterization of reactive, uniform, crosslinked microparticles based on free radical copolymerization of 4-vinylbenzyl azide

The introduction of functional groups into microparticles is commonly accomplished through, at times, low-yielding post-synthesis modification. In this detailed study, the introduction of azide functionality into uniform, crosslinked, macroporous microparticles through direct copolymerization of styrene, divinylbenzene (DVB), and 4-vinylbenzyl azide (VBA) in varying ratios inside swollen polystyrene seed (template) particles is investigated. Formulations contained up to 40 wt% of VBA in the monomer mixture. Resulting microspheres were characterised by SEM, porosimetry, FT-IR spectroscopy, and CHN elemental analysis. Uniform spherical particles with diameters ranging from 7.3 to 10.8 μm with diameter dispersities typically below 1.01 and with tuneable azide loadings from 0.11 to 1.17 mmol g−1 were obtained. Interestingly, severe effects of VBA addition on porosity, surface smoothness, and particle shape were observed. Specific surface areas and cumulative pore volumes increased with the amount of DVB in feed, decreased with increasing VBA feed ratio, and increased drastically for the use of azide-functional template particles with measured cumulative pore volumes reaching up to 0.60 cm3 g−1. With increasing VBA feed, formation of smaller, secondary particles was observed and attributed to an incomplete swelling of VBA into seed particles, which is discussed as a main reason for lower-than-expected azide contents in product particles. For high VBA feed ratios (>25 wt%), dented, hollow, or hollow collapsed azide-functional particles were found, presumably due to immiscibility of the growing azide-functional copolymer with the polystyrene seeds. Finally, successful click-modification is demonstrated with phenylacetylene and an alkyne-functional Rhodamine B dye allowing for mapping of functionalization density through confocal fluorescence microscopy.

Two in one: use of azide functionality for controlled photo-crosslinking and click-modification of polymer microspheres

Spherical, micrometer-sized, azide-functional particles were produced through dispersion copolymerization of styrene and vinylbenzyl azide (VBA, 1–100 wt% of monomer feed) in ethanol in the presence of stabilizers. The obtained microspheres were characterized by SEM, disc centrifuge, FT-IR and NMR spectroscopy, elemental analysis, DSC, and TGA, had measured azide loadings of up to 5.58 mmol g−1, and average diameters that decreased with increasing azide content from 2.8 to 0.8 μm. Microspheres were irradiated at a wavelength of 254 nm resulting in crosslinking based on azide-to-nitrene decomposition and subsequent C–H insertion and CC addition reactions. The conversion of azide functionality was monitored by FT-IR spectroscopy, elemental analysis, and DSC and was found to roughly follow first-order kinetics with increased rates found for microspheres with lower azide contents. Photo-crosslinking preserved shapes and size distributions and, above a crosslinking degree of 10%, prevented microsphere dissolution in good solvents. By controlling the irradiation time, the amount of azide consumed for photo-crosslinking could be precisely adjusted. Residual azide groups spared during the irradiation were shown to be amenable to highly efficient CuAAC click modification with a fluorescent dye, Rhodamine B propargyl ester. Given the demand for functional crosslinked microspheres and the inherent difficulties associated with common synthetic strategies in producing such materials, this methodology based on two orthogonal chemistries of the azide functionality provides simple access to well-defined microspheres with customizable degrees of crosslinking and functional group densities.

Reactor Concepts for Continuous Emulsion Polymerization

Emulsion polymers are products by process. Besides the process and recipe conditions the reactor itself is the main factor that determines the product properties. In today’s industrial environment the stirred tank reactor concept still is the main concept in use. This may be due to the easy design of the process and the fact that existing equipment in a plant can be used. But Emulsion Polymerization products are not uniform and reactor design can be taylored to the optimal process/product. This article reviews the wide variety in reactor concepts for continuous emulsion polymerization from the early beginning up to recent developments.

Numerical and Experimental Study of the Evaporation and Solid Layer Formation of a Bi-Component Droplet Under Various Drying Conditions

This paper presents a combined experimental and numerical study of the evaporation and solid layer formation of a single bi-component mannitol-water droplet in air. For spherically symmetric droplets, the problem is described mathematically by the unsteady, one-dimensional conservation equations of mass and energy. The effect of the formation of a solid layer at the droplet surface on the droplet evaporation and thermal diffusion rate is included in the present approach. The simulations are validated by comparison with experiments using acoustically levitated droplets. The study includes initial droplet diameters varying from 350 to 450 μm, gas temperatures ranging from 80 to 120 °C, and the initial mannitol mass fraction inside the droplet varies from 0.05 to 0.15. The numerical results are analyzed to identify the occurrence of solid layer formation, and the temporal evolutions of both the droplet size and mass are presented. A parameter study of the initial gas temperature, the initial droplet size, and the initial mannitol mass fraction inside the droplet on droplet evaporation and solid layer formation is presented. The present model accurately captures the initial stages of droplet drying under all conditions investigated.Copyright

One-pot preparation of poly(styrene-co-divinylbenzene)/silver nanoparticles composite microspheres with tunable porosity and their catalytic degradation of methylene blue in aqueous solution

A series of poly(styrene-co-divinylbenzene)/silver nanoparticle (P(St-DVB)/AgNP) composite microspheres with tunable porosity were synthesized by seed swelling polymerization in one pot using linear polystyrene seeds as templates and nanosized organic soluble AgNPs as the silver source. The polymerization of styrene and divinylbenzene took place inside the seed particles and AgNPs were dispersed directly into the mixture of styrene and divinylbenzene during the polymerization. The morphology properties of P(St-DVB)/AgNPs microspheres were characterized by optical microscope, scanning electron microscope (SEM) and nitrogen adsorption/desorption. The influencing parameters on the morphology of the microspheres, including feed ratio of styrene (monomer) and divinylbenzene (crosslinker), addictive amount of AgNPs and the variety of the porogens were systematically investigated. The porosity of the composite microspheres could be tuned from nonporous to mesoporous and to macroporous by control over these parameters. Meanwhile, AgNPs loading dispersity of the composite microspheres was characterized by transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDS), SEM and thermogravimetric analysis (TGA). TEM, EDS and SEM results demonstrated that the AgNPs had been loaded both inside and on the surface of the microspheres. The sizes of the loaded AgNPs were influenced by the addictive amount of AgNPs and the kinds of porogens used in the polymerization. Moreover, compared to the P(St-DVB) microspheres, the Ag-loading composite microspheres synthesized in this paper exhibited excellent catalytic activity and reusability in the degradation of methylene blue in the presence of NaBH4.

Acoustic Levitation: A Powerful Tool to Model Spray Processes

The acoustic levitation was used as a model system for spray drying processes within DFG Project SPP1423-Process Spray to elucidate the drying behavior of single droplets and the formation of the particle morphology in more detail. The gas flow characteristics inside the levitator were analyzed with computational fluid dynamics simulations. The obtained data were validated with experimental values and showed that the evaporation behavior of levitated droplets can correspond to sprayed droplets. Different reactive and nonreactive substance systems were investigated in single droplet experiments. The influence of process parameters like gas temperature, relative humidity, and droplet size on the drying behavior and particle morphology was elucidated. It was possible to track the conversion of N-Vinyl-2-pyrrolidone in droplet polymerization by using Raman spectroscopy and subsequent principal component analysis. It was found that the first principal component corresponds to the first drying stage and describes the evaporation of water, whereas the second principal component described the polymerization. The crystallinity of the obtained PVP particles increased when temperature and humidity were decreased. The polymerization of partially neutralized acrylic acid revealed the duality of polymerization and crystallization of monomer. Regarding the particle morphology, it was found that a higher amount of sodium acrylate led to a smoother particle surface. One reason for this result is that if sodium acrylate precipitates, it defines predominantly the particle structure. Another reason for this trend in particle morphology is a lower gas pressure within the particle due to a lower polymerization rate, which leads to fewer cracks within the shell. Mannitol served as a nonreactive model system. The drying rate increased with elevated gas temperatures and smaller initial droplet diameters. These experimental results corresponded well with the simulation carried out by Grosshans et al. Additionally, a categorization of morphological properties was introduced. This shows an increasing particle roughness at high temperatures and high initial mass fractions of mannitol. At high relative humidity, metastable, supersaturated mannitol solutions were formed during the drying process.