Beyond Mechanical Recycling: Giving New Life to Plastic Waste.

Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.

Repetitive on-demand drug release from polymeric matrices containing a macroscopic spherical iron core.

A system for multiple on-demand drug release has been prepared that can be activated with an alternating magnetic field as external trigger. The core/shell samples have been developed based on a macroscopic spherical iron core coated with a thermoresponsive polymer, poly(styrene-stat-butyl methacrylate), containing ibuprofen as a model drug. During exposure of the samples to the magnetic field (ON state), the release rate of ibuprofen is significantly increased, up to 35 times the release rate without the magnetic field (OFF state). Using one sample or two samples in line with the magnetic field does not influence the ON/OFF ratio of the system, showing the possibility of using multiple samples to increase and tune the drug dose. Increasing the concentration of ibuprofen in the polymer layer is shown to increase the release rate in both the ON and OFF states. Increasing the size of the iron core and, consequently, decreasing the polymer thickness, was found to only increase the release rate during exposure resulting in higher ON/OFF ratios. The developed on demand drug delivery systems represents a promising development towards on demand drug delivery implants. REFLECTIONS ON CAREER GOALS: During my chemical engineering studies, it was only during my master thesis work that I decided to continue with PhD research as I really enjoyed doing original research. When coming to the end of my PhD research under supervision of Prof. Ulrich S. Schubert, I developed the ambition to pursue an academic career. Fortunately, I got the opportunity to stay with Prof. Schubert as project leader for the Dutch Polymer Institute (DPI). Within this position, I supervised ten researchers and was able to start developing my independent research lines. Despite that I now advise students to not stay in the same laboratory, this first position allowed me to gain some initial independence and to publish a large number of papers that has been a great benefit in my further career. After two and a half years I needed a new challenge that I found by taking up a part-time position at a start-up company in Eindhoven, Dolphys Medical BV, while I also continued as part-time group leader for the DPI. As senior product developer, I was in charge of the research and learned to focus on the application rather than scientific curiosity. This experience made me realize that I prefer the freedom to do academic blue sky research and decided to fully go for an academic position. After personal discussions with some prominent professors in the Netherlands, I applied for a postdoc fellowship in the Netherlands with Prof. Roeland Nolte as well as a Humboldt fellowship in Germany with Prof. Martin Möller, which I both got. As a result, I went one year 'abroad' to Aachen and returned to Nijmegen where I intended to start my independent career. However, another opportunity came along. Via my personal network I was informed that I would make a good chance if I applied for a new professor scheme in Ghent. So I applied and the rest is history. Picture of the Supramolecular Chemistry Group (2017).

Reversible Calcium(II)-Ion Binding through an Apparent pKa Shift of Thermosensitive Block-Copolymer Micelles.

There is an increasing need for smart materials capable of removing multivalent ions from aqueous streams without the inconvenience of brine regeneration as in ion-exchange processes. Herein, we present a thermoresponsive micellar system consisting of polystyrene-poly(methoxy diethyleneglycol acrylate) block copolymer surfactants modified with carboxylic acid end groups (PS-PMDEGA-COOH) that can be used to switch between the adsorption and desorption of divalent calcium(II) cations by a mild temperature trigger, thus providing a new type of thermoregenerable ion-adsorbing materials. The switch of calcium(II)-binding capacity is demonstrated to result from a shift in the pKa value of the carboxylic acid groups by the collapse and redissolution of the PMDEGA block and the associated change in local polarity.

Dissolved gas and ultrasonic cavitation--a review.

The physics and chemistry of nonlinearly oscillating acoustic cavitation bubbles are strongly influenced by the dissolved gas in the surrounding liquid. Changing the gas alters among others the luminescence spectrum, and the radical production of the collapsing bubbles. An overview of experiments with various gas types and concentration described in literature is given and is compared to mechanisms that lead to the observed changes in luminescence spectra and radical production. The dissolved gas type changes the bubble adiabatic ratio, thermal conductivity, and the liquid surface tension, and consequently the hot spot temperature. The gas can also participate in chemical reactions, which can enhance radical production or luminescence of a cavitation bubble. With this knowledge, the gas content in cavitation can be tailored to obtain the desired output.

Mechanism of ultrasound scission of a silver-carbene coordination polymer.

Scission of a supramolecular polymer-metal complex can be carried out using collapsing cavitation bubbles created by ultrasound. Although the most plausible scission mechanism of the coordinative bonds is through mechanical force, the influence of radicals and high hot-spot temperatures on scission has to be considered. A silver(I)-N-heterocyclic carbene complex was exposed to 20 kHz ultrasound in argon, nitrogen, methane, and isobutane saturated toluene. Scission percentages were almost equal under argon, nitrogen, and methane. Radical production differs by a factor of 10 under these gases, indicating that radical production is not a significant contributor to the scission process. A model to describe the displacement of the bubble wall, strain rates, and temperature in the gas shows that critical strain rates for coil-to-stretch transition, needed for scission, are achieved at reactor temperatures of 298 K, an acoustic pressure of 1.2 × 10(5) Pa, and an acoustic frequency of 20 kHz. Lower scission percentages were measured under isobutane, which also shows lower strain rates in model simulations. The activation of the polymer-metal complexes in toluene under the influence of ultrasound occurs through mechanical force.

Effect of resonance frequency, power input, and saturation gas type on the oxidation efficiency of an ultrasound horn.

The sonochemical oxidation efficiency (η(ox)) of a commercial titanium alloy ultrasound horn has been measured using potassium iodide as a dosimeter at its main resonance frequency (20 kHz) and two higher resonance frequencies (41 and 62 kHz). Narrow power and frequency ranges have been chosen to minimise secondary effects such as changing bubble stability, and time available for radical diffusion from the bubble to the liquid. The oxidation efficiency, η(ox), is proportional to the frequency and to the power transmitted to the liquid (275 mL) in the applied power range (1-6 W) under argon. Luminol radical visualisation measurements show that the radical generation rate increases and a redistribution of radical producing zones is achieved at increasing frequency. Argon, helium, air, nitrogen, oxygen, and carbon dioxide have been used as saturation gases in potassium iodide oxidation experiments. The highest η(ox) has been observed at 5 W under air at 62 kHz. The presence of carbon dioxide in air gives enhanced nucleation at 41 and 62 kHz and has a strong influence on η(ox). This is supported by the luminol images, the measured dependence of η(ox) on input power, and bubble images recorded under carbon dioxide. The results give insight into the interplay between saturation gas and frequency, nucleation, and their effect on η(ox).

Sound-driven fluid dynamics in pressurized carbon dioxide.

Using high-speed visualization we demonstrate that ultrasound irradiation of pressurized carbon dioxide (CO(2)) induces phenomena that do not occur in ordinary liquids at ambient conditions. For a near-critical mixture of CO(2) and argon, sonication leads to extremely fast local phase separation, in which the system enters and leaves the two-phase region with the frequency of the imposed sound field. This phase transition can propagate with the speed of sound, but can also be located at fixed positions in the case of a standing sound wave. Sonication of a vapor-liquid interface creates a fine dispersion of liquid and vapor, irrespective whether the ultrasound horn is placed in the liquid or the vapor phase. In the absence of an interface, sonication of the liquid leads to ejection of a macroscopic vapor phase from the ultrasound horn with a velocity of several meters per second in the direction of wave propagation. The findings reported here potentially provide a tunable and noninvasive means for enhancing mass and heat transfer in high-pressure fluids.

Controlled methyl chloride synthesis at mild conditions using ultrasound irradiation.

A new route for the chlorination of methane is described using ultrasound irradiation, which allows for an intrinsically safe process at ambient pressure and temperature. By tuning the gas feed composition methyl chloride yields of up to 19% have been obtained.

Importance of acoustic shielding in sonochemistry.

It is well known that sonochemistry is less efficient at high acoustic intensities. Many authors have attributed this effect to decoupling losses and shielding of the acoustic wave. In this study we investigate both phenomena for a 20 kHz ultrasound field with an intensity ranging from 40 to 150 W/cm2. Visualization of the bubble cloud has demonstrated that the void fraction below the ultrasound horn increases more than proportional with increasing power input. Nevertheless, the energy coupling between the horn and the liquid remains constant; this implies that decoupling losses are not reinforced for larger bubble clouds. On the contrary, microphone measurements have shown that due to the larger bubble cloud a substantial part of the supplied energy is lost at high power inputs. In striving towards more efficient sonochemistry, reduction of shielding appears as one of the major challenges.

Phase behavior and micellar properties of carboxylic acid end group modified pluronic surfactants.

The micellar behavior of three different carboxylic acid end standing (CAE) surfactants has been characterized using conductometry, differential scanning calorimetry, isothermal titration calorimetry, and dynamic light scattering. The CAE surfactants are modified high molecular weight Pluronic (PEO-PPO-PEO triblock copolymer) surfactants. The influence of pH and salt additives on the critical micellization temperature (CMT) and the cloud point of the CAE surfactants have been studied. Both the CMT and the cloud points of the CAE surfactants increase as a function of pH and decrease as a function of ionic strength. For the CAE surfactants, the CMT varies by about 5 degrees C, and the cloud point shows a variation in the order of 20-30 degrees C, as compared to the unmodified Pluronics. From the different experimental techniques, it follows that at low pH values (pH<3.5), the CAE surfactants show the same micellar behavior as the unmodified Pluronic, while at high pH values (pH>6), the micellar properties of the CAE surfactants are considerably different from those observed for the corresponding Pluronic. It has been demonstrated that the CAE micelles are capable of removing simultaneously divalent ions and phenanthrane. The CAE surfactants are the first known anionic surfactants that show cloud point behavior with the addition of low concentrations of simple salts, such as, for example, NaCl.

Pressure-induced reduction of shielding for improving sonochemical activity.

The effect of hydrostatic pressure on chemical reactions induced by 20 kHz ultrasound has been studied using three different methods: the oxidation of potassium iodide, bubble cloud visualization studies, and sound attenuation measurements. The latter two have demonstrated that shielding of the ultrasonic wave is less pronounced at elevated pressures. Accordingly, the yield of iodine liberation increases with increasing pressure. At high static pressures, however, the less efficient cavitation dynamics dominate and the chemical reactivity decreases rapidly.

Inhibition of nonlinear acoustic cavitation dynamics in liquid CO2.

The authors present a model to study ultrasound-induced cavitation dynamics in liquid carbon dioxide (CO(2)), which includes descriptions for momentum, mass, and energy transport. To assist in the interpretation of these results, numerical simulations are presented for an argon cavity in water. For aqueous systems, inertia effects and force accumulation lead to a nonlinear radial motion, resulting in an almost adiabatic compression of the cavity interior. The simulations for liquid CO(2) suggest that transport limitations impede nonlinear cavitation dynamics and the corresponding temperature rise. Consequently, in liquid CO(2) the ultrasound-induced formation of radicals appears improbable.

Reversible binding of multivalent ions by surfactant self-assembly.

High molecular weight nonionic surfactants have been chemically modified to bind multivalent ions reversibly by using a moderate temperature stimulus as an on/off mechanism. Only above the critical micellization temperature (CMT) does binding of multivalent ions take place, whereas below the CMT, no binding occurs to the free surfactant molecules. Different calorimetric techniques have been used to prove the reversible binding of multivalent ions. This tunable binding of multivalent metal ions allows for the improvement of many ion-exchange processes and offers attractive opportunities in the biomedical field.

Protein partitioning driven by excluded-volume interactions in an aqueous nonionic micellar-gel system.

The separation and purification of biomolecules in aqueous media driven by excluded-volume interactions is a well-established concept. In this article we propose a new separations method, based on excluded-volume principles, consisting of an aqueous micellar-gel system (AMGS). Specifically, an outer aqueous phase containing cylindrically shaped micelles of the nonionic surfactant n-decyl tetra (ethylene oxide) (C10E4) is physically separated from an inner aqueous phase defined by the interior volume of gel beads, from which the micelles are completely excluded because of their shape and size. In the AMGS, the concentration of the micelles outside the gel beads is sufficiently high that the volume excluded to a biomolecule in the solution external to the gel beads is much larger than that within the gel beads. Accordingly, when biomolecules are introduced into the AMGS, they partition preferentially into the gel-bead phase, according to their sizes, as a result of the greater effect of the excluded-volume interactions with the C10E4 micelles present in the aqueous phase outside the gel beads. The new AMGS is more versatile and adaptable than the conventional two-phase aqueous C10E4 micellar system because the micelle volume fraction is independent of the temperature and because the effects of entrainment are eliminated. After demonstrating the experimental feasibility of creating the new AMGS, the three proteins myoglobin, ovalbumin, and BSA-FITC, and the enzyme G6PD, were partitioned in the AMGS and their partitioning behavior was found to follow the experimental excluded-volume trends dictated by the interactions of the biomolecules with the C10E4 micelles. Specifically, the measured partition coefficients of the four biomolecules into the micellar phase were found to be less than unity and to decrease with increasing biomolecule size. A theoretical description of the partitioning behavior of the biomolecules in the new AMGS was formulated, based on excluded-volume considerations, and the predicted biomolecule partition coefficients were found to compare favorably with those measured for the four biomolecules studied.