Reverse Engineering of Radical Polymerizations by Multi-Objective Optimization.

Reverse engineering is applied to identify optimum polymerization conditions for the synthesis of polymers with pre-defined properties. The proposed approach uses multi-objective optimization (MOO) and provides multiple candidate polymerization procedures to achieve the targeted polymer property. The objectives for optimization include the maximal similarity of molar mass distributions (MMDs) compared to the target MMDs, a minimal reaction time, and maximal monomer conversion. The method is tested for vinyl acetate radical polymerizations and can be adopted to other monomers. The data for the optimization procedure are generated by an in-house-developed kinetic Monte-Carlo (kMC) simulator for a selected recipe search space. The proposed reverse engineering algorithm comprises several steps: kMC simulations for the selected recipe search space to derive initial data, performing MOO for a targeted MMD, and the identification of the Pareto optimal space. The last step uses a weighted sum optimization function to calculate the weighted score of each candidate polymerization condition. To decrease the execution time, clustering of the search space based on MMDs is applied. The performance of the proposed approach is tested for various target MMDs. The suggested MOO-based reverse engineering provides multiple recipe candidates depending on competing objectives.

Temperature Dependence of the Number of Defect-Structures in Poly(vinylidene fluoride).

Poly(vinylidene fluoride) (PVDF) is predominantly characterized by alternating CH2 and CF2 units in a polymer backbone, originating from the head-to-tail addition of monomers or regular propagation. Due, to a small extent, to inverse monomer addition, so-called defect structures occur which influence the macroscopic properties of PVDF significantly. The amount of defect structures in the material is determined by the polymerization conditions. Here, the temperature dependence of the fraction of defect structures in PVDF obtained from polymerizations between 45 and 90 °C is reported. We utilized 19F-NMR spectroscopy to determine the fraction of defect structures as a function of temperature. To derive kinetic data, the polymerization of VDF is considered a quasi-copolymerization described by the Terminal Model involving four different propagation reactions. Based on the experimentally determined temperature-dependent fractions of defect structures, the known overall propagation rate coefficient, and taking into account the self-healing behavior of the macroradical, the Arrhenius parameters of the individual propagation rate coefficients were determined using the Monte Carlo methods.

Macromolecular Architecture-Dependent Polymorphous Crystallization Behavior of PVDF in the PVDF/γ-BL System via Thermally Induced Phase Separation.

This study investigates the effect of the macromolecular architecture of poly(vinylidene fluoride) (PVDF) on its thermally induced phase separation (TIPS) behavior and polymorphic crystallization in the PVDF/γ-butyrolactone (PVDF/γ-BL) system. Preparative PVDF fractions with specific macromolecular architecture and phase constitution are generated. The results show that PVDF's macromolecular architecture, particularly the degree of branching and regio-defects, plays a significant role in its temperature-dependent crystallization and resulting polymorphic phases. While regio-defects dominate crystallization in the temperature range between 30 and 25 °C, the degree of branching becomes decisive in the 25-20 °C interval. The developed fractions of PVDF are further analyzed in terms of their molecular weight distribution, revealing that the PVDF fractions crystallized out of solution have similar molecular weight distributions with lower dispersity compared with the feed polymer. These findings are crucial for macromolecular separation and adjustment of PVDF polymorphic properties and hence for the development of tailor-made PVDF matrix materials for composites and membranes. The findings suggest the possibility of polymorphous phase tailoring of PVDF based on macromolecular architecture due to temperature-controlled crystallization out of solution and strongly motivate further research to reveal deeper knowledge of regio-defect and branching influence of PVDF solution crystallization.

Controlled Release of Tea Tree Oil from a Chitosan Matrix Containing Gold Nanoparticles.

Chitosan is a biopolymer that, due to its versatile bioactive properties, has applications in several areas, including food, medicine and pharmaceuticals. In the field of tissue engineering, chitosan can be used, for example, as a dressing to treat wounds or dermal damage, such as burns or abrasions. This work deals with the controlled release of tea tree oil from chitosan-based polymeric films and droplets containing gold nanoparticles (AuNP). AuNPs were successfully incorporated into the chitosan matrix using two different approaches. Both solutions were loaded with tea tree oil, and from these solutions, it was possible to obtain drop-cast films and droplets. The controlled release of oil in water was performed both in the films and in the droplets. The addition of AuNP in the controlled release system of melaleuca oil favored a release time of around 25 h. A series of experiments was carried out to investigate the effects of different reaction temperatures and acetic acid concentrations on the formation of AuNPs in the presence of chitosan. For this purpose, images of the AuNP films and droplets were obtained using transmission electron microscopy. In addition, UV-vis spectra were recorded to investigate the release of tea tree oil from the different samples.

Extracellular matrix remodeling through endocytosis and resurfacing of Tenascin-R.

The brain extracellular matrix (ECM) consists of extremely long-lived proteins that assemble around neurons and synapses, to stabilize them. The ECM is thought to change only rarely, in relation to neuronal plasticity, through ECM proteolysis and renewed protein synthesis. We report here an alternative ECM remodeling mechanism, based on the recycling of ECM molecules. Using multiple ECM labeling and imaging assays, from super-resolution optical imaging to nanoscale secondary ion mass spectrometry, both in culture and in brain slices, we find that a key ECM protein, Tenascin-R, is frequently endocytosed, and later resurfaces, preferentially near synapses. The TNR molecules complete this cycle within ~3 days, in an activity-dependent fashion. Interfering with the recycling process perturbs severely neuronal function, strongly reducing synaptic vesicle exo- and endocytosis. We conclude that the neuronal ECM can be remodeled frequently through mechanisms that involve endocytosis and recycling of ECM proteins.

Determination of the through-plane profile of vanadium species in hydrated Nafion studied with micro X-ray absorption near-edge structure spectroscopy - proof of concept.

Vanadium-ion transport through the polymer membrane results in a significant decrease in the capacity of vanadium redox flow batteries. It is assumed that five vanadium species are involved in this process. Micro X-ray absorption near-edge structure spectroscopy (micro-XANES) is a potent method to study chemical reactions during vanadium transport inside the membrane. In this work, protocols for micro-XANES measurements were developed to enable through-plane characterization of the vanadium species in Nafion 117 on beamline P06 of the PETRA III synchrotron radiation facility (DESY, Hamburg, Germany). A Kapton tube diffusion cell with a diameter of 3 mm was constructed. The tube diameter was chosen in order to accommodate laminar flow for cryogenic cooling while allowing easy handling of the cell components by hand. A vertical step size of 2.5 µm and a horizontal step size of 5 µm provided sufficient resolution to resolve the profile and good statistics after summing up horizontal rows of scan points. The beam was confined in the horizontal plane to account for the waviness of the membrane. The diffusion of vanadium ions during measurement was inhibited by the cryogenic cooling. Vanadium oxidation, e.g. by water radiolysis (water percentage in the hydrated membrane ∼23 wt%), was mitigated by the cryogenic cooling and by minimizing the dwell time per pixel to 5 ms. Thus, the photo-induced oxidation of V3+ in the focused beam could be limited to 10%. In diffusion experiments, Nafion inside the diffusion cell was exposed on one side to V3+ electrolyte and on the other side to VO2+. The ions were allowed to diffuse across the through-plane orientation of the membrane during one of two short defrost times (200 s and 600 s). Subsequent micro-XANES measurements showed the formation of VO2+ from V3+ and VO2+ inside the water body of Nafion. This result proves the suitability of the experimental setup as a powerful tool for the determination of the profile of vanadium species in Nafion and other ionomeric membranes.

Characterization of Dimeric Vanadium Uptake and Species in Nafion™ and Novel Membranes from Vanadium Redox Flow Batteries Electrolytes.

A core component of energy storage systems like vanadium redox flow batteries (VRFB) is the polymer electrolyte membrane (PEM). In this work, the frequently used perfluorosulfonic-acid (PFSA) membrane Nafion™ 117 and a novel poly (vinylidene difluoride) (PVDF)-based membrane are investigated. A well-known problem in VRFBs is the vanadium permeation through the membrane. The consequence of this so-called vanadium crossover is a severe loss of capacity. For a better understanding of vanadium transport in membranes, the uptake of vanadium ions from electrolytes containing Vdimer(IV-V) and for comparison also V(II), V(III), V(IV), and V(V) by both membranes was studied. UV/VIS spectroscopy, X-ray absorption near edge structure spectroscopy (XANES), total reflection X-ray fluorescence spectroscopy (TXRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and micro X-ray fluorescence spectroscopy (microXRF) were used to determine the vanadium concentrations and the species inside the membrane. The results strongly support that Vdimer(IV-V), a dimer formed from V(IV) and V(V), enters the nanoscopic water-body of Nafion™ 117 as such. This is interesting, because as of now, only the individual ions V(IV) and V(V) were considered to be transported through the membrane. Additionally, it was found that the Vdimer(IV-V) dimer partly dissociates to the individual ions in the novel PVDF-based membrane. The Vdimer(IV-V) dimer concentration in Nafion™ was determined and compared to those of the other species. After three days of equilibration time, the concentration of the dimer is the lowest compared to the monomeric vanadium species. The concentration of vanadium in terms of the relative uptake λ = n(V)/n(SO3) are as follows: V(II) [λ = 0.155] > V(III) [λ = 0.137] > V(IV) [λ = 0.124] > V(V) [λ = 0.053] > Vdimer(IV-V) [λ = 0.039]. The results show that the Vdimer(IV-V) dimer needs to be considered in addition to the other monomeric species to properly describe the transport of vanadium through Nafion™ in VRFBs.

Self-Initiated Butyl Acrylate Polymerizations in Bulk and in Solution Monitored By In-Line Techniques.

High-temperature acrylate polymerizations are technically relevant, but yet not fully understood. In particular the mechanism and the kinetics of the thermal self-initiation is a topic of current research. To obtain more detailed information the conversion dependence of the polymerization rate, rbr, is determined via in-line DSC and FT-NIR spectroscopy for reactions in bulk and in solution at temperatures ranging from 80 to 160 °C. Solution polymerizations revealed that dioxane is associated with the highest rbr, while aromatic solvents result in the lowest values of rbr. Interestingly, rbr for polymerizations in solution with dioxane depends on the actual monomer concentration at a given time in the system, but is not depending on the initial monomer concentration. The overall rate of polymerization in bulk and in solution is well represented by an equation with three or four parameters, respectively, being estimated by multiple linear regression and the temperature as additional parameter.

Studies of Langmuir and Langmuir-Schaefer Films of Poly(3-Hexylthiophene) and Poly(Vinylidene Fluoride).

The synergistic use of blends of regioregular poly(3-hexylthiophene) (P3HT) and poly(vinylidene fluoride) (PVDF) or poly((vinylidene fluoride)-block-(methyl methacrylate)) (PVDF-PMMA) to form Langmuir and Langmuir-Schaefer (LS) films is reported. P3HT has wide applications in sensor devices because of its properties such as conductivity, luminescence, and chromism; however, the stiffness of the films and the difficulty in organizing the molecules may pose a problem in these applications. In this context, polymers based on PVDF can be used in the formation of thin P3HT films and present an alternative to improve the organization of P3HT molecules. In addition, PVDF acts as a plasticizer, making the film less rigid. The films were obtained from the blends of P3HT/PVDF and P3HT/PVDF-PMMA in a solution containing chloroform and DMAc (N,N-dimethylacetamide). Surface pressure isotherms, in situ ultraviolet-visible (UV-vis) spectroscopy, polarization-modulation infrared reflection-absorption spectroscopy, and Brewster angle microscopy techniques were used to analyze Langmuir films. The surface morphology of LS films was characterized by atomic force microscopy and UV-vis spectroscopy, and their degradation was analyzed by UV-vis spectroscopy after exposure to natural light under atmospheric conditions. The Langmuir films containing PVDF indicate a direct formation of the ferroelectric ß phase, with dipoles parallel to the water surface. The Langmuir films formed by P3HT presented dipoles of side chains parallel and aromatic groups perpendicular to the water surface. P3HT and PVDF or PVDF-PMMA films show high molecular organization compared with pure P3HT films. The results suggest that these films could be used to improve the properties of P3HT in several device applications, such as in optical and electrical sensors.

Polymer Electrolyte Membranes Prepared by Graft Copolymerization of 2-Acrylamido-2-Methylpropane Sulfonic Acid and Acrylic Acid on PVDF and ETFE Activated by Electron Beam Treatment.

Polymer electrolyte membranes (PEM) for potential applications in fuel cells or vanadium redox flow batteries were synthesized and characterized. ETFE (poly (ethylene-alt-tetrafluoroethylene)) and PVDF (poly (vinylidene fluoride)) serving as base materials were activated by electron beam treatment with doses ranging from 50 to 200 kGy and subsequently grafted via radical copolymerization with the functional monomers 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid in aqueous phase. Since protogenic groups are already contained in the monomers, a subsequent sulfonation step is omitted. The mechanical properties were studied via tensile strength measurements. The electrochemical performance of the PEMs was evaluated by electrochemical impedance spectroscopy and fuel cell tests. The proton conductivities and ion exchange capacities are competitive with Nafion 117, the standard material used today.

Transcription factor TAp73 and microRNA-449 complement each other to support multiciliogenesis.

Motile cilia serve vital functions in development, homeostasis, and regeneration. We recently demonstrated that TAp73 is an essential transcriptional regulator of respiratory multiciliogenesis. Here, we show that TAp73 is expressed in multiciliated cells (MCCs) of diverse tissues. Analysis of TAp73 mutant animals revealed that TAp73 regulates Foxj1, Rfx2, Rfx3, axonemal dyneins Dnali1 and Dnai1, plays a pivotal role in the generation of MCCs in male and female reproductive ducts, and contributes to fertility. However, the function of MCCs in the brain appears to be preserved despite the loss of TAp73, and robust activity of cilia-related networks is maintained in the absence of TAp73. Notably, TAp73 loss leads to distinct changes in ciliogenic microRNAs: miR34bc expression is reduced, whereas the miR449 cluster is induced in diverse multiciliated epithelia. Among different MCCs, choroid plexus (CP) epithelial cells in the brain display prominent miR449 expression, whereas brain ventricles exhibit significant increase in miR449 levels along with an increase in the activity of ciliogenic E2F4/MCIDAS circuit in TAp73 mutant animals. Conversely, E2F4 induces robust transcriptional response from miR449 genomic regions. To address whether increased miR449 levels in the brain maintain the multiciliogenesis program in the absence of TAp73, we deleted both TAp73 and miR449 in mice. Although loss of miR449 alone led to a mild ciliary defect in the CP, more pronounced ciliary defects and hydrocephalus were observed in the brain lacking both TAp73 and miR449. In contrast, miR449 loss in other MCCs failed to enhance ciliary defects associated with TAp73 loss. Together, our study shows that, in addition to the airways, TAp73 is essential for generation of MCCs in male and female reproductive ducts, whereas miR449 and TAp73 complement each other to support multiciliogenesis and CP development in the brain.

Preparation of Polymer Electrolyte Membranes via Radiation-Induced Graft Copolymerization on Poly(ethylene-alt-tetrafluoroethylene) (ETFE) Using the Crosslinker N,N'-Methylenebis(acrylamide).

Polymer electrolyte membranes (PEM) prepared by radiation-induced graft copolymerization are investigated. For this purpose, commercial poly(ethylene-alt-tetrafluoroethylene) (ETFE) films were activated by electron beam treatment and subsequently grafted with the monomers glycidyl methacrylate (GMA), hydroxyethyl methacrylate (HEMA) and N,N'-methylenebis(acrylamide) (MBAA) as crosslinker. The target is to achieve a high degree of grafting (DG) and high proton conductivity. To evaluate the electrochemical performance, the PEMs were tested in a fuel cell and in a vanadium redox-flow battery (VRFB). High power densities of 134 mW∙cm-2 and 474 mW∙cm-2 were observed, respectively.

Ab initio kinetic Monte Carlo simulation of seeded emulsion polymerizations of styrene.

Seeded emulsion polymerizations of styrene are modeled on the basis of a detailed kinetic scheme accounting for the chain length and conversion dependence of termination rate coefficients. A holistic kinetic Monte Carlo approach was developed, which simulates the elemental reactions in the aqueous phase, the transfer of radicals into individual particles, and the radical polymerization in each particle based on a complete kinetic model. Experimentally-derived particle size distributions are used as input for the simulations. The required rate coefficients were taken from literature. Without any adjustment of this data a very good agreement between simulation results and experimental data is found. The validation of the model is performed based on monomer conversion - time data and full molar mass distributions.

Kinetic Monte Carlo Simulation Based Detailed Understanding of the Transfer Processes in Semi-Batch Iodine Transfer Emulsion Polymerizations of Vinylidene Fluoride.

Semi-batch emulsion polymerizations of vinylidene fluoride (VDF) are reported. The molar mass control is achieved via iodine transfer polymerization (ITP) using IC4F8I as chain transfer agent. Polymerizations carried out at 75 °C and pressures ranging from 10 to 30 bar result in low dispersity polymers with respect to the molar mass distribution (MMD). At higher pressures a significant deviation from the ideal behavior expected for a reversible deactivation transfer polymerization occurs. As identified by kinetic Monte Carlo (kMC) simulations of the activation⁻deactivation equilibrium, during the initialization period of the chain transfer agent already significant propagation occurs due to the higher pressure, and thus, the higher monomer concentration available. Based on the kMC modeling results, semi-batch emulsion polymerizations were carried out as a two pressure process, which resulted in very good control of the MMD associated with a comparably high polymerization rate.

Propagation Kinetics of Isoprene-Glycidyl Methacrylate Copolymerizations Investigated via PLP-SEC.

Pulsed-laser polymerization combined with polymer analysis by NMR and size-exclusion chromatography is used to study the radical copolymerization kinetics of isoprene (IP) with glycidyl methacrylate (GMA). The copolymer is characterized by a close-to-alternating microstructure, with the addition of IP leading to a significant decrease in the composition-averaged propagation rate coefficient. A rigorous fitting strategy is developed to fit a mixed penultimate model to the data, with the selectivity of the IP, but not the GMA, macroradical dependent on the penultimate unit.

Inducing ß Phase Crystallinity in Block Copolymers of Vinylidene Fluoride with Methyl Methacrylate or Styrene.

Block copolymers of poly(vinylidene fluoride) (PVDF) with either styrene or methyl methacrylate (MMA) were synthesized and analyzed with respect to the type of the crystalline phase occurring. PVDF with iodine end groups (PVDF-I) was prepared by iodine transfer polymerization either in solution with supercritical CO2 or in emulsion. To activate all iodine end groups Mn2(CO)10 is employed. Upon UV irradiation Mn(CO)5 radicals are obtained, which abstract iodine from PVDF-I generating PVDF radicals. Subsequent polymerization with styrene or methyl methacrylate (MMA) yields block copolymers. Size exclusion chromatography and NMR results prove that the entire PVDF-I is converted. XRD, FT-IR, and differential scanning calorimetry (DSC) analyses allow for the identification of crystal phase transformation. It is clearly shown that the original α crystalline phase of PVDF-I is changed to the ß crystalline phase in case of the block copolymers. For ratios of the VDF block length to the MMA block length ranging from 1.4 to 5 only ß phase material was detected.

Propagation rate coefficients for homogeneous phase VDF-HFP copolymerization in supercritical CO2.

For the first time, propagation rate coefficients, k(p,COPO) , for the copolymerizations of vinylidene fluoride and hexafluoropropene have been determined. The kinetic data was determined via pulsed-laser polymerization in conjunction with polymer analysis via size-exclusion chromatography, the PLP-SEC technique. The experiments were carried out in homogeneous phase with supercritical CO(2) as solvent for temperatures ranging from 45 to 90 °C. Absolute polymer molecular weights were calculated on the basis of experimentally determined Mark-Houwink constants. The Arrhenius parameters of k(p,COPO) vary significantly compared with ethene, which is explained by the high electronegativity of fluorine and less intra- and intermolecular interactions between the partially fluorinated macroradicals.

Free radical copolymerization kinetics of γ-methyl-α-methylene-γ-butyrolactone (MeMBL).

The propagation kinetics and copolymerization behavior of the biorenewable monomer γ-methyl-α-methylene-γ-butyrolactone (MeMBL) are studied using the pulsed laser polymerization (PLP)/size exclusion chromatography (SEC) technique. The propagation rate coefficient for MeMBL is 15% higher than that of its structural analogue, methyl methacrylate (MMA), with a similar activation energy of 21.8 kJ·mol(-1). When compared to MMA, MeMBL is preferentially incorporated into copolymers when reacted with styrene (ST), MMA, and n-butyl acrylate (BA); the monomer reactivity ratios fit from bulk MeMBL/ST, MeMBL/MMA, and MeMBL/BA copolymerizations are r(MeMBL) = 0.80 ± 0.04 and r(ST) = 0.34 ± 0.04, r(MeMBL) = 3.0 ± 0.3 and r(MMA) = 0.33 ± 0.01, and r(MeMBL) = 7.0 ± 2.0 and r(BA) = 0.16 ± 0.03, respectively. In all cases, no significant variation with temperature was found between 50 and 90 °C. The implicit penultimate unit effect (IPUE) model was found to adequately fit the composition-averaged copolymerization propagation rate coefficient, k(p,cop), for the three systems.

Surprisingly high, bulk liquid-like mobility of silica-confined ionic liquids.

Mesoporous silica monoliths were prepared by the sol-gel technique and filled with 1-ethyl-3-methyl imidazolium [Emim]-X (X=dicyanamide [N(CN)2], ethyl sulfate [EtSO4], thiocyanate [SCN], and triflate [TfO]) ionic liquids (ILs) using a methanol-IL exchange technique. The structure and behavior of the ILs inside the silica monoliths were studied using X-ray scattering, nitrogen sorption, IR spectroscopy, solid-state NMR, and thermal analysis. DSC finds shifts in both the glass transition temperature and melting points (where applicable) of the ILs. Glass transition and melting occur well below room temperature. There is thus no conflict with the NMR and IR data, which show that the ILs are as mobile at room temperature as the bulk (not confined) ILs. The very narrow line widths of the NMR spectra suggest that the ILs in our materials have the highest mobility reported for confined ILs so far. As a result, our data suggest that it is possible to generate IL/silica hybrid materials (ionogels) with bulk-like properties of the IL. This could be interesting for applications in, e.g., the solar cell or membrane fields.

Solvent influence on propagation kinetics in radical polymerizations studied by pulsed laser initiated polymerizations.

The influence of the reaction medium (organic solvents, water, ionic liquids, supercritical CO(2) ) on the propagation rate in radical polymerizations has very different causes, e.g., hindered rotational modes, hydrogen bonding or electron pair donor/acceptor interactions. Depending on the origin of the solvent influence propagation rate coefficients, k(p) , may be enhanced by up to an order of magnitude associated with changes in the pre-exponential or the activation energy of k(p) . In contrast, non-specific interactions, size and steric effects lead to rather small changes in the vicinity of the radical chain end and are reflected by modest variations in k(p) .