Kinetics of alkoxysilanes and organoalkoxysilanes polymerization: A review

Scientists from various different fields use organo-trialkoxysilanes and tetraalkoxysilanes in a number of applications. The silica-based materials are sometimes synthesized without a good understanding of the underlying reaction kinetics. This literature review attempts to be a comprehensive and more technical article in which the kinetics of alkoxysilanes polymerization are discussed. The kinetics of polymerization are controlled by primary factors, such as catalysts, water/silane ratio, pH, and organo-functional groups, while secondary factors, such as temperature, solvent, ionic strength, leaving group, and silane concentration, also have an influence on the reaction rates. Experiments to find correlations between these factors and reaction rates are restricted to certain conditions and most of them disregard the properties of the solvent. In this review, polymerization kinetics are discussed in the first two sections, with the first section covering early stage reactions when the reaction medium is homogenous, and the second section covering when phase separation occurs and the reaction medium becomes heterogeneous. Nuclear magnetic resonance (NMR) spectroscopy and other techniques are discussed in the third section. The last section summarizes the study of reaction mechanisms by using ab initio and Density Functional Theory (DFT) methods alone, and in combination with molecular dynamics (MD) or Monte Carlo (MC) methods

Influence of the presence of medium-soft paraffin wax on the morphology and properties of iPP/silver nanocomposites

The aim of this study was to investigate the presence of wax, different Ag nanoparticle contents, and different cooling rates from the melt, on the morphology, thermal and electrical conductivity, and dynamic mechanical properties of iPP. The Ag particles were well dispersed in the polymer, and formed nucleation centres for the crystallization of iPP. They were also well dispersed in iPP/wax, but they were located in the wax phase which was dispersed between the iPP spherulites. Generally the extent of filler agglomeration increased with increasing filler content. The Ag particles, whether in the iPP or wax phase, had little influence on the crystallinities and melting temperatures of iPP. The presence of Ag particles in iPP had little influence on its modulus, but the presence of both wax and Ag particles significantly improved the modulus of these nanocomposites. The thermal and electrical conductivities of the samples more significantly improved when both wax and Ag were present. With increasing Ag particle contents in both iPP/Ag and iPP/wax/Ag, the thermal conductivities increased, but leveled off at higher filler contents, while the electrical conductivities continuously increased with increasing filler contents. The slowly cooled samples had higher crystallinities than the quenched samples and therefore they were more thermally conductive than the quenched samples.The National Research Foundation and Sasol Inzalo in South Africa financially supported the research, which was also supported in part by the Ministry of Education and Science, Republic of Serbia (Project No. 171029)

Crystallization and dielectric behavior of PLA and PHBV in PLA/PHBV blends and PLA/PHBV/TIO 2 nanocomposites

The morphology, crystallization behavior and mechanical properties of PLA, PHBV and their blends and nanocomposites with TiO 2 as filler were investigated. An uncommon morphological change was observed with increasing PLA content in the blends, and this complicated the isothermal crystallization kinetics analyses of the different samples. It was, for example, observed that PLA, which do not normally crystallize during the cooling of the sample, showed isothermal crystallization for certain blend compositions. TiO 2 was found to be mainly present in the PLA phase, which was also confirmed through broadband dielectric analysis. The blend composition, as well as the presence of TiO 2 nanoparticles, had an influence on the cold crystallization of the PLA. The tensile properties changed with blend and nanocomposite composition, and these changes could to a certain extent be related to the respective morphologies. Very little could be said about differences between the melting behavior of the different samples, because the PLA and PHBV melted at almost exactly the same temperature.Scopu

A review on electrospun bio-based polymers for water treatment

Over the past decades, electrospinning of biopolymers down to nanoscale garnered much interest to address most of the millennia issues related to water treatment. The fabrication of these nanostructured membranes added a new dimension to the current nanotechnologies where a wide range of materials can be processed to their nanosize. Electrospinning is a simple and versatile technique to fabricate unique nanostructured membranes with fascinating properties for a wide spectrum of applications such as filtration and others. These nanostructured membranes, fabricated by electrospinning, were found to be of a paramount importance because of their advanced inherited properties such as large surface-to-volume ratio, as well as tuneable porosity, stability, and high permeability. The extensive research conducted on these materials extended the success of electrospinning not only to bio-based polymer nanofibres, but to their hybrids and their derivatives. The technique also created avenues for advanced and massive production of nanofibres. This paper reviews the recent developments in the electrospinning technique. Electrospinning of biopolymers, their blends and functionalization using metals/metal oxides, and the potential applications of electrospun nanofibrous membranes in water filtration are discussed.National Research Foundation of South Africa (NRF), Professional Development Programme (PDP: UID-86101

Kinetics of alkoxysilanes hydrolysis: An empirical approach

Alkoxysilanes and organoalkoxysilanes are primary materials in several industries, e.g. coating, anti-corrosion treatment, fabrication of stationary phase for chromatography, and coupling agents. The hydrolytic polycondensation reactions and final product can be controlled by adjusting the hydrolysis reaction, which was investigated under a variety of conditions, such as different alkoxysilanes, solvents, and catalysts by using gas chromatography. The hydrolysis rate of alkoxysilanes shows a dependence on the alkoxysilane structure (especially the organic attachments), solvent properties, and the catalyst dissociation constant and solubility. Some of the alkoxysilanes exhibit intramolecular catalysis. Hydrogen bonding plays an important role in the enhancement of the hydrolysis reaction, as well as the dipole moment of the alkoxysilanes, especially in acetonitrile. There is a relationship between the experimentally calculated polarity by the Taft equation and the reactivity, but it shows different responses depending on the solvent. It was found that negative and positive charges are respectively accumulated in the transition state in alkaline and acidic media. The reaction mechanisms are somewhat different from those previously suggested. Finally, it was found that enthalpy–entropy compensation (EEC) effect and isokinetic relationships (IKR) are exhibited during the hydrolysis of CTES in different solvents and catalysts; therefore, the reaction has a linear free energy relationship (LFER).The publication of this article was funded by the Qatar National Library.Scopu

Isothermal crystallization kinetics and morphology of double crystalline PCL/PBS blends mixed with a polycarbonate/MWCNTs masterbatch

In this work, the 70/30 and 30/70 w/w polycaprolactone (PCL)/polybutylene succinate (PBS) blends and their corresponding PCL/PBS/(polycarbonate (PC)/multiwalled carbon nanotubes (MWCNTs) masterbatch) nanocomposites were prepared in a twin-screw extruder. The nanocomposites contained 1.0 and 4.0 wt% MWCNTs. The blends showed a sea-island morphology typical of immiscible blends. For the nanocomposites, three phases were formed: (i) The matrix (either PCL- or PBS-rich phase depending on the composition), (ii) dispersed polymer droplets of small size (either PCL- or PBS-rich phase depending on the composition), and (iii) dispersed aggregates of tens of micron sizes identified as PC/MWCNTs masterbatch. Atomic force microscopy (AFM) results showed that although most MWCNTs were located in the PC dispersed phase, some of them migrated to the polymer matrix. This is due to the partial miscibility and intimate contact at the interfaces between blend components. Non-isothermal differential scanning calorimetry (DSC) scans for the PCL/PBS blends showed an increase in the crystallization temperature (Tc) of the PCL-rich phase indicating a nucleation effect caused by the PBS-rich phase. For the nanocomposites, there was a decrease in Tc values. This was attributed to a competition between two effects: (1) The partial miscibility of the PC-rich and the PCL-rich and PBS-rich phases, and (2) the nucleation effect of the MWCNTs. The decrease in Tc values indicated that miscibility was the dominating effect. Isothermal crystallization results showed that the nanocomposites crystallized slower than the neat blends and the homopolymers. The introduction of the masterbatch generally increased the thermal conductivity of the blend nanocomposites and affected the mechanical properties.Thandi P. Gumede was financially supported by the National Research Foundation and the Sasol Inzalo Foundation in South Africa, while the POLYMAT/UPV/EHU team was funded by the following projects: UPV/EHU Infrastructure: INF 14/38; Mineco/FEDER: SINF 130I001726XV1/Ref: UNPV13-4E-1726 and MINECO MAT2017-83014-C2-1-P. The publication of this article was funded by the Qatar National Library.Scopu

Halogen-free flame-retardant compounds. Thermal decomposition and flammability behavior for alternative polyethylene grades

The effect of six halogen-free flame retardant (FR) formulations was investigated on the thermal stability of two low-density polyethylenes (LDPE) and one linear low-density polyethylene (LLDPE), by means of thermogravimetric analysis (TGA) under nitrogen and air atmosphere. The relative data were combined with flammability properties and the overall performance of the FRs was correlated with the type of branching in the polyethylene grades and to their processing behavior. The thermal degradation kinetics was further determined based on the Kissinger and Coats-Redfern methods. In terms of flammability, the addition of a triazine derivative and ammonium polyphosphate at a loading of 35 wt. %. was found to be the most efficient, leading to UL 94 V0 ranking in the case of the LDPE grade produced in an autoclave reactor. - 2019 by the authors.Funding: This publication was made possible by the NPRP award [NPRP 9-161-1-030] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the author(s)

Autoantibodies against type I IFNs in patients with life-threatening COVID-19

Interindividual clinical variability in the course of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is vast. We report that at least 101 of 987 patients with life-threatening coronavirus disease 2019 (COVID-19) pneumonia had neutralizing immunoglobulin G (IgG) autoantibodies (auto-Abs) against interferon-w (IFN-w) (13 patients), against the 13 types of IFN-a (36), or against both (52) at the onset of critical disease; a few also had auto-Abs against the other three type I IFNs. The auto-Abs neutralize the ability of the corresponding type I IFNs to block SARS-CoV-2 infection in vitro. These auto-Abs were not found in 663 individuals with asymptomatic or mild SARS-CoV-2 infection and were present in only 4 of 1227 healthy individuals. Patients with auto-Abs were aged 25 to 87 years and 95 of the 101 were men. A B cell autoimmune phenocopy of inborn errors of type I IFN immunity accounts for life-threatening COVID-19 pneumonia in at least 2.6% of women and 12.5% of men

Editorial: Using thermogravimetric analysis to determine polymer thermal stability: Relevance of changes in onset temperature of mass loss

Editoria