Crosslinking of Ethylene Copolymers

The chemical reactions taking place when crosslinking poly(ethylene-co-vinyltrimethoxysilane) (EVS) have been studied. Films of EVS were stored in water at 90 °C or treated at processing temperatures in a tubular oven. The different chemical groups involved in the reactions were then followed by FTIR after different treatment times. Gel content was determined in parallel. The investigation concerning water treatment of films shows that a further formation of Si-O-Si crosslinks takes place also after the point at which maximum gel content has been reached. Mechanical measurements indicate that the further crosslinks are formed within the existing gel. <p />For samples treated at processing temperatures the great importance of temperature, crosslinking catalyst, surrounding atmosphere and supply of external water on the rate of crosslinking was demonstrated. Another EVS copolymer containing butyl acrylate in the main chain (EVSBA) as well, was treated at processing temperature in pure nitrogen without catalyst. A considerable amount of gel was obtained despite the absence of external water. It was shown that the butyl acrylate groups in EVSBA are degraded and form carboxylic acid groups, which form anhydride under a simultaneous formation of water, enabling the crosslinking reaction and the gel formation. In addition carboxylic acid groups were shown to be active as crosslinking catalyst in the polymer. The catalytic activity of both stearic acid and polymer bound carboxylic acid groups were demonstrated, although less active that the commonly used dibutyltin dilaurate (DBTDL) on a molar basis. Indications were found that carboxylic acids probably catalyse both the hydrolysis and the condensation step of the crosslinking reaction. Furthermore DBTDL was found to strongly catalyse the first step of the crosslinking reaction, that is, the hydrolysis. <p />The response of poly(ethylene-co-1,9-decadiene) to peroxide and electron beam irradiation was investigated and compared to a reference. These polymers were produced in a low pressure process using a catalyst giving a relatively high level of inherent unsaturations. The copolymerisation with 1,9-decadiene gave an additional 63 % vinyl end groups in the polymer and a considerable improvement in the crosslinking response was observed. The reason for the improved crosslinking response was found to be the overall higher amount of vinyl groups and the placement of vinyl groups along the molecular weight distribution. A certain amount of vinyl unsaturations still remained after irradiation whereas almost all double bonds were consumed at high peroxide levels. Determination of the crosslinking density, Mc, at a certain degree of crosslinking, exhibited relatively small differences between the material containing decadiene and the reference material. These results were found to support the idea that the crosslinked network is mainly built up of the entanglements, i.e. physical crosslinks. At industrially used levels of degree of crosslinking Mc was found to be higher for irradiated polymer than for peroxide crosslinked polymer. Micrographs obtained from transmission electron microscopy (TEM) show clear differences in morphological structure between the different crosslinking technologies and this is likely to explain some observed differences in mechanical behaviour between the materials

Multiscale Colloidal Assembly of Silica Nanoparticles into Microspheres with Tunable Mesopores

Colloidal assembly of silica (nano)particles is a powerful method to design functional materials across multiple length scales. Although this method has enabled the fabrication of a wide range of silica‐based materials, attempts to design and synthesize porous materials with a high level of tuneability and control over pore dimensions have remained relatively unsuccessful. Here, the colloidal assembly of silica nanoparticles into mesoporous silica microspheres (MSMs) is reported using a discrete set of silica sols within the confinement of a water‐in‐oil emulsion system. By studying the independent manipulation of different assembly parameters during the sol–gel process, a design strategy is outlined to synthesize MSMs with excellent reproducibility and independent control over pore size and overall porosity, which does not require additional ageing or post‐treatment steps to reach pore sizes as large as 50 nm. The strategy presented here can provide the necessary tools for the microstructural design of the next generation of tailor‐made silica microspheres for use in separation applications and beyond

Multiscale Colloidal Assembly of Silica Nanoparticles into Microspheres with Tunable Mesopores

Colloidal assembly of silica (nano)particles is a powerful method to design functional materials across multiple length scales. Although this method has enabled the fabrication of a wide range of silica-based materials, attempts to design and synthesize porous materials with a high level of tuneability and control over pore dimensions have remained relatively unsuccessful. Here, the colloidal assembly of silica nanoparticles into mesoporous silica microspheres (MSMs) is reported using a discrete set of silica sols within the confinement of a water-in-oil emulsion system. By studying the independent manipulation of different assembly parameters during the sol–gel process, a design strategy is outlined to synthesize MSMs with excellent reproducibility and independent control over pore size and overall porosity, which does not require additional ageing or post-treatment steps to reach pore sizes as large as 50 nm. The strategy presented here can provide the necessary tools for the microstructural design of the next generation of tailor-made silica microspheres for use in separation applications and beyond

DTPA-Functionalized Silica Nano- and Microparticles for Adsorption and Chromatographic Separation of Rare Earth Elements

Silica nanoparticles and porous microparticles have been successfully functionalized with a monolayer of DTPA-derived ligands. The ligand grafting is chemically robust and does not appreciably influence the morphology or the structure of the material. The produced particles exhibit quick kinetics and high capacity for REE adsorption. The feasibility of using the DTPA-functionalized microparticles for chromatographic separation of rare earth elements has been investigated for different sample concentrations, elution modes, eluent concentrations, eluent flow rates, and column temperatures. Good separation of the La(III), Ce(III), Pr(III), Nd(III), and Dy(III) ions was achieved using HNO3 as eluent using a linear concentration gradient from 0 to 0.15 M over 55 min. The long-term performance of the functionalized column has been verified, with very little deterioration recorded over more than 50 experiments. The results of this study demonstrate the potential for using DTPA-functionalized silica particles in a chromatographic process for separating these valuable elements from waste sources, as an environmentally preferable alternative to standard solvent-intensive processes.QC 20180509</p

Local quantification of mesoporous silica microspheres using multiscale electron tomography and lattice Boltzmann simulations

The multiscale pore structure of mesoporous silica microspheres plays an important role for tuning mass transfer kinetics in technological applications such as liquid chromatography. While local analysis of a pore network in such materials has been previously achieved, multiscale quantification of microspheres down to the nanometer scale pore level is still lacking. Here we demonstrate for the first time, by combining low convergence angle scanning transmission electron microscopy tomography (LC-STEM tomography) with image analysis and lattice Boltzmann simulations, that the multiscale pore network of commercial mesoporous silica microspheres can be quantified. This includes comparing the local tortuosity and intraparticle diffusion coefficients between different regions within the same microsphere. The results, spanning more than two orders of magnitude between nanostructures and entire object, are in good agreement with bulk characterization techniques such as nitrogen gas physisorption and add valuable local information for tuning mass transfer behavior (in liquid chromatography or catalysis) on the single microsphere level