Chromophores from hexeneuronic acids: chemical behavior under peroxide bleaching conditions

Hexeneuronic acids (HexA) are a major cause of discoloration (yellowing/brightness reversion) in pulps from xylan-containing wood, being generated from the xylan’s 4-O-methylglucuronic acid residues. The HexA-derived chromophores, whose identification and structure confirmation have been described in the previous part of this series (Rosenau et al. in Cellulose, 2017), were subject to conditions of peroxide bleaching, i.e. treatment with hydrogen peroxide in alkaline medium. These chromophores, ladder-type oligomers of mixed aromatic-quinoid and mixed furanoid-benzoid character, are degraded relatively quickly to one major product, 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ), and a minor component, 2,5-dihydroxyacetophenone (DHA). These two compounds, which have already been identified as two of the three key chromophores (besides 5,8-dihydroxy-[1,4]-naphthoquinone, DHNQ) in aged cellulosics, are potent chromophores themselves and are subsequently more slowly degraded to non-colored degradation products, according to pathways already described in previous parts of this series. The occurrence of DHBQ and DHA in the bleaching treatment of the HexA-derived chromophores establishes the link between HexA chemistry and the key chromophore classes of residual chromophores found in aged cellulosic materials.     

Enantiomeric impurity determination in capillary electrophoresis using a highly-sulfated cyclodextrins-based method

Capillary electrophoresis (CE), using highly-sulfated cyclodextrins as chiral selectors, has been applied to determine the chiral purity of pharmaceutical compounds. A chiral separation strategy, developed earlier for racaemic mixtures, was applied on four basic drugs (propranolol, atenolol, chlorpheniramine and tryptophan methylester). The aim was to develop validated separation methods which allow determination of 0.1% impurity levels of the unwanted enantiomers (distomer) in the presence of 99.9% of the active compound (eutomer). The linearity, quantification limits for the trace enantiomers and the precision of the measurements were determined. In a second part, impurity separations have been simulated in order to evaluate the required resolution when assaying impurities. It is shown that a baseline resolution of 1.5, generally accepted for racaemic mixtures, does not always allow good impurity determinations. Two alternative methods to solve this problem have been proposed.     

Novel Chromophore‐Functionalized Poly[2‐(trifluoromethyl) adamantyl acrylate‐methyl vinyl urethane]s with High Poling Stabilities of the Nonlinear Optical Effect

Nonlinear optical vinyl polymers with high glass transition temperature (T_g) were prepared by the functionalization of a fluorinated acrylate‐methyl vinyl isocyanate copolymer. A modified pathway to obtain a thiophene bridged chromophore was worked out. Poled films of the polymers show a fairly high and stable nonlinear optical response, even at elevated temperatures.

The thiophene‐bridged chromophore, based on a substituted dicyanomethylene‐dihydrofuran acceptor, synthesized here.     

Stimuli‐Responsive Particle‐Based Amphiphiles as Active Colloids Prepared by Anisotropic Click Chemistry

Amphiphiles alter the energy of surfaces, but the extent of this feature is typically constant. Smart systems with amphiphilicity as a function of an external, physical trigger are desirable. As a trigger, the exposure to a magnetic field, in particular, is attractive because it is not shielded in water. Amphiphiles like surfactants are well known, but the magnetic response of molecules is typically weak. Vice‐versa, magnetic particles with strong response to magnetic triggers are fully established in nanoscience, but they are not amphiphilic. In this work colloids with Janus architecture and ultra‐small dimensions (25 nm) have been prepared by spatial control over the thiol‐yne click modification of organosilica‐magnetite core–shell nanoparticles. The amphiphilic properties of these anisotropically modified particles are proven. Finally, a pronounced and reversible change in interfacial stabilization results from the application of a weak (<1 T) magnetic field.     

A Low-Temperature Approach for the Phase-Pure Synthesis of MIL-140 Structured Metal–Organic Frameworks

In a systematic investigation, the synthesis of metal–organic frameworks (MOFs) with MIL-140 structure was studied. The precursors of this family of MOFs are the same as for the formation of the well-known UiO-type MOFs although the synthesis temperature for MIL-140 is significantly higher. This study is focused on the formation of Zr-based MIL-140 MOFs with terephthalic acid (H₂bdc), biphenyl-4,4′-dicarboxylic acid (H₂bpdc), and 4,4′-stilbenedicarboxylic acid (H₂sdc) and the introduction of synthesis field diagrams to discover parameters for phase-pure products. In this context, a MIL-140 network with H₂sdc as linker molecule is first reported. Additionally, an important aspect is the reduction of the synthesis temperature to make MIL-140 MOFs more accessible even though linkers with a more delicate nature are used. The solvothermal syntheses were conducted in highly concentrated reaction mixtures whereby a targeted synthesis to yield the MIL-140 phase is possible. Furthermore, the effect of the often-used modulator approach is examined for these systems. Finally, the characteristics of the synthesized MOFs are compared with physisorption measurements, thermogravimetric analyses, and scanning electron microscopy.     

Dijet angular distributions at \sqrt{s}=14 TeV

We present a Monte Carlo study of dijet angular distributions at $\sqrt{s}=14$  TeV. First we perform a next-to-leading order QCD study; we calculate the distributions in four different bins of dijet invariant mass using different Monte Carlo programs and different jet algorithms, and we also investigate the systematic uncertainties coming from the choice of the parton distribution functions and the renormalization and factorization scales. In the second part of this paper, we present the effects on the distributions coming from a model including gravitational scattering and black hole formation in a world with large extra dimensions. Assuming a 25% systematic uncertainty, we report a discovery potential for the mass bin 1<M _jj <2 TeV at 10 pb^?1 integrated luminosity.     

Quality of Brazilian vegetable oils evaluated by (modulated) differential scanning calorimetry

Vegetable oils are increasingly replacing fossil-oil-based polymers, and therefore aimed at being used in polymerization reactions from ?20 to 100?°C. Therefore, phase transitions and heat capacities in this temperature range should be well characterized to optimize processing conditions and energy inputs. By using the DSC analysis, only small primary correspondence or divergence between different oil types are seen as a function of their degree of unsaturation, but it does not clearly distinguish detailed features such as shoulder bands related to the separate melting processes of single fatty acid components. By using modulated DSC analysis, the combined analysis of reversing and non-reversing heat signals provides better results. The latter confirms that the melting is not a physical one-step process, but equilibrates between phase transitions and enthalpic reorganizations of the fatty acids that can be monitored separately. The specific heat capacities measured during modulated DSC are somewhat lower than traditional calorimetric measurements, but relate to the degree of unsaturation. The thermal behavior of palm-, soy-, sunflower-, corn-, castor-, and rapeseed-oil is discussed in relation to their composition, by applying a first or second heating scan.     

Micro- to nanoscale surface morphology and friction response of tribological polyimide surfaces

Sintered polyimide surfaces that were worn under macroscale conditions at different temperatures, were further characterised by contact-mode atomic force microscopy for getting insight in the tribophysical and -chemical processes at the micro- to nanoscale. Depending on the temperature, either mechanical interaction (23 °C < T < 100 °C), hydrolysis (120 °C < T < 140 °C), or imidisation (180 °C < T < 260 °C) results in different microscale surface characteristics. At low temperatures, surface brittleness and inter-grain fracture has been observed with an almost homogeneous friction pattern. At intermediate temperatures, the formation of a protecting local film leads to smoother surfaces with local lubricating properties. At high temperatures, different topographical and frictional patterns are observed depending on local imidisation or degradation. From AFM scans at the sub-micronscale, local debris depositions are observed and correspond to surface locations with locally reduced friction. From AFM scans at the nanoscale, polymer chain orientation is observed with formation of zig-zag or stretched molecular conformation: the latter is not induced by purely mechanical surface interactions or hydrolysis, but mainly results from tribochemically induced imidisation at high sliding temperatures. The present investigation describes the influences of local tribological interactions onto the macroscale wear behaviour of a polymer, and therefore aims at contributing to a better understanding of scaling between macro- to nanolevel tribological response.     

Nanomorphology and reaction to fire of polyurethane and polyamide nanocomposites containing flame retardants

The reaction to fire of polymer nanocomposites (thermoplastic polyurethane and polyamide-6) containing two different nanofillers (organoclay and carbon nanotube) has been investigated. Polymer nanocomposites exhibit significant reduction of peak of heat release rate but the nanomorphology (exfoliation, intercalation and presence of tactoids) does not play any significant role, although a reasonable level of nanodispersion is necessary to achieve good flame retardancy in specific cases (mass loss calorimetry experiment). Modelling aspects for the time to ignition are also proposed in the paper. It is shown that the approach ‘nanocomposite’ gives the best results combined with conventional flame retardants (phosphinate and phosphate) and leads to synergistic effects. The aspects of nanodispersion of the nanoparticle with the flame retardant (microfiller) are fully commented in the paper using TEM and solid state NMR. Mechanisms of action are finally proposed explaining the synergy when conventional flame retardants are combined with nanoparticles.