Layered double hydroxides intercalated with borate anions: Fire and thermal properties in ethylene vinyl acetate copolymer

Fire and thermal properties of ethylene vinyl acetate (EVA) composites prepared by melt blending with layered double hydroxides (LDH) have been studied. Two types of LDHs intercalated with borate anion were prepared using the coprecipitation method and the metals Mg2+, Zn2+ and Al3+. Characterization of the LDHs and the EVA composites was performed using X-ray diffraction, thermogravimetric analysis, and cone calorimetry. Thermal analyses show that the addition of LDHs improves the thermal stability of EVA. Fire properties evaluated using the cone calorimeter were significantly improved in the EVA/LDH composites. The peak heat release rate was reduced by about 40% when only 3% by weight of the LDH was added to the copolymer. Comparison of the fire properties of the LDHs with those of aluminum trihydrate (ATH), magnesium hydroxides (MDH), zinc hydroxide (ZH) and their combinations at 40% loading, reveal that the LDHs were more effective than when MDH and ZH are used alone

On Pi-Stacking, CH/Pi, and Halogen Bonding in Halobenzene Clusters: Resonant 2-Photon Ionization Studies of Chlorobenzene

Noncovalent interactions such as hydrogen bonding, π-π stacking, CH/π interactions, and halogen bonding play crucial roles in a broad spectrum of chemical and biochemical processes, and can exist in cooperation or competition. Here we report studies of the homoclusters of chlorobenzene, a prototypical system where π-π stacking, CH/π interactions, and halogen bonding interactions may all be present. The electronic spectra of chlorobenzene monomer and clusters (Clbz)n with n = 1-4 were obtained using resonant 2-photon ionization in the origin region of the S0–S1 (ππ*) state of the monomer. The cluster spectra show in all cases a broad spectrum whose center is redshifted from the monomer absorption. Electronic structure calculations aid in showing that the spectral broadening arises in large part from inhomogeneous sources, including the presence of multiple isomers and Franck-Condon (FC) activity associated with geometrical changes induced by electronic excitation. Calculations at the M06-2x/aug-cc-pVDZ level find in total five minimum energy structures for the dimer, four π-stacked structures, and one T-shaped, and six representative minimum energy structures were found for the trimer. The calculated time-dependent density functional theory spectra using range-separated and meta-GGA hybrid functionals show that these isomers absorb over a range that is roughly consistent with the breadth of the experimental spectra, and the calculated absorptions are redshifted with respect to the monomer transition, in agreement with experiment. Due to the significant geometry change in the electronic transition, where for the dimer a transition from a parallel displaced to sandwich structure occurs with a reduced separation of the two monomers, significant FC activity is predicted in low frequency intermolecular modes

Flame-retarded Polystyrene: Investigating Chemical Interactions between Ammonium Polyphosphate and MgAl Layered Double Hydroxide

Potential flame retardants, MgAl-LDH and ammonium polyphosphate (APP), were added to neat polystyrene (PS) individually or in combinations at weight fractions no greater than 10%. Structural morphologies of MgAl-LDH and the corresponding PS nanocomposites were established via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) and cone calorimetry were used to study the thermal stability and fire performance of the composites. Time to ignition is greatly reduced for PS composites when compared to the virgin polymer. Synergistic effects were observed in both TGA and cone calorimetry for formulations containing both MgAl-LDH and APP. Physical and chemical interactions between MgAl-LDH and APP are responsible for the observed synergy in thermal stability and fire performance

Resonant two-photon ionization studies of non covalent interactions in halo aromatic clusters and spin-orbit coupling modeling in mono-halocarbenes

Non-covalent interactions in halobenzenes (PhX) (X=F, Cl, Br) and phenylamine (C6 H5 NH2 ) have been studied here using resonance two-photon ionization (R2PI) spectroscopy combined with a linear TOF-mass spectrometer. Their interaction with polar molecules in form of ammonia (NH 3 ) and trifluorohalomethanes (CF3 X) has also been studied. DFT and TD-DFT calculations using M06-2X functionals were carried out on different cluster conformations to compliment experimental results. A general trend of broadness in homogenous dimers (PhX)2 , has been attributed to mainly the presence of multiple cluster isomers and Frank-Condon activity in the low frequency intermolecular vibrational modes. The van der Waals interactions between PhX and ammonia were all assigned to an in-plane σ-type geometry as compared to an out of plane π-type conformation. Stable halogen bonded dimer structures in the C 6 H5 NH2 ...CF 3 X system were also optimized. In the second part of this dissertation, a global analysis of spin-orbit coupling in the mono-halocarbenes, CH(D)X, where X = Cl, Br, I is presented. Mono-halocarbenes are model systems for examining carbene singlet-triplet energy gaps and spin-orbit coupling. Experiments probing the ground vibrational levels in these carbenes have clearly demonstrated the presence of perturbations involving a low-lying triplet. To model these interactions more globally, a diabatic treatment of the spin-orbit coupling is adopted, where matrix elements are written in terms of a purely electronic spin-orbit matrix element which is independent of nuclear coordinates, and an integral representing the overlap of the singlet and triplet vibrational wavefunctions. In this way, the structures, harmonic frequencies, and normal mode displacements from ab initio and DFT calculations were used to calculate the vibrational overlaps of the singlet and triplet state levels, incorporating the full effects of Duschinsky mixing. These results were then incorporated with the electronic spin-orbit matrix element into a matrix diagonalization routine that calculated the term energies of the mixed singlet-triplet levels, which were iteratively fit to the extensive experimental results from SVL emission and SEP spectroscopy for the halocarbenes. These calculations have allowed many new assignments to be made, particularly for CHI, and provided spin-orbit coupling matrix elements and improved values for the singlet-triplet gaps

Does organic modification of layered double hydroxides improve the fire performance of PMMA?

The effect of modified layered double hydroxides (LDHs) on fire properties of poly(methyl methacrylate) is investigated. Organically-modified LDHs were prepared via rehydration of calcined hydrotalcite in a palmitate solution. Composites consisting of the organo-LDHs, unmodified hydrotalcite and calcined oxides were prepared with poly(methyl methacrylate) using melt blending. Thermal and fire properties of the (nano)composites were studied. The thermogravimetric analyses of the composites show an increase in thermal stability. Fire performance, evaluated using cone calorimetry, show that organically-modified LDHs composites give the best reductions in peak heat release rate, PHRR, i.e., 51% at 10% weight loading. Dispersion of the LDHs was characterized using transmission electron microscopy and X–ray diffraction. Nanocomposite formation was observed with organically-modified LDHs, while the unmodified LDH composites gave only microcomposites

Variation of benzyl anions in MgAl-layered double hydroxides: Fire and thermal properties in PMMA

Magnesium aluminum layered double hydroxides (MgAl-LDHs) intercalated with a range of benzyl anions were prepared using the coprecipitation method. The benzyl anions differ in functionality (i.e. carboxylate, sulfonate, and phosphonate) and presence or absence of an amino substituent. Various methods for preparing LDHs (i.e. ion exchange, coprecipitation and rehydration of the calcined LDH methods) have been compared with the MgAl-benzene phosphonate and their effect on fire and thermal properties was studied. After characterization, the MgAl-LDHs were melt-blended with poly(methyl methacrylate) (PMMA) at loadings of 3 and 10% by weight to prepare composites. Characterization of the LDHs and the PMMA composites was performed using FTIR, XRD, TGA, transmission electron microscopy (TEM) and cone calorimetry. FTIR and XRD analyses confirmed the presence of the charge balancing benzyl anions in the galleries of the MgAl-LDHs. Improvements in fire and thermal properties of the PMMA composites were observed. The cone calorimeter revealed that the addition of 10% MgAl-LDHs reduces the peak heat release rate by more than 30%

Water footprint of growing vegetables in selected smallholder irrigation schemes in South Africa

Crop water footprint (WF) is the volume of fresh water used to produce a certain crop in all the steps in the production line. The CROPWAT model was used to calculate crop evapotranspiration, differentiating green and blue water in Zanyokwe (ZIS), Thabina (TIS) and Tugela Ferry (TFIS) Irrigation Schemes. Green beans had the highest water footprint in all three irrigation schemes with 3 535.7 m3/t in TIS, 2 753 m3/t in TFIS and 2 407.6 m3/t in ZIS. Cabbage had the lowest water footprint. The highest water footprint for growing cabbage was 254.5 m3/t in TFIS, followed by 223.1 m3/t in TIS, and the lowest was 217.8 m3/t in ZIS. Green WF represented the highest percentage of water use at ZIS (50.5%), followed by blue water at 26.5% while grey water constituted 22.9%. At TFIS blue, green and grey water use was 23.1%, 56.7% and 20.2%, respectively. The differences observed in the WF of different crops and different schemes were attributed to the differences in weather and environmental characteristics. Green beans had the highest grey water footprint, i.e., 373 m3/t and the lowest was cabbage with 37 m3/t. Potato, spinach and tomatoes had footprints of 156 m3/t, 214 m3/t and 132 m3/t, respectively. For future research it is necessary to consider the possibility and trade-offs of shifting production of each crop to the places where it is most efficient, and to focus on blue water scarcity in each of the case study locationsKeywords: smallholder irrigation schemes, water footprint, evapotranspiration, water us

MATI SPECTROSCOPY OF Ln(OH)2 (Ln = La AND Ce) FORMED BY O-H BOND ACTIVATION OF WATER.

Lanthanide (Ln = La and Ce) atom reactions with water are carried out in a pulsed-laser ablation molecular beam source and characterized by mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical calculations. Both reactions yield Ln(OH)$_{2}$ as the main product through a hydrogen atom elimination of each water molecule. The MATI spectra of Ln(OH)$_{2}$ are dominated by the origin band and metal-ligand symmetric stretching and bending vibronic progressions. Adiabatic ionization energies measured from the spectra are 40135 (5) \wn for La(OH)$_{2}$ and 40756 (5) \wn for the Ce(OH)$_{2}$. The molecular symmetry of Ln(OH)$_{2}$ is C$_{2}$$_{v}$, and the observed transitions are $^{1}$A$_{1}$ - $^{2}$A$_{1}$ for La(OH)$_{2}$ and $^{2}$B$_{1}$ - $^{3}$B$_{1}$ for Ce(OH)$_{2}$. The ground valence electron configurations of La(OH)$_{2}$ and Ce(OH)$_{2}$ are La 6s$^{1}$ and Ce 4f$^{1}$6s$^{1}$, respectively. Ionization of each species removes a Ln 6s-based electron, and the resultant ion also has C$_{2}$$_{v}$ symmetry. The spectrum of Ce(OH)$_{2}$ has a broader linewidth than that of La(OH)$_{2}$, which is attributed to the unresolved spin-orbit levels by comparing with relativistic quantum calculations at the level of spin-orbit multi-reference quasi-degenerated perturbation theory. The metal-mediated hydrogen elimination of water is predicted to be thermodynamically and kinetically favorable by the density functional theory calculations

Spectroscopic and Computational Characterization of Lanthanum-Mediated C-H and N-H Bond Activation of Amines

Metal-mediated bond activation of small organic and inorganic molecules plays critical roles in chemical transformation of small molecules into value-added products. This is because few of such chemical reactions would occur under mild conditions without the metal activation. In this work, lanthanum atom reactions with alkylamines are carried out in a laser-ablation supersonic molecular beam source; C-H and N-H bond activation in these species is investigated. The reaction products are observed with photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization (MATI) spectroscopy and theoretical calculations. Adiabatic ionization energy and metal-ligand and ligand-based vibrational frequencies of several short-lived lanthanum complexes are measured from MATI spectra. Molecular structures, electronic states, and formation mechanisms of these complexes are identified by combining the spectroscopic measurements with density functional theory calculations and spectral simulations

Mass-Analyzed Threshold Ionization of Lanthanide Imide LnNH (Ln = La and Ce) Radicals from N–H Bond Activation of Ammonia

Ln (Ln = La and Ce) atom reactions with ammonia are carried out in a pulsed laser vaporization supersonic molecular beam source. Lanthanide-containing species are observed with time-of-flight mass spectrometry, and LnNH molecules are characterized by mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical calculations. The theoretical calculations include density functional theory for both Ln species and a scalar relativity correction, electron correlation, and spin-orbit coupling for the Ce species. The MATI spectrum of LaNH exhibits a single vibronic band system with a strong origin band and two weak vibronic progressions, whereas the spectrum of CeNH displays two band systems separated by 75 cm−1 with each being like the LaNH spectrum. By comparing with the theoretical calculations, both LaNH and CeNH are identified as linear molecules with C∞v symmetry, and the two vibronic progressions are attributed to the excitations of Ln–N stretching and Ln–N–H bending modes in the ions. The additional band system observed for CeNH is due to the spin-orbit splitting from the interactions of triplet and singlet states. The ground valence electron configurations of LaNH and CeNH are La 6s1 and Ce 4f16s1, and the ionization of each species removes the Ln 6s1 electron. The remaining two electrons that are associated with the isolated Ln atoms or ions are in a doubly degenerate molecular orbital that is a bonding combination between Ln 5dπ and N pπ orbitals