Free-radical polymerization upon near-infrared light irradiation,merging photochemical and photothermal initiating methods

Polymerization of (meth)acrylate resins upon near-infrared (NIR) light remains a huge challenge. In this study, a new photoinduced method of polymerization of methacrylic monomers is presented, originally merging a photochemical and a photothermal pathway. A four-component system is proposed comprising an NIR dye combined with an iodonium salt, a phosphine, and a thermal initiator. A selection of dyes is suggested regarding electron transfer properties and/or light-to-heat conversion abilities. Several thermal initiators are studied: an alkoxyamine (BlocBuilder MA), an azo derivative, and a peroxide. For the first time, an NIR absorbing dye is used in photopolymerization using both its capacities of light-to-heat conversion and its ability to initiate an electron transfer reaction. Three wavelengths of irradiation will be presented here: 785, 940, and 1064 nm. These long wavelengths are challenging because the energy of photons is extremely low but these wavelengths offer significant advantages in term of light penetration (e.g., for the access to composites through photopolymerization processes). The different systems presented here exhibit high and rapid conversions of methacrylate functions. The underlying chemical mechanism will be fully depicted by real-time Fourier transform infrared spectroscopy and thermal imaging measurements. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 300–308     

Morphological transformation of a germanium layer grown on a silicon surface by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (<350°C) and studied using photoluminescence and atomic force microscopy. It is found that the germanium layer undergoes a morphological transformation when its thickness becomes equal to approximately five monolayers: an island relief transforms into a smooth undulating relief.     

B-DNA Structure and Stability: The Role of Nucleotide Composition and Order

We have quantum chemically analyzed the influence of nucleotide composition and sequence (that is, order) on the stability of double-stranded B-DNA triplets in aqueous solution. To this end, we have investigated the structure and bonding of all 32 possible DNA duplexes with Watson–Crick base pairing, using dispersion-corrected DFT at the BLYP-D3(BJ)/TZ2P level and COSMO for simulating aqueous solvation. We find enhanced stabilities for duplexes possessing a higher GC base pair content. Our activation strain analyses unexpectedly identify the loss of stacking interactions within individual strands as a destabilizing factor in the duplex formation, in addition to the better-known effects of partial desolvation. Furthermore, we show that the sequence-dependent differences in the interaction energy for duplexes of the same overall base pair composition result from the so-called “diagonal interactions” or “cross terms”. Whether cross terms are stabilizing or destabilizing depends on the nature of the electrostatic interaction between polar functional groups in the pertinent nucleobases.     

How the Chalcogen Atom Size Dictates the Hydrogen-Bond Donor Capability of Carboxamides,Thioamides, and Selenoamides

The amino groups of thio- and selenoamides can act as stronger hydrogen-bond donors than of carboxamides, despite the lower electronegativity of S and Se. This phenomenon has been experimentally explored, particularly in organocatalysis, but a sound electronic explanation is lacking. Our quantum chemical investigations show that the NH₂ groups in thio- and selenoamides are more positively charged than in carboxamides. This originates from the larger electronic density flow from the nitrogen lone pair of the NH₂ group towards the lower-lying π*_C=S and π*_C=Se orbitals than to the high-lying π*_C=O orbital. The relative energies of the π* orbitals result from the overlap between the chalcogen np and carbon 2p atomic orbitals, which is set by the carbon-chalcogen equilibrium distance, a consequence of the Pauli repulsion between the two bonded atoms. Thus, neither the electronegativity nor the often-suggested polarizability but the steric size of the chalcogen atom determines the amide's hydrogen-bond donor capability.     

Beneficial use of a cell coupling rheometry, conductimetry, and calorimetry to investigate the early age hydration of calcium sulfoaluminate cement

A specific cell was designed to monitor simultaneously the evolution of the viscoelastic properties, electrical conductivity, and temperature of a cement paste with ongoing hydration. Hydration of calcium sulfoaluminate cement by demineralised water or by a borated solution was then investigated as an example. Borate anions acted as set retarders but to a smaller extent than with ordinary Portland cement. The delay in cement hydration resulted from the precipitation of an amorphous or poorly crystallized calcium borate, which also caused a rapid stiffening (and thus a loss of workability) of the paste after mixing. The gypsum content of the CSA cement was shown to play a key role in the control of the cement reactivity.     

Pre-formulation studies on moisture absorption in microcrystalline cellulose using differential thermo-gravimetric analysis

A study on the differential thermo-gravimetric (DTG) measurements of microcrystalline cellulose (MCC) containing moisture indicated that particle size affected the amount of bound water and the flow indices. Thermal analysis of 6 commercial grades of MCC powders and MCC/water blends were performed using a thermo-gravimetric analyzer. These MCCs were differentiated by their particle size, bulk and tapped densities, crystallinity and micromeritic properties. From the DTG curves, it was observed that water loss from the MCC/water blends occurred in 3 phases which corresponded to the different states of water associated with the solid particles. Area under the third phase, or the falling rate phase, can be associated with the release of water that was physically shielded or bound to the solid. This water may be referred to as "structured" water. The large particle size grades of MCC-Avicel PH 102, PH 302 and Pharmacel 102 were found to possess smaller quantities of structured water. Water vapor sorption results revealed the monolayer capacities for the respective MCC grades. The amount of structured water appeared to correspond to the existence of bilayers on the surface of the small particle size MCC grades. Using the avalanche flow assessment method, flow properties of small particle size grades of MCC were found to be poorer as indicated by the significant correlation between their flow indices and size, in addition to the longer mean times to avalanche.