In practice, most of the studies about the interaction between cement and accelerators is performed with hand-mixed pastes. However, in many applications mixing occurs through spraying, which may affect accelerators reactivity and the microstructure of the hardened paste. The objective of this study is to analyze how the mixing process influences the early hydration of accelerated cement pastes. Isothermal calorimetry, X-ray diffraction, thermogravimetry and SEM imaging were performed on cement pastes produced by hand-mixing and by spraying, using equivalent doses of an alkali-free and an alkaline accelerator and two types of cement. Results showed a great influence of the spraying process on the reactivity of accelerators and on the morphology of the precipitated hydrates. Variations in hydration kinetics caused by the mixing method are explained and the results obtained might have a significant repercussion on how future research on the behavior of accelerated mixes will be performed.Peer ReviewedPostprint (author's final draft
A wide variety of ionic dendritic macromolecules based on bent-shaped structures have been synthesized and fully characterized. Regular polypropyleneimine dendrimers (PPI) of different generations and a random hyperbranched polymer (PEI) were selected as dendritic cores. Different bent-core structures derived from the 3,4 0 -biphenyl angular core with 5 or 6 aromatic rings as well as short and long terminal chains and spacers have been used for this study. The bent-core structure acts as strong driving force for the supramolecular arrangement of this type of macromolecules. All of the ionic dendritic polymers prepared show mesogenic behavior over broad temperature ranges, even if the carboxylic acid precursors were not mesomorphous. An extensive chemical structure-supramolecular organization relationship has been proposed for first time for this kind of bent-core dendrimer. Two types of supramolecular packing with lamellar and columnar order have been proposed on the basis of polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. Additionally, fibers that present anisotropic optical behavior can be drawn from the materials. These results open new and stimulating possibilities for both bent-core based and dendritic supramolecular systems, with interest in basic and applied supramolecular chemistry and materials science scenarios.
Control of the self-assembly of small molecules to generate architectures with diverse shapes and dimensions is a challenging research field. We report unprecedented results on the ability of ionic, bent dendritic molecules to aggregate in water. A range of analytical techniques (TEM, SEM, SAED, and XRD) provide evidence of the formation of rods, spheres, fibers, helical ribbons, or tubules from achiral molecules. The compact packing of the bent-core structures, which promotes the bent-core mesophases, also occurs in the presence of a poor solvent to provide products ranging from single objects to supramolecular gels. The subtle balance of molecule/solvent interactions and appropriate molecular designs also allows the transfer of molecular conformational chirality to morphological chirality in the overall superstructure. Functional motifs and controlled morphologies can be combined, thereby opening up new prospects for the generation of nanostructured materials through a bottom-up strategy.
Mesomorphic salicylaldimines and related Cu(II), Pd(II), Zn(II), and VO(IV) complexes modified with acrylate groups have been synthesized. The liquid crystalline properties of the acrylic monomers are reported and, except for the Zn(II) complex, all are mesogenic. Their free-radical polymerization has been undertaken under several conditions, in solution, by using a thermal initiator or a photoinitiator. Use of visible light and a fluorinated diaryltitanocene photoinitiator (Irgacure 784-DC from Ciba-Geigy) have been proved the most adequate conditions to polymerize either the organic or the metal-containing acrylate Schiff bases. The light-induced polymerization has been investigated by optical microscopy, DSC, and in cells, and only Cu(II) inhibits the process. The process has been applied to the preparation of metal-containing elastomers and networks, but the change of mesophase during polymerization and the high transition temperatures of diacrylates have limited the preparation of macroscopically oriented films. Light-activated polymerization has also been attempted for related vinyl-ether Schiff bases but it was unsuccessful.
Several first- or second-generation Janus-type codendrimers that combine promesogenic bent-core and rodlike molecular segments have been synthesized by the versatile CuAAC reaction and the materials have been characterized by POM, DSC, XRD, and SHG studies. Depending on the ratio between the number of rod- and bent-core units, these compounds form mesophases ranging from nematic to a complex polar smectic mesophase in which the two types of mesogenic moieties are segregated on a nanometric scale. In the case of a 1:1 bent/rod ratio, the materials form ferroelectric mesophases and the macroscopic polarization is stable in the absence of an applied electric field. The appearance of the different mesophases is explained in terms of the relative values of the transverse areas of the mesogenic cores.
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