In situ investigation of phosphonate retarder interaction in oil well cements at elevated temperature and pressure conditions.

The effect of a high-performance retarding additive in oil well cements was investigated under elevated temperature (165°C) and pressure (1000 psi) conditions via in situ synchrotron-based X-ray diffraction (XRD) and quasielastic neutron scattering (QENS) techniques. Under these temperature and pressure conditions, crystalline calcium silicate hydrates (C-S-H) are formed through the cement hydration process. From in situ XRD experiments, the retardation effect was observed by a change in the rate of the appearance of 11 Å tobermorites as well as a change in the rate of the α-C2SH generation and depletion. QENS analysis revealed that the retardation effect was related to the non-conversion of free water to chemical and constrained water components. A high presence of free water components was attributed to a decrease in 11 Å tobermorites along with slower consumption of the quartz and portlandite phases. Furthermore, QENS results infer that the water molecules experienced confinement in the restricted pore spaces. The retarder inhibited this initial water confinement by slowing the bulk diffusion of free water in the confined region.

3D Printed Tubulanes as Lightweight Hypervelocity Impact Resistant Structures.

Lightweight materials with high ballistic impact resistance and load-bearing capabilities are regarded as a holy grail in materials design. Nature builds these complementary properties into materials using soft organic materials with optimized, complex geometries. Here, the compressive deformation and ballistic impact properties of three different 3D printed polymer structures, named tubulanes, are reported, which are the architectural analogues of cross-linked carbon nanotubes. The results show that macroscopic tubulanes are remarkable high load-bearing, hypervelocity impact-resistant lightweight structures. They exhibit a lamellar deformation mechanism, arising from the tubulane ordered pore structure, manifested across multiple length scales from nano to macro dimensions. This approach of using complex geometries inspired by atomic and nanoscale models to generate macroscale printed structures allows innovative morphological engineering of materials with tunable mechanical responses.

Phase Change Transformations with Dynamically Addressable Aminal Metallogels.

Dynamic polymers assembled through hemiaminal and aminal functionalities reversibly fragment upon binding to trivalent metals. Gels produced with these dynamic polymers are broken down to liquids after the addition of metal salts. Nuclear magnetic resonance spectroscopy studies and density functional theory calculations of intermediates reveal that the presence of these metals causes shifts in the energetic landscape of the intermediates in the condensation pathway to render stable nonequilibrium products. These species remain stable in the liquid phase at room temperature but convert to gels upon heating. With thermal activation, the fragmented ligands transform catalytically into closed-ring hexahydrotriazine products, which are macroscopically observable as new gels with distinct physical properties. The interplay between equilibrium and nonequilibrium gels and liquids and the ligands responsible for these transformations has been observed rheologically, giving controlled gel times dictated by the thermodynamics and kinetics of the system. This constitutionally dynamic macromolecular system offers the possibility of harnessing an equilibrium/nonequilibrium system in tandem with its inherent self-healing properties and triggered release functionality.

Theoretical and structural analysis of long C-C bonds in the adducts of polycyanoethylene and anthracene derivatives and their connection to the reversibility of Diels-Alder reactions.

X-ray structure determinations on four Diels-Alder adducts derived from the reactions of cyano- and ester-substituted alkenes with anthracene and 9,10-dimethylanthracene have shown the bonds formed in the adduction to be particularly long. Their lengths range from 1.58 to 1.62 Å, some of the longest known for Diels-Alder adducts. Formation of the four adducts is detectably reversible at ambient temperature and is associated with free energies of reaction ranging from -2.5 to -40.6 kJ mol(-1). The solution equilibria have been experimentally characterised by NMR spectroscopy. Density-functional-theory calculations at the MPW1K/6-31+G(d,p) level with PCM solvation agree with experiment with average errors of 6 kJ mol(-1) in free energies of reaction and structural agreement in adduct bond lengths of 0.013 Å. To understand more fully the cause of the reversibility and its relationship to the long adduct bond lengths, natural-bond-orbital (NBO) analysis was applied to quantify donor-acceptor interactions within the molecules. Both electron donation into the σ*-anti-bonding orbital of the adduct bond and electron withdrawal from the σ-bonding orbital are found to be responsible for this bond elongation.

Deformation resilient cement structures using 3D-printed molds.

Cementitious structures exhibit high compression strength but suffer from inherent brittleness. Conversely, nature creates structures using mostly brittle phases that overcome the strength-toughness trade-off, mainly through internalized packaging of brittle phases with soft organic binders. Here, we develop complex architectures of cementitious materials using an inverse replica approach where a soft polymer phase emerges as an external conformal coating. Architected polymer templates are printed, cement pastes are molded into these templates, and cementitious structures with thin polymer surface coating are achieved after the solubilization of sacrificial templates. These polymer-coated architected cementitious structures display unusual mechanical behavior with considerably higher toughness compared to conventional non-porous structures. They resist catastrophic failure through delayed damage propagation. Most interestingly, the architected structures show significant deformation recovery after releasing quasi-static loading, atypical in conventional cementitious structures. This approach allows a simple strategy to build more deformation resilient cementitious structures than their traditional counterparts.

Coordination Dynamics and Thermal Stability with Aminal Metallogels and Liquids.

In this article, we review a dynamic covalent gel system developed as a high temperature well construction fluid. The key gel/fluid phase changes and related materials properties are addressable via the constitutional and coordination dynamics of the equilibrium and non-equilibrium molecular species comprising the material. The interplay between these species and external stimuli leads to material adaptability. Specifically, the introduction of metal ions into a non-equilibrium hemiaminal gel reverts this phase into a non-equilibrium liquid. When heated, this liquid transforms itself catalytically into the thermodynamically favoured closed-ring polyhexahydrotriazine (PHT) gel product. The temperature stability of different PHT gel formulations is evaluated as a function of the inclusion of various salts. It is possible to revert this thermodynamic PHT gel back into a liquid. This pH dependent transformation depends on the R groups linking the hexahydrotriazines (HTs) to one another. While polyethylene glycol (PEG) based PHT gels revert to liquids with water and mild protonation conditions, in comparison, polypropylene glycol (PPG) based gels require stronger acid conditions with heat, or a different more nucleophilically driven ring-opening mechanism by, for example, phosphines. The covalent dynamic chemistry in this chemical system gives way to many possible applications in addition to the high temperature solution-gelation (sol-gels) for which it has been primarily designed.

Reversible Diels-Alder reactions for the generation of dynamic combinatorial libraries.

Condensation reactions between various dienes and dienophiles have been screened for reversibility. Functionalized fulvenes, bearing in particular biological groups, and cyanolefins have been found to react rapidly and reversibly, in the temperature range from -10 to +50 degrees C. These results pave the way for the development of dynamic combinatorial libraries based on reversible Diels-Alder chemistry.

Sapphyrin-nanotube assemblies.

Single wall carbon nanotubes (SWNTs) bind strongly to sapphyrins, quintessential pentapyrrolic "expanded porphyrin" macrocycles, through donor-acceptor stacking interactions. The specific use of a functionalized sapphyrin diol yields stable water-suspendable nanotubes and also permits the formation of well-defined assemblies in ionic liquids. The absorption and steady-state fluorescence spectra of the resulting noncovalently functionalized nanotube complexes have been analyzed in aqueous media and ionic liquids, yielding a description of the photophysical properties of the nanotube-sapphyrin complexes as donor-acceptor species for light-harvesting.

Band gap fluorescence from individual single-walled carbon nanotubes.

Fluorescence has been observed directly across the band gap of semiconducting carbon nanotubes. We obtained individual nanotubes, each encased in a cylindrical micelle, by ultrasonically agitating an aqueous dispersion of raw single-walled carbon nanotubes in sodium dodecyl sulfate and then centrifuging to remove tube bundles, ropes, and residual catalyst. Aggregation of nanotubes into bundles otherwise quenches the fluorescence through interactions with metallic tubes and substantially broadens the absorption spectra. At pH less than 5, the absorption and emission spectra of individual nanotubes show evidence of band gap-selective protonation of the side walls of the tube. This protonation is readily reversed by treatment with base or ultraviolet light.