A chemo-mechanical model for describing sorption hysteresis in a glassy polyurethane.

Hysteretic sorption and desorption of water is observed from 0 to 95% relative humidity and 298-333 K on a glassy polyurethane foam. It is postulated that sorption-induced swelling of the glassy polyurethane increases the concentration of accessible hydrogen-bonding adsorption sites for water. The accessibility of sites is kinetically controlled due to the restricted thermal motions of chains in the glassy polymer, causing a difference in accessible site concentrations during sorption and desorption. This discrepancy leads to hysteresis in the sorbed concentrations of water. A coupled chemo-mechanical model relating volumetric strain, adsorption site concentration, and sorbed water concentration is employed to describe water sorption hysteresis in the glassy polyurethane. This model not only describes the final mass uptake for each relative humidity step, but also captures the dynamics of water uptake, which exhibit diffusion and relaxation rate-controlled regimes.

Predicting 3D moisture sorption behavior of materials from 1D investigations.

Moisture in materials can be a source of future outgassing and exacerbate unwanted changes in physical and chemical properties. Here, we investigate the effect of sample size and shape on the moisture transport phenomena through a combined experimental and modeling approach. Several different materials varying in size and shape were investigated over a wide range of relative humidities (0-90%) and temperatures ([Formula: see text]) using gravimetric type dynamic vapor sorption (DVS). A dynamic triple-mode sorption model, developed previously, was employed to describe the experimental results with good success; the model includes absorption, adsorption, pooling (clustering) of species, and molecular diffusion. Here we show that the full triple-mode sorption model is robust enough to predict the dynamic uptake and outgassing of 3-dimensional (3D) samples using parameters derived from quasi-1D samples. This successful demonstration on three different materials (filled polydimethylsiloxane (PDMS), unfilled PDMS, and ceramic inorganic composite) illustrates that the model is robust at describing the scale-independent physics and chemistry of moisture sorption and diffusion materials. This work demonstrates that while sorption mechanisms manifest in testing of all sample sizes, some of these mechanisms were so subtle that they were overlooked in our initial modeling and assessment, illustrating the importance of multi-scale experiments in the development of robust predictive capabilities. Our study also outlines the challenges and viable solutions for global optimization of a multi-parameter model. The ability to quantify moisture sorption and diffusion, independent of scale, using 1D lab-scale experiments enables prediction of long-term bulk materials behavior in real applications.

Role of filler and its heterostructure on moisture sorption mechanisms in polyimide films.

Moisture sorption and diffusion exacerbate hygrothermal aging and can significantly alter the chemical and mechanical properties of polymeric-based components over time. In this study, we employ a multi-pronged multi-scale approach to model and understand moisture diffusion and sorption processes in polyimide polymers. A reactive transport model with triple-mode sorption (i.e., Henry's, Langmuir, and pooling), experiments, and first principles atomistic computations were combined to synergistically explore representative systems of Kapton H and Kapton HN polymers. We find that the CaHPO4 processing aid used in Kapton HN increases the total moisture uptake (~0.5 wt%) relative to Kapton H. Henry's mode is found to play a major role in moisture uptake for both materials, accounting for >90% contribution to total uptake.However, the pooling mode uptake in Kapton HN was ~5 times higher than in Kapton H. First principles thermodynamics calculations based on density functional theory predict that water molecules chemisorb (with binding energy ~17-25 kcal/mol) on CaHPO4 crystal surfaces. We identify significant anisotropy in surface binding affinity, suggesting a possible route to tune and mitigate moisture uptake in Kapton-based systems through controlled crystal growth favoring exposure of CaHPO4 (101) surfaces during manufacturing.

Dynamic Triple-Mode Sorption and Outgassing in Materials.

Moisture uptake and outgassing can be detrimental to a system by altering the chemical and mechanical properties of materials within the system over time. In this work, we conducted isotherm experiments to investigate dynamic moisture sorption and desorption in markedly different materials, i.e., a polymeric material, Sylgard-184 and a ceramic aluminosilicate material, Zircar RS-1200, at different temperatures (30 °C-70 °C) by varying the water activity (0.0-0.90). Sylgard-184 showed a linear sorption and outgassing behavior with no-hysteresis over the entire temperature and water activity range considered here. Whereas, the sorption and outgassing of Zircar RS-1200 was highly non-linear with significant hysteresis, especially at higher water activities, at all temperatures considered here. The type of hysteresis suggested the presence of mesopores in Zircar RS-1200, whereas the lack of hysteresis in Sylgard-184 indicates that it has a nonporous structure. A diffusion model coupled with a dynamic, triple-mode sorption (Langmuir, Henry, and pooling modes) model employed in this study matched our experimental data very well and provides mechanistic insight into the processes. Our triple-mode sorption model was adaptive enough to (1) model these distinctly different materials and (2) predict sorption and outgassing under conditions that are distinctly different from the parameterization experiments.

Modeling and Uncertainty Quantification of Vapor Sorption and Diffusion in Heterogeneous Polymers.

A high-fidelity model of kinetic and equilibrium sorption and diffusion is developed and exercised. The gas-diffusion model is coupled with a triple-sorption mechanism: Henry's law absorption, Langmuir adsorption, and pooling or clustering of molecules at higher partial pressures. Sorption experiments are conducted and span a range of relative humidities (0-95 %) and temperatures (30-60 °C). Kinetic and equilibrium sorption properties and effective diffusivity are determined by minimizing the absolute difference between measured and modeled uptakes. Uncertainty quantification and sensitivity analysis methods are described and exercised herein to demonstrate the capability of this modeling approach. Water uptake in silica-filled and unfilled poly(dimethylsiloxane) networks is investigated; however, the model is versatile enough to be used with a wide range of materials and vapors.

Advances in modeling sorption and diffusion of moisture in porous reactive materials.

Water-vapor-uptake experiments were performed on a silica-filled poly(dimethylsiloxane) (PDMS) network and modeled by using two different approaches. The data was modeled by using established methods and the model parameters were used to predict moisture uptake in a sample. The predictions are reasonably good, but not outstanding; many of the shortcomings of the modeling are discussed. A high-fidelity modeling approach is derived and used to improve the modeling of moisture uptake and diffusion. Our modeling approach captures the physics and kinetics of diffusion and adsorption/desorption, simultaneously. It predicts uptake better than the established method; more importantly, it is also able to predict outgassing. The material used for these studies is a filled-PDMS network; physical interpretations concerning the sorption and diffusion of moisture in this network are discussed.

Thermodynamic study on dynamic water vapor sorption in Sylgard-184.

The dynamic and equilibrium water vapor sorption properties of Sylgard-184, a commercially available poly(dimethylsiloxane) elastomer (PDMS), were determined via gravimetric analysis from 30 to 70 °C. Described here is a methodology for quantitatively assessing how water vapor diffuses and ad/absorbs into polymeric materials that are traditionally considered hydrophobic. PDMS materials are frequently chosen for their moisture barrier properties; our results, however, demonstrate that moisture is able to penetrate the material over a range of temperatures and humidities. The sorption values measured here ranged from ca. 0.1 to 1.4 cm(3) (STP) H(2)O/g Sylgard. The isotherms exhibited sigmoidal character and were fit to a triple mode sorption model. Asymptotic behavior at low water activities was characterized using a Langmuir type adsorption model, linear behavior was fit to a Henry's law type dependence, and the convex portion at higher activities was fit with good agreement to Park's equation for pooling or clustering. The thermal dependence of these sorption modes was also explored and reported. The dynamics of the sorption process were fit to a Fickian model and effective diffusivities are reported along with corresponding activation energies. The diffusivity values measured here ranged from ca. 0.5 to 3.5 × 10(-5) cm(2)/s depending on the temperature and relative humidity. The concentration dependence of the diffusivity showed a direct correlation with the three modes of uptake obtained from the isotherms. Corrections to the diffusivities were calculated using existing models that take into account adsorption and pooling.

Moisture desorption rates from TATB formulations: experiments and kinetic models.

The rate of water desorption from PBX-9502, a formulation containing 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), is measured using temperature-programmed desorption and modeled using conventional kinetic modeling methods. The results of these studies show two stages of moisture release. At lower temperatures, the release is likely assisted by thermal expansion of the TATB and melting of the Kel-F binder. At higher temperatures, a considerable amount of water is released and is attributed to sublimation of the TATB, which exposes new surfaces for water desorption.

Time-resolved IR studies on the mechanism for the functionalization of primary C-H bonds by photoactivated Cp*W(CO)3(Bpin).

Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the well-defined systems that reacts under photochemical conditions using both density functional theory calculations and pico- through microsecond time-resolved IR spectroscopy. UV irradiation of Cp*W(CO)(3)(Bpin) (Cp* = C(5)(CH(3))(5); pin = 1,2-O(2)C(2)-(CH(3))(4)) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-Cp*W(CO)(2)(Bpin)(pentane), undergoes C-H bond activation by a sigma-bond metathesis mechanism--in 16 micros, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a sigma-borane complex, Cp*W(CO)(2)(H-Bpin)(C(5)H(11)). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C(5)H(11) ligand. In this configuration, the B-H sigma-bond is broken in favor of a B-C sigma-bond, forming Cp*W(CO)(2)(H)(C(5)H(11)-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form Cp*W(CO)(2)(H) in approximately 170 micros. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex.

Pressure-dependent decomposition kinetics of the energetic material HMX up to 3.6 GPa.

The effect of pressure on the global thermal decomposition rate of the energetic material HMX was studied. HMX was precompressed in a diamond anvil cell (DAC) and heated at various rates. The parent species population was monitored as a function of time and temperature using Fourier transform infrared (FTIR) spectroscopy. Global decomposition rates were determined by fitting the fraction reacted to the extended-Prout-Tompkins nucleation-growth model and the Friedman isoconversional method. The results of these experiments and analysis indicate that pressure accelerates the decomposition at low-to-moderate pressures (i.e., between ambient pressure and 0.1 GPa) and decelerates the decomposition at higher pressures. The decomposition acceleration is attributed to pressure-enhanced autocatalysis, whereas the deceleration at high pressures is attributed to pressure-inhibiting bond homolysis step(s), which would result in an increase in volume. These results indicate that both the beta- and delta-polymorphs of HMX are sensitive to pressure in the thermally induced decomposition kinetics.

Nanosecond time-resolved and steady-state infrared studies of photoinduced decomposition of TATB at ambient and elevated pressure.

The time scale and/or products of photoinduced decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were investigated at ambient pressure and compared with products formed at 8 GPa. Ultrafast time-resolved infrared and steady-state Fourier transform IR (FTIR) spectroscopies were used to probe TATB and its products after photoexcitation with a 5 ns pulse of 532 nm light. At ambient pressure, transient spectra of TATB indicate that the molecule has significantly decomposed within 60 ns; transient spectra also indicate that formation of CO(2), an observed decomposition product, is complete within 30-40 mus. Proof of principle time-resolved experiments at elevated pressures were performed and are discussed briefly. Comparison of steady-state FTIR spectra obtained at ambient and elevated pressure (ca. 8 GPa) indicate that the decomposition products vary with pressure. We find evidence for water as a decomposition product only at elevated pressure.

Direct observation of photoinduced bent nitrosyl excited-state complexes.

Ground-state structures with side-on nitrosyl (eta (2)-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO) 3(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and eta (2)-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO) 3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.

Ligand rearrangement reactions of Cr(CO)6 in alcohol solutions: experiment and theory.

The ligand rearrangement reaction of Cr(CO)6 is studied in a series of alcohol solutions using ultrafast infrared spectroscopy and Brownian dynamics simulations. Excitation with 266 nm light gives Cr(CO)5 which is quickly solvated by a ligand from the bath. In alcohol solutions, solvation by an alkyl or hydroxyl site can occur; all alkyl bound complexes eventually rearrange to hydroxyl bound complexes. This rearrangement has been described using both an intermolecular (stochastic) and intramolecular (chainwalk) mechanism. Experiments alone do not allow for characterization of the mechanism, and therefore, theoretical calculations were carried out for the first time by modeling the ligand rearrangement as a diffusive walk along a potential defined by the different interaction possibilities. Experiments and simulations were carried out for Cr(CO)6 in 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 2-pentanol, 2-methylbutanol, and 3-methylbutanol. The trends in the theoretical and experimental rearrangement times are similar for all simulations carried out indicating that the two mechanisms have very similar ensemble behavior when bath effects are taken into account. The nature of the mechanism responsible for motion along the alcohol chain is not of primary importance in isolating the kinetic behavior because of the highly diffusive nature of the reaction. Future experimental and theoretical work will be directed at identifying a definitive assignment of the reaction mechanism.

The role of odd-electron intermediates and in-cage electron transfer in ultrafast photochemical disproportionation reactions in Lewis bases.

Femtosecond visible pump-IR probe studies of Cp2W2(CO)6 in P(OMe)3 and CH2Cl2 have allowed direct observation of a 19-electron intermediate and of disproportionation into CpW(CO)3- and CpW(CO)3P(OMe)3+ on the ultrafast time scale. A new disproportionation mechanism involving in-cage electron transfer between a 19-electron intermediate and a 17-electron radical has been proposed.