Cross-Linking Methodology for Fully Atomistic Models of Hydroxyl-Terminated Polybutadiene and Determination of Mechanical Properties

Quantum Chemical Modeling of Propellant Degradation

An ab initio quantum chemical approach for the modeling of propellant degradation is presented. Using state-of-the-art bonding analysis techniques and composite methods, a series of potential degradation reactions are devised for a sample hydroxyl-terminated-polybutadiene (HTPB) type solid fuel. By applying thermochemical procedures and isodesmic reactions, accurate thermochemical quantities are obtained using a modified G3 composite method based on the resolution of the identity. The calculated heats of formation for the different structures produced presents an ∼2 kcal/mol average error when compared against experimental values.

Hexogen Coating Kinetics with Polyurethane-Based Hydroxyl-Terminated Polybutadiene (HTPB) Using Infrared Spectroscopy

The kinetics of hexogen coating with polyurethane-based hydroxyl-terminated polybutadiene (HTPB) using infrared spectrometry was investigated. The kinetics model was evaluated through reaction steps: (1) hydroxyl and isocyanate to produce urethane, (2) urethane and isocyanate to produce allophanate, and (3) nitro and isocyanate to produce diazene oxide and carbon dioxide. HTPB, ethyl acetate, TDI (toluene diisocyanate), and hexogen were mixed for 60 min at 40 °C. The sample was withdrawn and analyzed with infrared spectroscopy every ten minutes at reference wavelengths of 2270 (the specific absorption for isocyanate groups) and 1768 cm⁻¹ (the specific absorption for N=N groups). The solvent was vaporized; then, the coated hexogen was cured in the oven for 7 days at 60 °C. The effect of temperature on the coating kinetics was studied by adjusting the reaction temperature at 40, 50, and 60 °C. This procedure was repeated with IPDI (isophorone diisocyanate) as a curing agent. The reaction rate constant, k₃, was calculated from an independent graphic based on increasing diazene oxide concentration every ten minutes. The reaction rate constants, k₁ and k₂, were numerically calculated using the Newton–Raphson and Runge–Kutta methods based on decreasing isocyanate concentrations. The activation energy of those steps was 1178, 1021, and 912 kJ mole⁻¹. The reaction rate of hexogen coating with IPDI was slightly faster than with TDI.

Atomistic simulation of volumetric properties of epoxy networks: effect of monomer length

Properties of epoxy thermosets can be varied broadly to suit design requirements by altering the chemistry of the component agents. Atomistically-detailed molecular dynamics simulations are well-suited for molecular insight into the structure-property relationship for a rational tailoring of the chemistry. Since the macroscopic properties of interest for applications emerge hierarchically from molecular-scale chemical interactions, seamless integration of experiment, computation, and theory is of great interest. Recently, a Specific Volume-Cooling Rate analysis protocol was successfully developed to quantitatively compare the volumetric properties of an epoxy network model with experimental results in the literature, in spite of the nine orders of magnitude mismatch in the accessible time-scales. Here, we extend the application of the method for two epoxy networks in the same class of chemistry but whose monomers have a higher number of repeating units compared to the previous one for validating the generality of our approach. We observed that atomistic simulations are able to predict the experimental temperature trend of the specific volume within 0.4% for both these networks. Using the William-Landel-Ferry equation to account for rate differences, we also see good agreement between the computational and experimental values of the glass transition temperature.

A Molecular Simulation Approach to Bond Reorganization in Epoxy Resins: From Curing to Deformation and Fracture

We model bond formation and dissociation processes in thermosetting polymer networks from molecular dynamics simulations. For this, a coarsened molecular mechanics model is derived from quantum calculations to provide effective interaction potentials that enable million-atoms scale simulations. The importance of bond (re)organization is demonstrated for (i) simulating epoxy resin formation-for which our approach leads to realistic network models which can now account for degrees of curing up to 98%. Moreover, (ii) we elucidate the competition of bond dissociation and bond reformation during plastic deformation and fracture. On this basis, we rationalize the molecular mechanisms that account for the irreversible nature of damaging epoxy polymers by mechanical load.