Mechanical Property of Phenolic Resin via Molecular Dynamics Using a Reactive Force Field

Abstract

During terrestrial atmospheric re-entry, aerospace vehicles can be subjected to temperatures greater than 1800 K which requires thermal insulation to protect the aircraft. Thermal protection systems (TPS) were developed to address this challenge with two goals in mind: (1) provide insulation to inner materials from high temperatures and (2) maintain low weight to reduce payload costs. One class of TPS materials commonly used as ablatives are carbon-carbon composites (CCCs), which consist of a carbon matrix embedded with carbon fillers. Common aerospace materials lose mechanical integrity at high temperatures, while CCCs continue to perform. The manufacturing process of CCCs is cyclic and begins with a net-shape uncured prepreg which is subjected to polymerization and carbonization via pyrolysis. Due to the shrinkage and mass loss associated with these processes, the resulting structure is subjected to several cycles of uncured resin re-impregnation, polymerization, and carbonization until a sufficiently dense structure is achieved. Carbon yield is defined as the mass remaining after carbonization divided by the mass before carbonization and is used as a metric for selecting carbon matrix precursors. A high carbon yield translates into fewer re-impregnation cycles, thus economizing the process. Phenolic resin is a common carbon matrix precursor material due to its relatively high carbon yield of 50-55%. The goal of this project is to establish molecular dynamics (MD) protocols for simulating the manufacturing process of a phenolic-derived carbon matrix. In the present work, an atomistic MD model of the polymerization of phenolic resin was simulated using a reactive force field. By using a well-known material system, resulting MD properties can be compared to experimental values for validation. Since the end goal is a carbonized product, a poly- merized phenolic structure was created using pre-polymerized monomers as the initial structures which are then crosslinked at high temperature (1000 K) via an openvalence approach. A total of 5 replicates were built to account for statistical variation. To keep track of the gelation behavior of the system, the extent of reaction is calculated using Carothers' equation for a trifunctional branching polycondensation reaction, as well as the percent mass of the largest cluster relative to the system mass. After obtaining a gelled structure, the system was equilibrated and subjected to uniaxial tensile simulations. True stress and true strain values from the tensile simulations were plotted to calculate Young's modulus (E) based on the slope of the initial linear portion. The average equilibrated mass density was found to be 1.24 g/cc, while average E was calculated as 3.60 GPa. These properties agree with the experimental values of a mass density of 1.2 – 1.25 g/cc and E of 2.36 – 4.83 GPa.