AI-experiment-theory integrated study on char formation kinetics of polymeric ablative materials: Comparative study of phenolic resin vs. poly(p-phenylene oxide)

Abstract

The present study aims to provide a fundamental understanding of the structure-property relationship that determines the char yield (Y_C) of polymeric ablative materials (PAM) for spacecraft thermal protection systems. We selected phenolic resin (PR) and poly(p-phenylene oxide) (PPO) as a model system for their similarity in molecular structure while having drastically different Y_C (55 wt.% for PR and 25 wt.% for PPO). First, a graph neural network (GNN) was trained to predict the Y_C of polymers, while statistically estimating how each bond in a polymer contributes to its Y_C. The GNN model indicated that PPO’s low Y_C comes from the para-substituted structure and two methyl groups attached to the aromatic. Second, an experimental study using a pyrolizer and gas-chromatography/mass-spectrometry (Py-GC/MS) revealed that PR forms tricyclics during pyrolysis, while PPO does not, which may be related to PPO’s lower Y_C. Next, theoretical analysis on the pyrolysis mechanism of PR and PPO revealed that the para-substituted structure of PPO increases the energy barrier for cyclization reactions to form tricyclics. In addition, the methyl groups cannot promote cyclization due to their significant spatial separation from adjacent aromatic rings, which collectively reduces the yield of the tricyclic species. Following the MD simulation, it was confirmed that tricyclics at the initial stage of pyrolysis are key to accelerating the growth of char precursors, explaining the higher Y_C of PR compared to PPO. The comprehensive insights into the structure-property relationships of Y_C will facilitate the discovery of novel PAM for superior thermal protection performance.