De Novo Design Of Ablative Polymers For Aerospace Thermal Protection Systems Through An Integrated AI-Theory-Experiment Approach

Novolac phenolic resin (NPR) is a widely used polymeric ablative material (PAM) for thermal protection systems (TPS) due to its high thermal decomposition temperature (TD, 500�°C) and char yield (YC, 55�wt%). However, its drawbacks (including high thermal conductivity, low mechanical properties and water uptake) prompt the need for improved PAMs. This study explores the pyrolysis behavior and char-forming capabilities of NPR and a potential alternative PAM, poly(1,6-dimethyl phenol) (PPO), through both theoretical and experimental approaches. A graph neural network (GNN) was developed to predict structure-property relationships, revealing that benzyl group positioning and oxygen atom placement significantly influence YC. PPO's para-positioned chains and second methyl group also hinder its char formation. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) confirmed distinct decomposition products between NPR and PPO, correlating with their TD differences. Density functional theory (DFT) showed NPR requires higher energy for initial dissociation (92.4 vs. 76.4�kcal/mol), contributing to its superior thermal resistance and TD. The experimental study and molecular dynamics (MD) simulations revealed that NPR forms large polycyclic structures, while PPO forms smaller ones, limiting char formation. DFT mechanistic analysis indicated that NPR undergoes intramolecular cyclization to tricyclic intermediates regardless of the initiation pathway (bond fission or H-abstraction). On the other hand, PPO forms similar structures only under H-abstraction due to higher bond energy barriers encountered through bond fission. The key to the formation of polycyclic compounds is the radical center to the adjacent aromatic ring distance. Finally, poly(diphenyl phenol) (PPPO), a PPO analog, demonstrated better thermal properties (TD 530�°C, YC 45�wt%) by enabling cyclization through phenyl substitution. Overall, the study underscores that char yield depends on a polymer's ability to generate radicals capable of cyclization, providing valuable insights for the rational design of next-generation PAMs.