Toward reliable solid-solid phase change materials for thermal energy storage: Polymer Fibrillar network-reinforced organic plastic crystals

Solid-solid phase change materials (PCMs) offer attractive advantages for thermal energy storage (TES), including leakage-free operation, reduced encapsulation requirements, and design flexibility. Among them, organic plastic crystals (OPCs) such as pentaglycerine (PG) and pentaerythritol (PE) display high latent heat and tunable transition temperatures. However, their practical use remains limited by material-level constraints, particularly brittleness in the crystalline pase, limited processability, and macroscopic deformation during thermal cycling. This work proposes a materials-engineering strategy based on the incorporation of a fibrillating polytetrafluoroethylene (PTFE) binder. PG and PE composites were fabricated through shear-induced PTFE fibrillation followed by tableting, generating an interconnected fibrillar network within the OPC matrix. This network provides mechanical reinforcement and improves ductility in the crystalline phase while preserving the intrinsic solid-solid thermal storage functionality of the OPCs. The PTFE-reinforced composites show markedly improved shape stability under repeated thermal cycling, with reduced permanent deformation and mitigated expansion-contraction damage. At the same time, the phase transition temperatures and latent heat remain essentially governed by the OPC phase, while subcooling values fall within the lower range reported for these systems. Additional effects include moderate thermal conductivity enhancement and reduced hygroscopicity. Overall, this work demonstrates that PTFE fibrillation can overcome a key material-level bottleneck of OPCs: their loss of macroscopic shape stability during cycling. The resulting composites therefore provide an enabling platform for future TES integration strategies, including protective coatings and additional reinforcement.