Structure of different polymers and the dielectric breakdown characteristics in terms of FVE size distribution.
Structure of different polymers and the dielectric breakdown characteristics in terms of FVE size distribution.

Polymer films’ dielectric properties, combined with their lightweight and easy processability, are highly attractive for capacitors. Their performance in capacitors is determined by dielectric breakdown, beyond which they become electrically conducting. The factors and mechanisms determining dielectric breakdown are, however, not fully understood. Now researchers from Stanford University and the University of Connecticut have demonstrated that the intrinsic dielectric breakdown of polymers is associated with voids or empty spaces in films arising because of the imperfect way in which molecules fit together [Fica-Contreras et al., Materials Today (2023),].

“Polymers are made of long chains of individual molecules, or monomers. Because polymers are long, they do not pack together as well as small molecules do. Large packing imperfections leave voids in a polymeric solid [known as] free volume elements (FVEs),” explains Michael D. Fayer, Professor of Chemistry at Stanford, who has pioneered the study of FVEs.

Depending on the type of monomer, FVEs can be large enough to fit in other types of molecules, which is useful in processes like gas diffusion and separation. The team show, in addition, that large FVEs play a role in dielectric breakdown. Using polyetherimide (PEI), a high-quality dielectric with a high breakdown field, the researchers synthesized polymers with different end groups, which keep the electronic properties largely intact while changing the size distribution of FVEs.

With a custom test setup designed by Yang Cao, FVE size distribution was measured using the restricted orientation anisotropy method (ROAM). The approach is like twirling a baton in a room, explain the researchers. The baton can be twirled in all angles in a large room but will be restricted in a very small room, hitting the walls and only moving through limited angles. The IR probe molecule in ROAM act like the baton, while the distribution of angles of anisotropic decay indicate the size of the FVEs. The team’s observations indicate that polymer films with a high probability of large FVEs have a lower dielectric breakdown field. Conversely, polymers with a probability of smaller FVEs demonstrate higher dielectric breakdown fields.

“We observed that even a small amount of large radius FVEs will cause diminishing breakdown,” points out Gregory Sotzing of the University of Connecticut.

Designing polymer films with increased monomer packing efficiency could, therefore, reduce the probability of large FVEs and improve dielectric breakdown strength. The breakdown strength of PEI could be optimized by simply changing the end groups for polymerization.