Main-Chain Linkage Effects on Field-Induced Conformation and High-Frequency Dielectric Dissipation in Polyimides

Abstract High-frequency dielectric dissipation in aromatic polyimides (PIs) is generally discussed in terms of monomer polarity, while the role of field-induced chain conformation remains less clear. In this study, statistical learning and molecular dynamics simulations under alternating electric fields were combined to elucidate linkage-dependent dielectric dissipation in BPDA-based PIs. A random forest model indicated that ether- and ester-containing fragments were associated with low dielectric dissipation, as identified by SHAP analysis. Guided by this result, BPDA-based PIs containing ether (BPDA-ODA), ketone (BPDA-DABP), ester (BPDA-APAB), and amide (BPDA-DABA) linkages were constructed and examined in terms of chain conformations and dielectric properties at 10 and 50 GHz. BPDA-ODA and BPDA-DABP exhibited coiled shapes, while BPDA-APAB and BPDA-DABA exhibited extended shapes. Under alternating electric fields, the PIs with flexible linkages and coiled shapes exhibited symmetric expansion, while the PIs with rigid linkages and extended shapes mainly exhibited anisotropic expansion. Compared with BPDA-ODA and BPDA-DABP, which have flexible linkages, BPDA-DABA showed the highest induced polarization density because of its rigid, polar amide linkage. The maximum induced polarization density indicated a strong positive linear correlation with dielectric dissipation at 10 and 50 GHz. In addition, the calculation showed that PIs with flexible linkages and coiled shapes exhibited larger increases in dielectric dissipation from 10 to 50 GHz than PIs with extended shapes. These findings establish a molecular-level framework linking functionalized linkage, chain conformation, field-induced anisotropy, induced polarization, and dielectric dissipation, suggesting that PIs with low dielectric dissipation require suppressing excessive anisotropic expansion and induced polarization under alternating electric fields.