Enhancing optical, dielectric, and thermal properties of bio-based polyimides incorporating isomannide with a bent and sterically constrained conformation

Abstract

Novel semi-alicyclic copolyimides (CoPIs) containing appropriate ratios of isomeric 1,4;3,6-dianhydrohexitols (isohexides), namely isomannide (ISM) and isosorbide (ISS), were synthesized from isomannide- and isosorbide-3,6-diyl-bis(trimellitic anhydride), ISMDA and ISSDA, and 2,2′-bis(trifluoromethyl)benzidine (TFDB). The ISM-containing CoPIs exhibited enhanced optical properties, such as higher optical transparency, lower refractive index (n_av), and smaller birefringence (Δn), than conventional PIs and even homopolyimide (HomoPI) from ISSDA. From the systematic conformational analysis using density functional theory (DFT) calculations, ISMDA has a boat-like and sterically constrained structure which inhibits the main-chain orientation and reduces the polarizability anisotropy of the PI chain. The ISMDA moiety also improves the dielectric properties with lowering the dielectric constant (D_k) and dissipation factor (D_f) at 10–20 GHz due to the reduced dipolar polarization and suppression of local molecular motion. In particular, CoPI containing 30 mol% ISM, which provides a good balance between the rigidity and molecular packing, demonstrated enhanced optical, dielectric, and thermal properties with n_av of 1.5537 at 1310 nm, an optical transmittance of 84% at 400 nm, a D_k of 2.92 at 10 GHz, a glass transition temperature (T_g) of 265 °C, and 5 wt% weight-loss temperature (T⁵_d) of 417 °C. This study clarifies the mechanism underlying the enhancement of various physical properties induced by the incorporation of a specific structure of ISM.