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 (nav), 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 (Dk) and dissipation factor (Df) 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 nav of 1.5537 at 1310 nm, an optical transmittance of 84% at 400 nm, a Dk of 2.92 at 10 GHz, a glass transition temperature (Tg) of 265 °C, and 5 wt% weight-loss temperature (T5d) 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.