High refractive index, photocurable resins for inkjet-printable optical devices

High refractive index (HRI), photocurable resins are advancing polymer-based optical materials for inkjet 3D-printable optical devices. Majority of commercial resins are acrylate-based systems and have lower refractive indices (RI ≈ 1.45–1.50); therefore, possess a limited scope in high-performance optical devices. This study focuses on the synthesis of a high-performance, fluorene-based cardo-type diurethane dimethacrylate (DUDMA) monomer. The monomer was mixed as a reactive functional component into a commercial UV-curable acrylate-based resin to formulate high refractive index, photocurable resin mixtures for inkjet 3D-printable optics. Modified resins exhibited improved optical performance: refractive index increased from 1.514 to 1.549 at 589 nm while maintaining excellent visible-light transparency (≥ 88%). Incorporation of DUDMA enhanced the mechanical performance of cured resin formulations: tensile strength (62%), Young's modulus (34%), and strain at break (55%). Interestingly, a 32% reduction in the curing shrinkage was observed; thus, mitigating polymerization stresses. Furthermore, the cured materials showed high thermal stability (T d > 350 °C), and enhanced surface hardness (Shore-D values increasing from 77 to 82); thus, reflecting an improvement in scratch and abrasion resistance. Although, the monomer addition elevated room-temperature viscosity of modified formulations, an optimal inkjet printing window was established between 40 and 55 °C via temperature-dependent rheological measurements. A drop-on-demand (DOD) inkjet printing demonstrated that these modified resins could have potential for precise and dimensionally stable fabrication of complex micro- to macro-scale optical constructs. This study reflects that fluorene-based DUDMA material chemistry can be seen as potential class of reactive functional components for meeting high-performance optical and mechanical requirements of next-generation 3D printed optics.