Jong Chan Won

Cross-linkable spirobisindane-based polyimides were synthesized and investigated as low-dielectric materials for high-frequency applications. The polymer composition was controlled by varying the molar ratio of diamines, and allyl groups were introduced through postpolymerization functionalization to enable subsequent thermal cross-linking. The successful introduction and cross-linking of the allyl groups were confirmed by 1H NMR and FT-IR spectroscopy, together with gel fraction measurements. The resulting films were readily fabricated by solution-based bar coating and exhibited enhanced hydrophobicity and thermal stability after cross-linking. Positron annihilation lifetime spectroscopy (PALS) revealed a composition-dependent reorganization of the local free-volume structure upon cross-linking, rather than a uniform increase in free volume. In particular, the 55 series exhibited a pronounced decrease in fractional free volume after cross-linking, whereas the 73 series showed relatively minor changes. Despite these different free-volume responses, the cross-linked X-BMDA films exhibited improved dielectric properties compared with their uncross-linked counterparts, indicating that the dielectric enhancement cannot be attributed solely to an increase in free volume. X-BMDA-55 exhibited the lowest dielectric constant (Dk = 2.20 at 28 GHz) and dissipation factor (Df = 0.0063 at 10 GHz). These results demonstrate that the combination of spirobisindane-based molecular design and post-cross-linking provides an effective strategy for controlling local molecular structure and achieving low dielectric properties in polyimide films.

Lightweight, flexible, and thermally robust neutron-shielding materials are essential for nuclear, aerospace, space, and defense applications, yet conventional polymer composites struggle to balance filler loading with flexibility, thermal stability, and processability. Here, we report an aqueous in-situ polymerization strategy for fabricating boron nitride nanotube (BNNT)-incorporated polyimide (PI) composite films (PIBT) with BNNT loadings up to 90 wt%. BNNTs are incorporated during water-borne poly(amic acid) salt (W-PAAS) precursor formation, yielding free-standing PI–BNNT films without post-synthetic functionalization or toxic aprotic solvents. Composition-dependent analysis shows that shielding efficiency depends not on nominal BNNT content alone, but on the balance among boron-site density, absorber distribution, and matrix continuity. The optimized PIBT-60 film achieves a linear neutron attenuation coefficient of 1.67 mm⁻¹, a mass attenuation coefficient of 9.23 cm² g⁻¹, an in-plane thermal conductivity of 8.23 W m⁻¹ K⁻¹, a 2.5 mm minimum bending radius, and a 5% weight-loss decomposition temperature above 600 °C in a sub-100 μm film. Neutron transport simulations of a lunar extravehicular mobility unit show that PIBT-60 reduces low-energy neutron flux within a realistic multilayer protective structure. This work establishes aqueous in-situ processing as a route for multifunctional PI-based composite films integrating neutron shielding, heat spreading, flexibility, and thermal endurance for harsh-environment applications.

Fluorene-containing polyimides with pendant hydroxyl groups were synthesized via azeotropic imidization from BPADA, mTB, and APFD in varying feed ratios. Post-polymerization cross-linking with terephthaloyl chloride (TPC) generates covalent junctions between polymer chains, disrupting dense packing and arresting expanded inter-chain free space. Wide-angle X-ray Scattering (WAXS) analysis confirmed that the formation of cross-linked networks increased free volume in the polymer matrix, leading to a reduction in dielectric constant (D_k) and dissipation factor (D_f). The CBMAD73(1.0) film exhibited the advanced low dielectric properties with D_k = 2.66 and D_f = 0.0056 at 10 GHz, compared to 3.13 and 0.0246 at 10 GHz for the polymeric precursor, BMAD73. Thermal analysis demonstrated that the cross-linked polymer network retained thermal stability, which is comparable to that of the neat polyimide precursor, demonstrating structural integrity of the cross-linked network. These results highlight post-polymerization cross-linking as an effective approach for modulating nanoscale free volume to achieve low dielectric performances while maintaining thermal stability required for high-frequency electronic applications.

Wireless powering of implantable medical devices (IMDs) using triboelectric nanogenerators (TENGs) has gained attention as a sustainable and battery-free energy strategy. Ultrasound-driven TENGs (US-TENGs) offer favorable biocompatibility and deep tissue accessibility, but their practical application remains limited by unstable acoustic vibration, filler aggregation at low contents, and complex electrode integration. Here, we report a porous polyimide composite-based TENG (PPC-TENG) designed to overcome these limitations through material and structural innovations. The triboelectric layer is fabricated via a Pickering emulsion templating approach using a water-borne poly(amic acid) salt (W-PAAS) and high-permittivity CaCu₃Ti₄O₁₂ (CCTO) nanoparticles, enabling homogeneous dispersion up to 10 wt % without aggregation. The resulting porous architecture concentrates mechanical deformation at branch-like pore supports, promoting uniform vibration and enhancing charge generation under acoustic actuation. A laser-induced graphene (LIG) electrode is directly patterned on the porous surface, enabling a double-electrode configuration through a simplified, metal-free process. The PPC-TENG exhibits high electrical output, stable capacitor and battery charging performance, and confirmed biocompatibility in both in vitro and in vivo tests. This work presents a scalable and structurally adaptive triboelectric platform for wirelessly powered biomedical electronics.

A critical challenge in flexible high-performance thin-film transistors (TFTs) is ensuring the reliability of the dielectric layer with a high-mobility semiconductor, which must maintain its insulating properties while withstanding repeated mechanical deformation. In this study, we investigate photo-cross-linkable photosensitive polyimide (PSPI), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride-3,5-diaminobenzyl cinnamate (6FDA/DABC), as a dielectric material in oxide TFTs using zinc tin oxide or indium gallium zinc oxide as the channel materials. The photo-cross-linked PSPI dielectric exhibited a high areal capacitance of 17.5 nF cm^-2 at 1 kHz, an ultralow leakage current density of 10^-10 A cm^-2 at 2 MV cm^-1, and a breakdown field exceeding 6.7 MV cm^-1 under static conditions. Under repeated mechanical stress, the dielectric maintained its low leakage current and structural integrity after 10,000 bending cycles, ensuring a stable electrical performance. The photo-cross-linked PSPI and zinc tin oxide-based TFT device demonstrated excellent electrical characteristics, achieving a high mobility of 15.5 cm² V^-1·s^-1, an on/off current ratio of 1.5 × 10⁹, and good electrical stability under positive and negative bias stress, confirming its potential for high-performance, flexible TFT applications.

Thin-film transistors offer excellent and uniform electrical properties over large areas, making them a promising option for various future electronic devices. Polyimide dielectrics are already widely used in various electronic devices because of their exceptional dielectric properties, thermal stability, and desirable mechanical flexibility, which make them suitable for harsh environments. However, the current research on polyimide dielectric materials has certain limitations, such as the use of toxic solvents, high-temperature processes, and deficient coating properties. Herein, we introduce an aromatic polyimide dielectric, which exhibits excellent electrical properties even when processed at a low temperature of 250 °C using environmentally friendly water-based "one-step" polymerization. Despite its thin thickness of -1. Furthermore, we successfully achieved a large-area coating of uniform dielectric layers with no pinholes using only water as a solvent and a simple solution process without any additional processing steps. These results demonstrate that the water-borne polyimide gated indium-gallium-zinc oxide transistor exhibits excellent and stable device performance. Moreover, we used the transistor to successfully demonstrate various logic gates (NOT, NAND, and NOR). Overall, this study provides guidelines for the eco-friendly and sustainable use of water-borne polyimide dielectric materials with high electrical performance and large-processing window advantages.

Macroporous polymers have gained significant attention due to their unique mass transport and size-selective properties. In this study, we focused on Polyimide (PI), a high-performance polymer, as an ideal candidate for macroporous structures. Despite various attempts to create macroporous PI (Macro PI) using emulsion templates, challenges remained, including limited chemical diversity and poor control over pore size and porosity. To address these issues, we systematically investigated the role of poly(amic acid) salt (PAAS) polymers as macrosurfactants and matrices. By designing 12 different PAAS polymers with diverse chemical structures, we achieved stable high internal phase emulsions (HIPEs) with >80 vol % internal volume. The resulting Macro PIs exhibited exceptional porosity (>99 vol %) after thermal imidization. We explored the structure–property relationships of these Macro PIs, emphasizing the importance of controlling pore size distribution. Furthermore, our study demonstrated the utility of these Macro PIs as separators in Li-metal batteries, providing stable charging–discharging cycles. Our findings not only enhance the understanding of emulsion-based macroporous polymers but also pave the way for their applications in advanced energy storage systems and beyond.