Mixed Rigid and Flexible Component Design for High-Performance Polyimide Films

One-Dimensional Crystal-Structure Te-Se Alloy for Flexible Shortwave Infrared Photodetector and Imaging

Flexible shortwave infrared detectors play a crucial role in wearable devices, bioimaging, automatic control, etc. Commercial shortwave infrared detectors face challenges in achieving flexibility due to the high fabrication temperature and rigid material properties. Herein, we develop a high-performance flexible Te0.7Se0.3 photodetector, resulting from the unique 1D crystal structure and small elastic modulus of Te-Se alloying. The flexible photodetector exhibits a broad-spectrum response ranging from 365 to 1650 nm, a fast response time of 6 μs, a broad linear dynamic range of 76 dB, and a specific detectivity of 4.8 × 1010 Jones at room temperature. The responsivity of the flexible detector remains at 93% of its initial value after bending with a small curvature of 3 mm. Based on the optimized flexible detector, we demonstrate its application in shortwave infrared imaging. These results showcase the great potential of Te0.7Se0.3 photodetectors for flexible electronics.

Progress of research on the bonding-strength improvement of two-layer adhesive-free flexible copper-clad laminates

The arrival of the 5G era has placed high demands on the electronic products. Developing thin, light, and portable electronic products capable of simultaneously improving the transmission rate and reducing the signal delay and transmission loss is necessary to meet such demands. The traditional three-layer, adhesive, flexible copper-clad laminate (3L-FCCL) cannot satisfy these demands because of its adhesive component. The large thickness and poor heat resistance disadvantages of 3L-FCCL can be avoided with a two-layer, adhesive-free, flexible copper-clad laminate (2L-FCCL). However, 2L-FCCL has low bonding strength. This work introduces the selection of conductor materials and insulating base films for flexible copper-clad laminates. Modification studies aimed at increasing the bonding performance of 2L-FCCL are summarized based on three aspects. These modification techniques include the surface treatment of copper foils, modification and surface treatment of polyimide films, and surface treatment of liquid-crystal polymers. Prospects are further provided.

The Pivotal Role of Benzimidazole in Improving the Thermal and Dielectric Performance of Upilex-Type Polyimide

Polyimide (PI) with ultra-high thermal resistance and stability is essential for application as a flexible substrate in electronic devices. Here, the Upilex-type polyimides, which contained flexibly "twisted" 4,4'-oxydianiline (ODA), have achieved various performance improvements via copolymerization with a diamine containing benzimidazole structure. With the rigid benzimidazole-based diamine bearing conjugated heterocyclic moieties and hydrogen bond donors fused into the PI backbone, the benzimidazole-containing PI showed outstanding thermal, mechanical, and dielectric performance. Specifically, the PI containing 50% bis-benzimidazole diamine achieved a 5% decomposition temperature at 554 °C, an excellent high glass transition temperature of 448 °C, and a coefficient of thermal expansion lowered to 16.1 ppm/K. Meanwhile, the tensile strength and modulus of the PI films containing 50% mono-benzimidazole diamine increased to 148.6 MPa and 4.1 GPa, respectively. Due to the synergistic effect of rigid benzimidazole and hinged, flexible ODA, all PI films exhibited an elongation at break above 4.3%. The electrical insulation of the PI films was also improved with a dielectric constant lowered to 1.29. In summary, with appropriate mixing of rigid and flexible moieties in the PI backbone, all the PI films showed superior thermal stability, excellent flexibility, and acceptable electrical insulation.

Enhancing the Toughness of Free-Standing Polyimide Films for Advanced Electronics Applications: A Study on the Impact of Film-Forming Processes

High-quality and free-standing polyimide (PI) film with desirable mechanical properties and uniformity is in high demand due to its widespread applications in highly precise flexible and chip-integrated sensors. In this study, a free-standing PI film with high toughness was successfully prepared using a diamine monomer with ether linkages. The prepared PI films exhibited significantly superior mechanical properties compared to PI films of the same molecular structure, which can be attributed to the systematic exploration of the film-forming process. The exploration of the film-forming process includes the curing procedures, film-forming substrates, and annealing treatments. Additionally, the thickness uniformity and surface homogeneity of free-standing films were crucial for toughness. Increasing the crystallinity of the PI films by eliminating residual stress also contributed to their high strength. The results demonstrate that by adjusting the above-mentioned factors, the prepared PI films possess excellent mechanical properties, with tensile strength and elongation at break of 194.71 MPa and 130.13%, respectively.

Research Progress and Application of Polyimide-Based Nanocomposites

Polyimide (PI) is one of the most dominant engineering plastics with excellent thermal, mechanical, chemical stability and dielectric performance. Further improving the versatility of PIs is of great significance, broadening their application prospects. Thus, integrating functional nanofillers can finely tune the individual characteristic to a certain extent as required by the function. Integrating the two complementary benefits, PI-based composites strongly expand applications, such as aerospace, microelectronic devices, separation membranes, catalysis, and sensors. Here, from the perspective of system science, the recent studies of PI-based composites for molecular design, manufacturing process, combination methods, and the relevant applications are reviewed, more relevantly on the mechanism underlying the phenomena. Additionally, a systematic summary of the current challenges and further directions for PI nanocomposites is presented. Hence, the review will pave the way for future studies.

Core-Shell Sr2CeO4@SiO2 Filled COC-Based Composites with Low Dielectric Loss for High-Frequency Substrates

High-frequency communication equipment urgently needs substrate materials with lower dielectric loss, better heat dissipation, and higher stability, to ensure real-time low-loss and high-speed signal transmission. The core-shell structure of Sr2CeO4@SiO2 was prepared by the sol-gel method, and the modified powders with different volume contents were introduced into the cyclic olefin copolymer (COC) to prepare hydrocarbon resin-based composites. Due to the protective effect of the SiO2 shell, the stability of the powders is significantly improved, and the moisture barrier and corrosion resistance of the composites are enhanced, which is conducive to the normal operation of electronic equipment in harsh and complex environments. When the filler content is 20 vol%, the composite has a dielectric loss of 0.0023 at 10 GHz, a dielectric constant of 3.5, a thermal conductivity of 0.9 W·m-1·K-1, a water absorption of 0.32% and a coefficient of thermal expansion of 37.7 ppm/°C. The COC/Sr2CeO4@SiO2 composites exhibit excellent dielectric properties and thermal conductivity, while maintaining good moisture resistance and dimensional stability, which shows potential application prospects in the field of high-frequency substrates.

Polyimides containing a novel bisbenzoxazole with high T g and low CTE

A novel diamine named (2,2'-bibenzoxazole)-5,5'-diamine (DBOA) and derived polyimides (PIs) were successfully synthesized. The rigid, linear, symmetrical molecular structure and the strong charge transfer complex (CTC) were considered to be the reasons for the improved molecular packing and enhanced thermal properties of the polymers. These DBOA based PIs exhibited a higher glass transition temperature (T g) and lower coefficient of thermal expansion (CTE) than traditional benzoxazole (BOA) based PIs. Meanwhile, the PI derived from DBOA and BPDA (3,3',4,4'-biphenyltetracarboxylic dianhydride) exhibited high T g (395 °C) and low CTE (8.9 ppm per °C), and is expected to be applied in organic light-emitting diode (OLED) displays.

MPPE/SEBS Composites with Low Dielectric Loss for High-Frequency Copper Clad Laminates Applications

Copper clad laminates (CCLs) with low dissipation factor (Df) are urgently needed in the fields of high-frequency communications devices. A novel resin matrix of modified poly (2,6-dimethyl-1,4-phenylene ether) (MPPE) and styrene-ethylene/butylene-styrene (SEBS) was employed in the fabrication of high-frequency copper clad laminates (CCLs). The composites were reinforced by E-glass fabrics, which were modified with phenyltriethoxysilane (PhTES). The composite laminates obtained exhibited impressive dielectric loss of 0.0027 at 10 GHz when the weight ratio of MPPE to SEBS was 5:1. In order to modify the dielectric constant (Dk), coefficient of thermal expansion (CTE) and other performances of laminates, Li₂TiO₃ (LT) ceramic powders were introduced into the resin matrix. The composite laminates showed low dielectric loss of 0.0026 at 10 GHz and relatively high flexural strength of 125 MPa when the mass ratio of LT fillers to resin is 0.4. Moreover, the composite laminates all maintain low water uptake (<0.5%). The microstructure and thermal properties of composite laminates filled with LT ceramic powders were also tested. These results show that copper clad laminates prepared with modified polyphenylene ether (MPPE)/SEBS and LT ceramic fillers have strong competitiveness to fabricate printed circuit boards (PCBs) for high-frequency and high-speed applications.

Synthesis of Superheat-Resistant Polyimides with Enhanced Dielectric Constant by Introduction of Cu(ΙΙ)-Coordination

To achieve polyimide-metal complexes with enhanced properties, 5-amine-2-(5-aminopyridin-2-yl)-1-methyl-benzimidazole (PyMePABZ) that contains stiff 2-(2'-pyridyl)benzimidazole (PyBI) was synthesized and exploited to construct the Cu(ΙΙ)-crosslinked polyimides (Cu-PIs). These Cu-PIs exhibited higher dielectric, thermal, and mechanical properties with an increase in Cu2+ content. Among them, their dielectric constants (εrS) were up to 43% superior to that of the neat PI, glass transition temperatures (Tgs) were all over 400 °C, and 5% weight loss temperature (T5%) maintained beyond 500 °C. These data indicate that the metal coordination crosslinking provided a useful guide to develop high performance PIs which possess potential application as useful high temperature capacitors.

High performance polyimide films containing benzimidazole moieties for thin film solar cells

In order to match the fabrication process of flexible Copper-Indium-Gallium-Selenide (CIGS) solar cell, a series of polyimides (PIs) with high initial decomposition temperatures (Td) were prepared from 6,4′-diamino-2′-trifluoromethyl-2-phenylbenzimidazole (DATFPBI), p-phenylenediamine (p-PPD), and S-type biphenyl dianhydride (s-BPDA) using a sequential copolymerization, casting, and thermal imidization process. The physical properties of the PIs were found to be effectively modified by adjusting both the ratio of the rigid momomers and the thermal imidization process. With the introduction of DATFPBI, the polymers showed significant improvements in thermal stability, thermal expansion, moisture absorption and mechanical properties. PIPBId, one of the synthesized PI film, exhibited an excellent comprehensive performance: a glass transition temperature of 368°C, a tensile modulus of 6.8 GPa, a linar coefficient thermal expansion (CTE) of 16.8 ppm/K, and a moisture absorption of 1.42%. Furthermore, Td of this thin film was up to 524°C,which indicated that the PIPBId film is a competitive candidate as the flexible substrate for CIGS, Copper-Zinc-Tin-Sulphide (CZTS) solar cell and flexible printed circuit boards (FPCB) where high process temperature is necessary.

High Performance Soluble Polyimides from Ladder-Type Fluorinated Dianhydride with Polymorphism

A novel rigid semi-alicyclic dianhydride 9,10-difluoro-9,10-bis(trifluoromethyl)-9,10-dihydroanthracene-2,3,6,7-tetracarboxylic acid dianhydride (8FDA) was reported, and its single crystal X-ray diffraction result revealed the existence of the polymorphic structure in this compound. The detail geometric configuration transition during the synthesized process was investigated, exhibiting a transition of from trans- to cis- when the hydroxyl groups were substituted by fluoride with diethylaminosulfur trifluoride (DAST). Compared with the dianhydride 4,4'-(Hexaflouroisopropylidene) diphthalic anhydride (6FDA) and 1S,2R,4S,5R-cyclohexanetetracarboxylic dianhydride (HPMDA), the resulting polyimide (PI) films based on 8FDA exhibited an obviously higher glass transition temperature (Tg, 401 °C) and a much lower coefficient of thermal expansion (CTE, 14 ppm K-1). This indicates that 8FDA is an ideal building block in high-performance soluble PIs with low CTE.

Understanding the Effect of the Dianhydride Structure on the Properties of Semiaromatic Polyimides Containing a Biobased Fatty Diamine

In this work we report the effect of the hard block dianhydride structure on the overall properties of partially biobased semiaromatic polyimides. For the study, four polyimides were synthesized using aliphatic fatty dimer diamine (DD1) as the soft block and four different commercially available aromatic dianhydrides as the hard block: 4,4'-(4,4'-isopropylidenediphenoxy) bis(phthalic anhydride) (BPADA), 4,4'-oxidiphthalic anhydride (ODPA), 4,4'-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA), and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). The polymers synthesized were fully organo-soluble thermoplastic branched polyimides with glass transition temperatures close to room temperature. The detailed analysis took into account several aspects of the dianhydrides structure (planarity, rigidity, bridging group between the phtalimides, and electronic properties) and related them to the results obtained by differential scanning calorimetry, rheology, fluorescence and broadband dielectric spectroscopy. Moreover, the effects of physical parameters (crystallization and electronic interactions) on the relaxation behavior are discussed. Despite the presence of the bulky branched soft block given by the dimer diamine, all polyimides showed intermolecular charge transfer complexes, whose extent depends on the electronic properties of the dianhydride hard block. Furthermore, the results showed that polyimides containing flexible and bulky hard blocks turned out fully amorphous while the more rigid dianhydride (BPDA) led to a nanophase separated morphology with low degree of crystallinity resulting in constrained segmental relaxation with high effect on its mechanical response with the annealing time. This work represents the first detailed report on the development and characterization of polyimides based on a biobased fatty dimer diamine. The results highlight the potential of polymer property design by controlled engineering of the aromatic dianhydride blocks.