Highly Transparent and Colorless Polyimide Film with Low Dielectric Constant by Introducing Meta-substituted Structure and Trifluoromethyl Groups

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.

Preparation and Properties of Low-Dielectric Polyimide Films Containing Tert-Butyl

The design of high-performance polyimide (PI) films and understanding the relationship of the structure-dielectric property are of great significance in the field of the microelectronics industry, but are challenging. Herein, we describe the first work to construct a series of novel tert-butyl PI films (denoted as PI-1, PI-2, PI-3, and PI-4) based on a low-temperature polymerization strategy, which employed tetracarboxylic dianhydride (pyromellitic anhydride, 3,3',4,4'-biphenyl tetracarboxylic anhydride, 4,4'-diphenyl ether dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic anhydride) and 4,4'-diamino-3,5-ditert butyl biphenyl ether as monomers. The results indicate that introducing tert-butyl branches in the main chain of PIs can enhance the free volume of the molecular chain and reduce the interaction between molecular chains of PI, resulting in a low dielectric constant. Particularly, the optimized PI-4 exhibits an excellent comprehensive performance with a high (5) wt% loss temperature (454 °C), tensile strength (117.40 MPa), and maximum hydrophobic angle (80.16°), and a low dielectric constant (2.90), which outperforms most of the results reported to date.

The Influence of Large Pendent Groups on Chain Anisotropy and Electrical Energy Loss of Polyimides at High Frequency through All-Atomic Molecular Simulation

Polyimide is a potential material for high-performance printed circuit boards because of its chemical stability and excellent thermal and mechanical properties. Flexible printed circuit boards must have a low static dielectric constant and dielectric loss to reduce signal loss in high-speed communication devices. Engineering the molecular structure of polyimides with large pendant groups is a strategy to reduce their dielectric constant. However, there is no systematic study on how the large pendant groups influence electrical energy loss. We integrated all-atomic molecular dynamics and semi-empirical quantum mechanical calculations to examine the influence of pendant groups on polymer chain anisotropy and electrical energy loss at high frequencies. We analyzed the radius of gyration, relative shape anisotropy, dipole moment, and degree of polarization of the selected polyimides (TPAHF, TmBPHF, TpBPHF, MPDA, TriPMPDA, m-PDA, and m-TFPDA). The simulation results show that anisotropy perpendicular to chain direction and local chain rigidity correlate to electrical energy loss rather than dipole moment magnitudes. Polyimides with anisotropic pendant groups and significant local chain rigidity reduce electrical energy loss. The degree of polarization correlated well with the dielectric loss with a moderate computational cost, and difficulties in directly calculating the dielectric loss were circumvented.

Ultralow Coefficient of Thermal Expansion and a High Colorless Transparent Polyimide Film Realized Through a Reinforced Hydrogen-Bond Network by In Situ Polymerization of Aromatic Polyamide in Colorless Polyimide

Colorless polyimides (CPIs) are a key substrate material for flexible organic light-emitting diode (OLED) displays and have attracted worldwide attention. Here, in this paper, the dispersion and interfacial interaction of aromatic polyamide (PA) in CPI (synthesized from 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 2,2'-bis(trifluoromethyl)benzidine (TFMB)) were significantly improved by in situ polymerization, and colorless transparent macromolecular polyimide composites (CPI-PAx) were successfully prepared by PA and CPI. By adjusting the ratio of PA to CPI, a high-performance engineering plastic with excellent film-forming properties was obtained. Molecular simulations confirmed the uniform distribution of PA in CPI and its interaction in polymers. In CPI-PAx, the CPI was locked by the PA chain, and numerous molecular chains were mutually entangled to form a hydrogen-bond network structure. Due to the strong interaction between the chains imparted by the hydrogen bonds of the PA, they do not slide under external forces and heating. In addition, the additive PA has excellent dimensional stability, thermal, and mechanical properties, and CPI has outstanding optical properties, so the synthesized CPI-PAx combines the comprehensive properties of PA and CPI. The CPI-PAx has excellent thermal and mechanical properties, with a thermal decomposition temperature of 499 °C, a glass transition temperature of 385 °C, a coefficient of thermal expansion of 0.8 ppm K-1, a tensile strength of 50.9 MPa, and an elastic modulus of 3.9 GPa. Particularly, CPI-PAx has a 90% transmittance in the visible region. These data prove that the strategy of combining PA and CPI by in situ polymerization is an effective method to circumvent the bottleneck of CPI in the current flexible window application, and this design strategy is universal.

Synthesis and Characterization of Organo-Soluble Polyimides Based on Polycondensation Chemistry of Fluorene-Containing Dianhydride and Amide-Bridged Diamines with Good Optical Transparency and Glass Transition Temperatures over 400 °C

Polymeric optical films with light colors, good optical transparency and high thermal resistance have gained increasing attention in advanced optoelectronic areas in recent years. However, it is somewhat inter-conflicting for achieving the good optical properties to the conventional thermal resistant polymers, such as the standard aromatic polyimide (PI) films, which are well known for the excellent combined properties and also the deep colors. In this work, a series of wholly aromatic PI films were prepared via the polycondensation chemistry of one fluorene-containing dianhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (FDAn) and several aromatic diamines with amide linkages in the main chain, including 9,9-bis [4-(4-aminobenzamide)phenyl]fluorene (FDAADA), 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-aminobenzamide)] biphenyl (ABTFMB), and 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-amino-3-methyl)benzamide] biphenyl (MABTFMB). The derived FLPI-1 (FDAn-FDAADA), FLPI-2 (FDAn-ABTFMB) and FLPI-3 (FDAn-MABTFMB) resins showed good solubility in the polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). The solution-processing FDAn-PI films exhibited good optical transmittance over 80.0% at a wavelength of 500 nm (T500), yellow indices (b*) in the range of 1.01-5.20, and haze values lower than 1.0%. In addition, the FDAn-PI films showed low optical retardance with optical retardation (Rth) values in the range of 31.7-390.6 nm. At the same time, the FDAn-PI films exhibited extremely high glass transition temperatures (Tg) over 420 °C according to dynamic mechanical analysis (DMA) tests. The FDAn-PI films showed good dimensional stability at elevated temperatures with linear coefficients of thermal expansion (CTE) in the range of (31.8-45.8) × 10-6/K.

Structural Designs of Transparent Polyimide Films with Low Dielectric Properties and Low Water Absorption: A Review

The rapid development of communication networks (5G and 6G) that rely on high-speed devices requiring fast and high-quality intra- and inter-terminal signal transmission media has led to a steady increase in the need for high-performance, low-dielectric-constant (D_k) (<2.5) materials. Consequently, low-dielectric polymeric materials, particularly polyimides (PIs), are very attractive materials that are capable of meeting the requirements of high-performance terminal devices that transmit broadband high-frequency signals. However, such a PI needs to be properly designed with appropriate properties, including a low D_k, low dielectric loss (D_f), and low water absorptivity. PI materials are broadly used in various fields owing to their superior property/processibility combinations. This review summarizes the structural designs of PIs with low D_k and D_f values, low water-absorbing capacity, and high optical transparency intended for communication applications. Furthermore, we characterize structure-property relationships for various PI types and finally propose structural modifications required to obtain useful values of the abovementioned parameters.

Synthesis and Properties of Optically Transparent Fluoro-Containing Polyimide Films with Reduced Linear Coefficients of Thermal Expansion from Organo-Soluble Resins Derived from Aromatic Diamine with Benzanilide Units

Wholly aromatic polyimide (PI) films with good solution processability, light colors, good optical transparency, high storage modulus, and improved heat resistance were prepared and characterized. For this purpose, a multi-component copolymerization methodology was performed from a fluoro-containing dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), a rigid dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and a fluoro-containing diamine, 2,2'-bis(trifluoromethyl)-4,4'-bis [4-(4-amino-3-methyl)benzamide]biphenyl (MABTFMB). One homopolymer, FPI-1 (6FDA-MABTFMB), and five copolymers, FPI-2~FPI-6, containing the BPDA units from 10 mol% to 50 mol% in the dianhydride moieties, were prepared, respectively. The derived PI resins showed good solubility in the polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). The flexible PI films obtained by the solution casting procedure showed good optical properties with the transmittances higher than 74.0% at the wavelength of 450 nm. The PI films exhibited excellent thermal properties, including 5% weight loss temperatures (T_5%) over 510 °C, together with glass transition temperatures (T_g) over 350.0 °C according to the peak temperatures of the loss modulus in dynamical mechanical analysis (DMA) measurements. The FPI-6 film also showed the lowest linear coefficient of thermal expansion (CTE) value of 23.4 × 10^-6/K from 50 to 250 °C according to the thermomechanical analysis (TMA) measurements, which was obviously lower than that of FPI-1 (CTE = 30.6 × 10^-6/K).

Synthesis and characterization of an adamantane-based copolyimides with high transparency

In this work, a copolyimide (Co-PI) film with high transparency was prepared by the copolymerization of hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 2,2′-Bis(trifluoromethyl)benzidine (TFMB) and adamantane-1,3-diamine (DAA). The effects of DAA monomers on the optical, thermal, and mechanical properties of the co-PIs were discussed in detail. We found that the preparation of polyimide (PI) based on the combination of two dianhydrides and two diamines could obtain the co-PI film with excellent comprehensive performance due to the synergy between the -CF3 group, the aliphatic ring and the aromatic structure. Through the structure and composition optimization, the co-PI film with 1.30% DAA (Q3) has a Tg of 374oC, T5 higher than 530oC, T430 of 82% and the tensile strength higher than 145 MPa. These results indicate that the Co-PI films can be successfully utilized in the development of novel heat-resistant plastic substrates for the optoelectronic engineering applications.

The role of intramolecular and intermolecular hydrogen bonding effect for adenine-containing polyimide films

A series of adenine-containing polyimides (APIs) with intramolecular hydrogen bonding (HB) were prepared from N 1 -(9-(4-aminophenyl)- 9H-purin-6-yl)-2-methoxybenzene-1,4-diamine (b-H-MeO-APA) and commercial dianhydrides. And a systematic comparison between intramolecular and intermolecular HB effects was made using APIs with (b-H-MeO-APIs) and without (b-H-H-API) methoxy group. FTIR indicated that the occurrence of intramolecular HB originated from the side MeO group ortho to -NH-. XRD indicated that the introduction of side group MeO and corresponding intramolecular HB could destroy the orderly stacking of b-H-H-APIs. Birefringence and polarized ATR FTIR studies showed that the groups and chains in b-H-MeO-APIs films could also be simultaneously self-arranged in the in-plane direction. In contrast, the orientation degree in the in-plane direction is generally lower than that of b-H-H-APIs. The b-H-MeO-APIs films showed lower water absorption, higher Tg, and lower CTE compared to b-H-H-APIs films, which indicates that the introduction of intramolecular HB may have some positive influence in controlling water absorption, Tg, and CTE.

Surface Modification of Matrimid® 5218 Polyimide Membrane with Fluorine-Containing Diamines for Efficient Gas Separation

Polyimide membranes have been widely investigated in gas separation applications due to their high separation abilities, excellent processability, relatively low cost, and stabilities. Unfortunately, it is extremely challenging to simultaneously achieve both improved gas permeability and selectivity due to the trade-off relationship in common polymer membranes. Diamine modification is a simple strategy to tune the separation performance of polyimide membranes, but an excessive loss in permeability is also generally observed. In the present work, we reported the effects of diamine type (i.e., non-fluorinated and fluorinated) on the physicochemical properties and the corresponding separation performance of a modified membrane using a commercial Matrimid^® 5218 polyimide. Detailed spectroscopic, thermal, and surface analyses reveal that the bulky fluorine groups are responsible for the balanced chain packing modes in the resulting Matrimid membranes compared to the non-fluorinated diamines. Consequently, the modified Matrimid membranes using fluorinated diamines exhibit both higher gas permeability and selectivity than those of pristine Matrimid, making them especially effective for improving the separation performance towards H₂/CH₄ and CO₂/CH₄ pairs. The results indicate that the use of fluorinated modifiers may offer new opportunities to tune the gas transport properties of polyimide membranes.

Reduced coefficient of linear thermal expansion for colorless and transparent polyimide by introducing rigid-rod amide units: synthesis and properties

Polyimide films with high optical transparency and dimensional stability and low linear thermal expansion were synthesized by introducing rigid-rod amide units.

Benzocyclobutene-functionalized hyperbranched polysiloxane for low-k materials with good thermostability

Although hyperbranched polysiloxanes have been extensively studied, they have limited practical applications because of their low glass transition temperatures. In this study, we synthesized benzocyclobutene-functionalized hyperbranched polysiloxane (HB-BCB) via the Piers-Rubinsztajn reaction. The synthesized material was cured and crosslinking occurred at temperatures greater than 200 °C, forming a low-k thermoset resin with high thermostability. The structure of the resin was characterized using nuclear magnetic resonance (NMR) spectroscopy, viz. ¹H NMR and ¹³C NMR spectroscopy. ²⁹Si NMR spectroscopy was used to calculate the degree of branching. Differential scanning calorimetry, dynamic mechanical analysis, and thermogravimetric analysis revealed that the cured resin possesses good high-temperature mechanical properties and exhibits a high thermal decomposition temperature (T_d5 = 512 °C). In addition, the cured resin has a low dielectric constant (k = 2.70 at 1 MHz) and low dissipation factor (2.13 × 10^-3 at 1 MHz). Thus, the prepared resin can function as a low-k material with excellent high-temperature performance. These findings indicate that the performance of crosslinked siloxane is significantly attributed to the introduction of BCB groups and the formation of the highly crosslinked structure.

Design and fabrication of photoactive imidazole-based poly(ether-imide)s and a polyimide/HBP-modified SiO2 composite: toward high heat-resistance, antimicrobial activity and removal of heavy metal ions

This article describes the synthesis and properties of novel imidazole-based aromatic polyimides (PIs) containing bulky groups from direct polycondensation of two diamines with 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and (hexafluoroisopropylidene)diphthalic anhydride (6FDA). The structure–property relationship of the prepared samples was fully determined via FT-IR, ¹H and ¹³C NMR and elemental analysis (CHN) techniques. The inherent viscosity values of the polyimides ranged from 0.51 to 0.73 dL g⁻¹. These PIs showed glass transition temperatures ranging from 273 to 306 °C, and 10% mass loss temperatures within the range of 478–504 °C in a N₂ atmosphere. High transparency with a UV-visible absorption cut-off wavelength was found to range between 285 and 300 nm. Good antimicrobial activity can be correlated with the presence of xanthene and imidazole units into the main structure of PIs. Next, SiO₂ nanoparticles as inorganic nanoparticles were added to one of the synthesized polyimides (BTDA-PIb), causing changes in the attributes of both the nanoparticles and PI. The data obtained from examining the properties of the prepared BTDA-PIb/HBP@SiO₂ demonstrated increased heat resistance, photoluminescence intensity, and antimicrobial inhibition compared to pure PI. Also, in this article, the polymeric samples as adsorbents were evaluated for extraction of heavy metal ions (Hg²⁺ and Co²⁺) from water sources.

Novel thermostable, photoactive, and solvable poly(ether-imide)s containing imidazole-based pendant groups and a BTDA-PIb/HBP@SiO₂ composite were synthesized and used due to their antimicrobial activity and as an adsorbent to remove heavy metal ions.