Synthesis and properties of ultralow dielectric constant porous polyimide films containing trifluoromethyl groups

In this research, a series of porous copolyimide (co‐PI) films containing trifluoromethyl group (CF₃) were facilely prepared via a phase separation process. The co‐PI were synthesized by the reaction of benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) with two diamines of 4,4′‐diaminodiphenyl ether (ODA) and 3‐trifluoromethyl‐4,4'‐diaminodiphenyl ether (FODA) with various molar ratios. The flexible and tough porous co‐PI films with about 300 μm thickness and 8～10 μm average diameter could be obtained by solution casting conveniently. The thermal properties of the obtained porous co‐PI films were excellent with a glass transition temperature at 270 °C ～ 280 °C and only 5% weight loss in temperature from 530 °C to 560 °C under nitrogen atmosphere. In addition, the dielectric and hydrophobic properties of porous co‐PI films were remarkably improved owing to the presence of trifluoromethyl groups (CF₃) in the polymer chains. Moreover, our synthesized porous co‐PI films also showed good mechanical properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44494.     

Revealing the correlation between molecular structure and dielectric properties of carbonyl-containing polyimide dielectrics

Polymer dielectrics with outstanding heat resistance and advanced dielectric properties are of great importance for high-temperature capacitors in the applications of hostile circumstances. In this work, a series of aromatic carbonyl-containing polyimides (CPI) are prepared from the carbonyl dianhydride and different diamines. The correlation between molecular structure (i.e., different linked structure (─O─, ─CH2─, ─SO2─) in diamines, the length of repeating unit and the linked position (para-para or meta-meta), and properties is revealed in detail to obtain CPI dielectrics with excellent thermal resistance (glass transition temperature, T_g: 241~352°C), reasonably high dielectric constant (3.99~5.23), low dissipation factor (0.00307~0.00395), and admirable breakdown strength (425~552 MV/m) simultaneously. Particularly, CPI-5 with carbonyl structure in dianhydride and sulfonyl group in diamine proves to exhibit discharged energy density and charge–discharge efficiency of 6.34 J/cm³ and 92.3% at 500 MV/m, respectively. In addition, CPI-5 also displays stable dielectric properties in temperature range of −50‑200°C. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47883.     

Hyperbranched-polysiloxane-based hyperbranched polyimide films with low dielectric permittivity and high mechanical and thermal properties

A series of hyperbranched polysiloxane (HBPSi)-based hyperbranched polyimide (HBPI) films with low dielectric permittivity and multiple branched structures are fabricated by copolymerizing 2,4,6-triaminopyrimidine (TAP) with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 4,4′-diaminodiphenyl ether, and HBPSi via the two-step polymerization method. The dielectric permittivity of HBPSi hyperbranched polyimide films decreases with increasing TAP fraction, namely, from 3.28 for sample PI-1 to 2.80 for PI-4, mainly owing to the enlarged free volume created by the incorporation of multiple branched structures. Moreover, HBPSi HBPI possesses desirable solubility and good mechanical properties and thermal stability. PI-4 not only has low dielectric permittivity (2.80, 1 MHz), excellent solubility (soluble in several common organic solvents), and remarkable thermal properties (glass-transition temperature of 273 °C, 5% weight loss temperature of 498 °C in N₂ and 486 °C in O₂), but it also demonstrates admirable mechanical properties with a tensile strength of 103 MPa, elongation at break of 7.3%, and a tensile modulus of 2.16 GPa. HBPSi HBPI might have potential applications in interlayer dielectrics and other microelectronics fields. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47771.     

Novel polyimide/graphene oxide composite films with ultralow dielectric constants

Graphene is generally used for conductive material; it can also be used as a key nanofiller for the insulation material of inverter motors. In this study, a series of polyimide (PI) films were prepared successfully by a conventional two‐step polymerization method based on bis[3,5‐dimethyl‐4‐(4‐aminophenoxy)phenyl]methane as a diamine and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride as a dihydride with different weight percentage graphene oxide (GO) nanosheets as nanofillers. The dielectric constant (ε) and dielectric loss (tan δ) of these films were measured. The results show good dielectric properties, especially an ultralow ε value of 1.41 at 1 MHz with 0.19% GO. This showed that the low ε value was caused by a high free volume led by the GO nanosheets and the C? F bond. The structure and micromorphology of the PIs were characterized by X‐ray diffraction and scanning electron microscopy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41385.     

Preparation and properties of layered silicate/polyimide hybrid films

Layered silicate/polyimide (PI) hybrid films were prepared from 4,4′‐oxydianiline, 3,3′,4,4′‐oxydiphthalic anhydride, and chemically modified montmorillonite via an in situ intercalation polymerization pathway. The X‐ray diffraction and transmission electron microscopy results indicated that the silicates were homogeneously dispersed as exfoliated layers in the PI hybrid film with 2% silicate. The mechanical properties and thermal stabilities of the PI hybrid films changed with the content of the layered silicates. The coefficient of thermal expansion and water uptake of the PI hybrid films decreased with increasing silicate contents because of the barrier effect of platelike silicate layers, which prevented the diffusion and penetration of water. The dielectric strength and electrical aging performance of the PI hybrid films could also be improved as the silicate layer highly dispersed in the films. For the hybrid PI film with 5% layered silicate, the time to failure during electrical aging exceeded 280 h, which was 2.5 times as long as that of pure PI film. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1176–1183, 2005     

Electrical properties of conventional polyimide films: Effects of chemical structure and water uptake

A series of conventional polyimide (CPI) films, based on pyromellitic dianhydride (PMDA) and benzophenonetetracarboxylicdianhydride (BTDA), were prepared by a two step process, and their dielectrical constant, dielectrical loss, and DC conduction behaviors were studied at different frequencies and voltages. Their dielectrical breakdown voltage, water uptake, and solubility properties were also investigated. The effects of chemical structure and water uptake on the electrical properties of the films are discussed in detail. The dielectric constants of the CPI films vary between 2.93 and 3.72 at 1 MHz frequency and they are in the following decreasing order: BTDA‐DDS > BTDA‐DDE > PMDA‐DDS > PMDA‐DDE. The structure and thermal and oxidative stability of films were analyzed by FTIR‐ATR and TGA, respectively. The results showed that all CPI films have good insulating properties, such as high dielectric breakdown voltage, low dielectric constant with stability for long period of frequency, and low leakage density. Our results concerning electrical properties also suggest that electron hopping is responsible for AC conduction and Poole‐Frenkel mechanism is predominant for DC conduction of all CPI films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 810–818, 2006     

Reduction of dielectric constant by nanovoids formed through chemical treatment on silica crosslinked polyimide and its effect on properties

In this study, 2,7‐diamino‐9‐fluorenol (DAF) has been introduced to bond silica to the main chain of the polyimide (PI) copolymer. DAF contains a hydroxyl group that could covalently bond with silica particles. 4,4′‐(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4,4′‐oxydianiline (4,4′‐ODA) have been used as monomers to form a copolymer with DAF. The variation of silica content was controlled as 5%, 7.5, 10, 12.5 wt %. Variation in silica content contributes to the formation of various size (100–410 nm) of macroporous voids after hydrofluoric acid (HF) treatment. HF etching process was introduced to dissolve the silica and form voids in the structure of PI copolymer films. Compared with conventional PI films, air voids that were formed in the PI copolymer film reduced the dielectric from 4.40 to 1.86. The reduction in the dielectric constants can be explained in terms of creating silica particles that increase the presence of air voids after HF treatment. The thermal stability was stable up to 500 °C and the modulus change was confirmed with a dynamic mechanical analysis (DMA) to evaluate the effect of silica on thermal and mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45982.     

Fabrication and properties of anisotropic polyimide aerogels with aligned tube-like pore structure

Polyimide (PI) aerogels with highly aligned tube-like pores were fabricated by unidirectional ice crystal-induced self-assembly method. During this process, the mold bottom contacted with the freezing medium, the aqueous solution of poly(amic acid) (PAA) ammonium salt in the mold was unidirectionally frozen, the ice crystals grew from the bottom to top of PAA ammonium salt (PAS) solution along the freezing direction, which endowed PI aerogels with aligned tube-like pores after sublimation of ice crystals and thermal imidization of PAS. The obtained aerogels had low densities (0.077–0.222 g cm⁻³) and high porosities (83.8–94.2%) and exhibited anisotropic morphology and properties. Their compression strength in vertical direction (parallel to freezing direction) was higher than that in horizontal direction (perpendicular to freezing direction). Their heat transport in horizontal direction was much slower than that in vertical direction; the aerogels had better thermal insulating property in horizontal direction. This facile approach contributed to prepare new type of PI aerogel materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48769.     

Dielectric properties of graphene–iron oxide/polyimide films with oriented graphene

The preparation of high‐dielectric‐constant (k) materials is important in the field of electronics. However, how to effectively use the function of fillers to enhance k is still a challenge. In this study, anisotropic graphene (GNS)–iron oxide (Fe₃O₄)/polyimide (PI) nanocomposite films with oriented GNSs were prepared by the in situ polymerization of 4,4′‐oxydianiline and pyromellitic anhydride in the presence of GNS–Fe₃O₄. Films of the precursors were fabricated, and this was followed by stepwise imidization under a magnetic field at a higher temperature to orient the magnetic sheets. The orientation of GNS–Fe₃O₄ and the relationships of the GNS–Fe₃O₄ content and measurement frequency with the dielectric properties of the GNS–Fe₃O₄/PI films were studied in detail. The dielectric property differences of the GNS–Fe₃O₄/PIs with GNS–Fe₃O₄ parallel or perpendicular to the film surface were not obvious, when the content of GNS–Fe₃O₄ was lower than 5 wt %. However, at the percolation threshold, the k values of GNS–Fe₃O₄/PI films with horizontal GNS–Fe₃O₄ were much higher than those of the other two kinds of films at 10³ Hz; this was derived from the contribution of more effective microcapcitors parallel to the film surface. So, making the GNS–Fe₃O₄ parallel to the film surface greatly enhanced k of GNS–Fe₃O₄. However, switching the charges on the large lateral surface of the parallel GNSs with the electric field also caused a higher dielectric loss and the frequency dependence of k and the dielectric loss at low frequency. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43041.     

The effect of titania content on the physical properties of polyimide/titania nanohybrid films

In this study, a series of PI/TiO₂ nanohybrid materials were prepared from polyamic acid of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride/3,3′-diaminodiphenyl sulfone, and titania precursor by the sol-gel method. The titania content in the hybrid system was varied from 0 to 5 wt %. The physical and mechanical properties of the hybrids such as refractive index, optical transmission, and tensile strength were investigated. It was determined that incorporation of titania precursor into the PI matrix improved the refractive indices and tensile modulus of the hybrid films. It was observed that the optical transmittance and tensile strength of the nanohybrids were slightly decreased with the increasing titania content. It was determined that the hybrid films might have enhanced the UV shielding properties compare to the PI films. Furhermore, the hybrid materials showed better thermal stability than the PI. SEM studies demonstrated that titania particles (1 and 3 wt %) were distributed homogeneously through the PI matrix. The effect of the titania content in the PI on DC conductivity and dielectric constant were also analyzed. For the PI film containing 5 wt % titania, activation energy value increased to 1.0 eV from the value of 0.65 eV. DC conductivity value of the films depending on titania content varied between 3.0 × 10⁻¹¹ and 1.4 × 10⁻¹⁰ S/cm at room temperature. Relative dielectric constants of the films were calculated from capacitance measurements depending on frequency (40–100 kHz) at different temperatures (303–360 K). The values increased with the increasing titania content. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Porous polyimide films prepared by thermolysis of porogens with hyperbranched structure

Two hyperbranched molecules, benzenetricarboxylic acid dendritic benzyl ether ester (BTRC–BE) and benzenetricarboxylic acid polyethylene glycol ester (BTRC–PEG), were prepared and tested as pore‐generating agents (porogens) for the preparation of porous polyimide. The hyperbranched molecules were thermally stable during the imidization process and completely decomposed well below the degradation temperature of polyimides, indicating that they possessed desirable thermal decomposition characteristics as porogens for the porous polyimide. From the SEM analysis dispersed domains were observed in the poly(amic acid) films containing BTRC–BE, whereas no phase separation was observed in the poly(amic acid) films containing BTRC–PEG. This may be attributable to the different polarities of the porogens because the hydrophobic BTRC–BE was phase‐separated in the hydrophilic poly(amic acid) matrix but BTRC–PEG was well mixed with poly(amic acid). The morphology developed in the poly(amic acid) film was retained after imidization and decomposition of the porogens; thus only BTRC–BE produced the porous polyimide. As the content of BTRC–BE decreased from 20 to 5 wt %, the pore size decreased from 390 ± 100 to 90 ± 50 nm and the pore density also decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1711–1718, 2004     

Gradient structure polyimide/graphene composite aerogels fabricated by layer-by-layer assembly and unidirectional freezing

In this work, gradient polyimide (PI)/graphene composite aerogels were prepared with poly(amic acid) ammonium salt/graphene aqueous solution through layer-by-layer assembly, unidirectional freezing, freezing drying, and thermal imidization process. Each layer of gradient PI aerogels was consisted of oriented channel-like pores along the freezing direction. The gradient PI/graphene composite aerogels exhibited anisotropic conductivity and heat transfer property. The conductivity of composite aerogels in the perpendicular direction of oriented channel-like pores was higher than that along the direction of oriented pores. The heat transfer from the high-density end to the low-density end of gradient density composite aerogels was faster. Compared with those of homogeneous composite aerogel with same density, the compression yield stress of gradient density composite aerogels obviously decreased, and their compression platform region also obviously shortened. Moreover, when the compressive strain exceeded 35%, the compressive strength of gradient composite aerogel with more layers was much higher.     

SiC whiskers-reinforced polyimide aerogel composites with robust compressive properties and efficient thermal insulation performance

Polyimide (PI) aerogels enable promising application in many fields but are always inhibited by their weak resistance to high temperature and low mechanical properties. Herein, novel PI aerogel composites with efficient heat insulation and high compressive performance are prepared by introducing Silicon carbide whiskers (SiCw) as reinforcement. The SiCw in aerogel function as shrinkage inhibitor and high temperature stabilizer. The addition of SiCw exhibits obvious enhancement and toughening effect, the compressive strength of the PI aerogel composites increases from 1.09 to 1.96 MPa. These PI aerogel composites also show enhanced high temperature stability. The as-prepared PI aerogel composites possess integrated properties of high-temperature resistance, low thermal conductivity, and high compressive strength, which can be the candidate for the application in aerospace.     

Effect of chemical structure and preparation process on the aggregation structure and properties of polyimide film

In this article, polyimide (PI) films were fabricated via the three‐step method including the reactions of condensation polymerization, chemical imidization, and thermal imidization. In comparison with the conventional two‐step method to produce PI films, there was an additional step in the present method, i.e., chemical imidization. The aim of chemical imidization was to get PI intermediates with different pre‐imidization degree (pre‐ID). And PI component in PI intermediates acted as in‐situ rigid‐rod segments and induced orientation in the films of PI intermediates. Then the orientations of molecular chains were preserved in the following thermal imidization, and caused the difference in aggregation structure and property of the final PI films. The test results indicated that the orderly degree of molecular chains and mechanical properties of PI films increased with pre‐ID increasing. Furthermore, this tendency was much more obvious for more rigid backbone structure. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Inclusion of covalent triazine framework into fluorinated polyimides to obtain composites with low dielectric constant

Polyimides possess good mechanical properties, favorable dielectric properties, and chemical inertness, which enabled them to find applications in microelectronic industries. The dielectric constant of the polyimides varies between 2.5 and 4, which is rather high for such applications. Hence, synthesizing polyimides with still lower dielectric constant has become one of the critical research confronts. As the properties of a terpolyimides (TPI) could be altered as per the requirement, it was synthesized by combining the dianhydrides 3,3′,4,4′-biphenyldianhydride, 3,3′,4,4′-oxydiphthalicdianhydride, and 4,4′-(hexafluoroisopropylidene) with a diamine 4,4′-(hexafluoroisopropylidene)dianiline or 2,2-bis[4-(4-amino phenoxy)phenyl]hexafluoropropane. As porous covalent triazine framework (CTF-1) is capable of capturing much air within its pores and interfacial voids, it was combined with the TPI matrix in different loadings to obtain CTF-1/TPI composite films with low dielectric constant. The composites exhibited high thermal stability, as their thermal decomposition occurred above 520°C. The tensile properties and the dielectric constant of the composites declined with the raise in CTF-1 loading up to 4%. The decrease in dielectric constant is essentially due to the incorporation of air voids (dielectric constant of air ~1) in the TPI matrix due to the inclusion of porous CTF-1.     

Functionalized KIT‐6/Terpolyimide composites with ultra‐low dielectric constant

Polyimides with low dielectric constants are important raw materials for the fabrication of flexible printed circuit boards and other microelectronic applications. As creation of voids in polyimide matrix could decrease dielectric constant, in this study mesoporous KIT‐6, synthesized hydrothermally, was functionalized with 3‐aminopropyltriethoxysilane (APTS) and mixed with 4,4′‐oxydianiline (ODA) in the synthesis of terpoly(amic acid) using 3,3′,4,4′‐biphenyldianhydride (BPDA), 3,3′,4,4′‐oxydiphthalic dianhydride (ODPA) and 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and subsequently stage‐cured to obtain APTS‐KIT‐6/Terpolyimide composites (APTS‐KIT‐6/TPI). The asymmetric and symmetric vibrations of imide O?C? N? C?O groups of APTS‐KIT‐6/TPI composites showed their peaks at 1772 and 1713 cm^?1. The dielectric constant decreased with the increase in KIT‐6 loading from 2 to 4%, but increased at higher loadings, and at 4% loading it was 1.42. Its tensile strength (103 MPa), tensile modulus (2.5 GPa), and percentage elongation (8.2) and high thermal stability (>540°C) were also adequate for application in microelectronics such as flexible printed circuits. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40508.     

Inkjet printing of polyimide insulators for the 3D printing of dielectric materials for microelectronic applications

In this article, we report the first continuous fabrication of inkjet‐printed polyimide films, which were used as insulating layers for the production of capacitors. The polyimide ink was prepared from its precursor poly(amic) acid, and directly printed on to a hot substrate (at around 160 °C) to initialize a rapid thermal imidization. By carefully adjusting the substrate temperature, droplet spacing, droplet velocity, and other printing parameters, polyimide films with good surface morphologies were printed between two conducting layers to fabricate capacitors. In this work, the highest capacitance value, 2.82 ± 0.64 nF, was achieved by capacitors (10 mm × 10 mm) with polyimide insulating layers thinner than 1 μm, suggesting that the polyimide inkjet printing approach is an efficient way for producing dielectric components of microelectronic devices. © 2016 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43361.     

Preparation of porous thin films of a partially aliphatic polyimide

A thermally labile polymer, poly(propylene glycol), was modified to obtain PPG having an amino end group. PPG was incorporated into a partially aliphatic polyimide based on an alicyclic dianhydride, and this afforded triblock copolymers containing various amounts of PPG blocks. The thermal properties of the copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry. The thermal decomposition of the PPG block in the copolymers was carried out at 240°C under various pressures to obtain porous polyimide films. The pores remained during the thermolysis under a reduced pressure of 710 mmHg, whereas they collapsed under (near) atmospheric pressure. The pore size increased as the amount of the PPG block in the copolymers increased. The dielectric constants of the porous polyimides varied from 2.60 to 2.42 with the original copolymer composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 532–538, 2006     

Enhanced thermal and electrical insulation properties of polyimide films determined via a two‐dimensional layered double hydroxide–potassium perfluorooctane sulfonate material

Polyimide (PI) films used in aerospace and rail applications are degraded by thermal, chemical, and electric power under the effect of insulation aging. To prevent these types of degradation, we prepared nanocomposite films of PI and layered double hydroxide (LDH) modified with the potassium perfluorooctane sulfonate (FS) by an in situ method. On the whole, the glass‐transition temperature, dielectric constant (?′), and corona‐resistance lifetime of the nanocomposite films increased over those of the pure PI film, but the temperature at 10 wt % weight loss, breakdown strength, and volume resistance decreased correspondingly as a whole. The PI matrix was protected by the inorganic material of LDH–FS with a high ?′ from corona corrosion. Furthermore, the heat and chemical interactions of the composite films improved the corona‐resistance lifetime, despite the decrease in the breakdown strength; this should result in promising applications for insulation parts of variable‐frequency motors. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46528.     

Construction of fluorenyl-modified low dielectric constant polyimide films with excellent mechanical properties and optical transparency

In order to meet the requirements of highly integrated and miniaturized electronic components, there is an urgent need for low dielectric materials with high mechanical properties and optical transparency in the field of microelectronics. In this study, a series of novel polyimide films (FPI) containing fluorenyl were prepared, and the effects of the fluorenyl content on the thermal, mechanical, and dielectric properties of the copolymerized films were investigated and discussed. The results demonstrate a significant decrease in the dielectric constant of the FPI films following the introduction of fluorenyl into polyimide (PI) chain segment. The FPI films also exhibited high mechanical properties, including tensile strengths between 92 and 106 MPa and elongation at break in the range of 8.4%–13.0%. Additionally, the introduction of the noncoplanar fluorenyl considerably improved the optical transparency and solubility of the FPI film. It is noteworthy that the FPI-3 has the best dielectric properties, with a low dielectric constant of 2.61 at 10 MHz and shows low water absorption (0.49%). The results show that we have prepared a novel low dielectric PI material film with excellent mechanical properties and optical transparency by introducing fluorenyl into the PI chain segment. These FPI films with satisfactory properties may be good candidates for dielectric materials for electronic components.