Inkjet printing of polyvinyl alcohol multilayers for additive manufacturing applications

Here we demonstrate that inkjet printing technology is capable of producing polyvinyl alcohol (PVOH) multilayer structures. PVOH water‐based inks were formulated with the addition of additives such as humectant and pigments. The intrinsic properties of the inks, such as surface tension, rheological behavior, pH, wetting, and time stability were investigated. The ink's surface tension was in the range 30–40 mN/m. All formulated inks displayed a pseudoplastic (non‐Newtonian shear thinning and thixotropic) behavior at low‐shear rates and a Newtonian behavior at high‐shear rates; were neutral solutions (pH7) and demonstrated a good time stability. A proprietary 3D inkjet printing system was utilized to print polymer multilayer structures. The morphology, surface profile, and the thickness uniformity of inkjet printed multilayers were evaluated by optical microscopy and FT‐IR microscopy. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43572.     

High-temperature polyimide nanofoams for microelectronic applications

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term ‘nanofoam’. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block, leaving pores of the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials were investigated as well as from a number of thermally labile polymers. The foams were characterized by a variety of experiments including TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and low dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.     

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.     

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.     

Effect of polymer structure on the morphologies and dielectric properties of nanoporous polyimide films

Low dielectric constant polyimide (PI) films have potential applications in integrated circuit. In this study, poly(methyl methacrylate), poly(ethylene oxide), and polystyrene as thermally labile materials were used as templates to generate PI films with nanopores by first mixing the polymer templates with the precursor of PI, poly(amic acid), followed by imidization of poly(amic acid) together with degradation of the polymer templates. The sizes of the formed pores, the thermal and dielectric constant of the nanofoamed PI films were studied and compared in detail. It is concluded that the dielectric constant of PI films using poly(ethylene oxide) as pore template is more stable because of the formation of uniform pores which is from the great accordance of imidization temperature of poly(amic acid) with the degradation temperature of poly(ethylene oxide). But that using poly(methyl methacrylate) as pore template is frequency dependent as the influence of inhomogeneous pores and PMMA residue from incompletely degradation of poly(methyl methacrylate). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41480.     

Fluorinated graphene/polyimide nanocomposites for advanced electronic packaging applications

Low dielectric constant and low dissipation factor, superior hydrophobicity, excellent thermal and mechanical property, and good optical performance are desired for advanced electronic packaging of fan-out wafer level package (FO-WLP). An effective approach was introduced to fabricate novel fluorinated graphene/polyimide(FG/PI) nanocomposite films in this paper. FG nanosheets exhibit excellent dispersion in the PI matrix due to their high surface area with some oxygen-containing functional groups, and individual graphitized planar structure. Besides, the effects of the addition of FG on the dielectric, optical, mechanical, thermal properties as well as the hydrophobicity of the films are investigated with controlled amounts of FG. The dielectric constant and loss can be as low as 2.64 (at 10⁶ Hz) and 0.00176 for PI-0.5wt% FG. And with the increase of the loading of FG, the thermal stability (T₅ = 514°C) and mechanical property (tensile modulus = 2.11GPa, tensile strength = 93.23 MPa, elongation at break = 11.60%) has been improved successfully. Besides, the contact angles have been increased from 83° to 92° showing a superior hydrophobicity which is essential for the FO-WLP. Moreover, the incorporation of FG also proved excellent optical performance of 88% transmittance at 550 nm. Therefore, with the excellent comprehensive performance, the as-prepared FG/PI films possess widespread applications in the microelectronics especially in FO-WLP.     

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.     

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.     

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     

Dependence of the dielectric constant on the fluorine content and porosity of polyimides

The trend toward miniaturization in integrated circuit fabrication demands good interlayer dielectric materials. This need can be met by polyimide (PI), which has extreme thermal and chemical stability and, most importantly, a low dielectric constant. Four porous PIs with symmetrically substituted fluorine contents were synthesized. Different porosity levels were achieved with a sol–gel technique through the incorporation of 10 or 20% tetraethyl orthosilicate into the polymer matrix and then acid etching. Their dielectric constants were correlated with the fluorine contents and porosity levels. High porosity levels and higher fluorine contents induced substantial decreases in the PI dielectric constants (2.4–2.7). The resultant values were within the applicable range for dielectric materials in integrated circuits. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structure–property relationship of polyimides based on pyromellitic dianhydride and short‐chain aliphatic diamines for dielectric material applications

Most polyolefins that are used for dielectric materials exhibit a low dielectric constant and operating temperatures up to 70°C. Polyimides offer a means to a higher dielectric constant material by the introduction of a polar group in the polymer backbone and are thermally stable at temperatures exceeding 250°C. A common dianhydride, pyromellitic dianhydride (PMDA), is reacted with various short‐chain diamines to produce polymers with high imide density. Homopolymers and copolymers synthesized had dielectric constants ranging from 3.96 to 6.57. These materials exhibit a dielectric constant twice that of biaxially oriented polypropylene and therefore a twofold increase in capacitance as well as maintaining low dissipation factors that are acceptable for this application. The experimental dielectric constants of these materials are also compared to density functional theory calculations and exhibit a close relationship. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1276‐1280, 2013     

Fabrication of bulk piezoelectric and dielectric BaTiO3 ceramics using paste extrusion 3D printing technique

A simple and facile method was developed to fabricate functional bulk barium titanate (BaTiO₃, BT) ceramics using the paste extrusion 3D printing technique. The BT ceramic is a lead-free ferroelectric material widely used for various applications in sensors, energy storage, and harvesting. There are several traditional methods (eg, tape casting) to process bulk BT ceramics but they have disadvantages such as difficult handing without shape deformation, demolding, complex geometric shapes, expansive molds, etc. In this research, we utilized the paste extrusion 3D printing technique to overcome the traditional issues and developed printable ceramic suspensions containing BT ceramic powder, polyvinylidene fluoride (PVDF), N,N-dimethylformamide (DMF) through simple mixing method and chemical formulation. This PVDF solution erformed multiple roles of binder, plasticizer, and dispersant for excellent manufacturability while providing high volume percent and density of the final bulk ceramic. Based on empirical data, it was found that the maximum binder ratio with good viscosity and retention for desired geometry is 1:8.8, while the maximum BT content is 35.45 vol% (77.01 wt%) in order to achieve maximum density of 3.93 g/cm³ (65.3%) for 3D printed BT ceramic. Among different sintering temperatures, it was observed that the sintered BT ceramic at 1400°C had highest grain growth and tetragonality which affected high performing piezoelectric and dielectric properties, 200 pC/N and 4730 at 10³ Hz respectively. This paste extrusion 3D printing technique and simple synthesis method for ceramic suspensions are expected to enable rapid massive production, customization, design flexibility of the bulk piezoelectric and dielectric devices for next generation technology.     

Three-dimensional printing of high-performance polyimide by direct ink writing of hydrogel precursor

Three-dimensional (3D) printing of all-aromatic polyimides (PIs) is attracting extensive attention due to the advantages of excellent comprehensive performances and complex structures. It still remains challenges because of some drawbacks, such as poor solubility and infusibility, and the environmental unfriendly preparation. Here, polyamide acid salt (PAAS) hydrogels were prepared by the reaction of polyamide acid (PAA) with organic amines in water. PAAS hydrogels had remarkable rheological properties, which made it applied in direct ink writing 3D printing. The printed objects were finally converted into high-performances PI objects with prominent mechanical properties and thermal stability. Not only so, the advantages also include simple preparation, less organic solvent, low thermal imidization temperature (250°C), controllable macrostructure and porous microstructure, and the present 3D printed PI by PAAS was expected to significantly promote the application potentials. In this study, thermal conductivity (100°C) of the PI objects after structure design and 3D printing was reduced from 0.102 W m⁻¹ K⁻¹ to 0.061 W m⁻¹ K⁻¹, and density was reduced from 0.4562 g cm⁻³ to 0.2731 g cm⁻³.     

Optimization of 3D printing parameters for high-performance biodegradable materials

Developing 3D printing high-performance biodegradable materials is important to protect the environment and deal with emergencies such as COVID-19. Fused deposition modeling (FDM), one of the 3D printing methods, has many advantages, such as low cost and wide range of materials. However, the weak interlayer adhesion is an important factor restricting the development of FDM. In addition to the influence of material properties, the optimization of 3D printing parameters is also an important means to give full play to the inherent properties of materials. The optimal 3D printing parameters are conducive to the diffusion and entanglement of molecular chains between adjacent layers. PLA/PBAT/PLA-g-GMA (70/30/10 wt%, PLA-g-GMA was a compatibilizer synthesized in our lab) was used as the research object. This work aims to analyze the mechanical properties response of biodegradable polymers products manufactured through FDM. Herein, the effect of 3D printing parameters including layer thickness, nozzle temperature, printing speed and platform temperature have been systematically investigated by orthogonal experimental design. The result showed that the excellent performance of 3D printing specimen was obtained when the layer thickness was 0.15 mm, the printing speed was 50 mm·s⁻¹, the nozzle temperature was 200°C and the platform temperature was 50°C. The SEM images showed that the optimal 3D printing products had the best interlayer adhesion and the lowest porosity. Undergoing optimization of 3D printing processing, the yield strength and elongation at break of specimen increased by 115% and 229%, respectively. In this paper, the interlayer adhesion and mechanical properties of 3D printing products can be significantly improved by simply optimizing the 3D printing parameters without complex material modification. This work provided a new method for improving the interlayer adhesion of FDM and the mechanical properties of FDM products.     

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     

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.     

Improved processability of PA66-polyimide copolymers with different polyimide contents

Limitations in the properties of polyamide PA66, such as low glass transition temperature and high water absorptivity, limit its applications. Introduction of amorphous polyimide segments into the PA66 main chain lowers the glass transition temperature and melting temperature and also improves its processability. PA66-polyimide (PA-PI) copolymers with different weight ratios of PI are prepared by high temperature and high-pressure solution polymerization. The degree of crystallization of PA-PI copolymers decreases with increasing PI content. The melting point decreases from 261°C for PA66 to 223°C for PA-PI-4. Dynamic mechanical analysis shows that the T_g increases from 70 to 90°C, and the storage modulus can be well maintained. Rheological studies show that the temperature for processing can reach 70°C. Copolymers with different PI contents show different processing viscosities. In addition, water absorptivity (about 1.8%) and dielectric constant values of PA-PI and PA6/6T are similar.     

Dielectric behavior of three phase polyimide percolative nanocomposites

Three phase ceramic–metal–plastic (cermetplas) percolative nanocomposite films were prepared using polyimide, as the matrix to prepare the composite films. Silver nanoparticles with a mean particle size of about 41 nm were prepared and used as the metal phase. Nanocomposites of barium titanate (BaTiO₃) particles in polyimide matrix, with silver as metallic inclusion, were formulated using effective medium theory and percolation theory. The concentration and frequency dependence of dielectric permittivity (ε) and loss tangent (tan δ) measurements were reported and discussed for cermetplas below the percolation threshold. Experimental results for cermetplas show that a ε of above 500 can be achieved below the percolation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

3D printing dielectric elastomers for advanced functional structures: A mini-review

Three-dimensional (3D) printed bionic products play an important role in intelligent robotics, microelectronics, and polymers. The printing and manufacturing process of 3D printers is conducive to obtaining soft structures that meet specific requirements, and saves time and cost. Soft intelligent robotics, an emerging research field, has always been developed based on soft materials and actuators with their biological properties. This article reviews the current understanding of 3D bioprinting technologies for dielectric elastomers (DEs), DE actuators (DEAs) and soft robots, such as inkjet, extrusion, laser-induced and stereolithography bioprinting. 3D printers for fabricating soft materials are presented and classified. The approaches to exploit 3D bioprinters for DEs/DEAs are as follows: (1) 3D printing DEAs utilize ionic hydrogel–elastomer hybrids that are analogous to human muscles, and the DEAs usually have flexible structures and large deformations with multiple functionalities. (2) An electrohydrodynamic (EHD) 3D printer confers high printing resolution and high production efficiency, which offers advantages such as full automation and flexible design. The optimal printing conditions are mainly determined by the effects of printing voltages and ink properties, which are related to the formation of the liquid cone and the printed line width. Furthermore, the advantages of 3D bioprinting technologies have accelerated their development and applications.     

Room temperature densified H3BO3 microwave dielectric ceramics with ultra-low permittivity and high quality factor for dielectric substrate applications

The densification and microwave dielectric properties of H₃BO₃ ceramics prepared by dry pressing at room temperature were studied. The results show that pressure is the key factor of densification of H₃BO₃ ceramics. No second phase appears in all the as-fabricated H₃BO₃ ceramic samples. A dense H₃BO₃ ceramic (relative density~97.6%) was obtained by uniaxial compression of 384 MPa for 300s and the optimal microwave dielectric properties are ε_r = 2.83, Q × f = 59,400 GHz (f = 16 GHz), τ_f = −91 ppm/°C, which make it as a prospective candidate for microwave and millimeter-wave devices such as substrates for 5G communication technology.