A novel cross-linked polyimide film: synthesis and dielectric properties

A novel cross-linked polyimide (CPI) has been prepared by imidization of cross-linked poly(amic acid) (CPAA). In this work, the Ac conductivity and dielectric properties of this polyimide are presented comparitively with those of conventional polyimide (PI), in the 0.2-100 kHz frequency range and 300-463 K temperature interval. Although the frequency and temperature dependencies of dielectric constant of both conventional and cross-linked polyimides show the same behaviour, the dielectric constant of CPI takes lower values. The Ac conduction studies suggest that electron hopping is responsible for conduction of the PI and CPI films. The activation energy calculated in 296-353 K temperature interval and the ß-relaxation was also observed for CPI.     

Synthesis and characterization of polyimides with low dielectric constants from aromatic dianhydrides and aromatic diamine containing phenylene ether unit

New aromatic polyimides (PIs) having the lowest dielectric constant in nonfluorinated PIs have been developed from aromatic dianhydrides and an aromatic diamine containing phenylene ether units. The diamine monomer was prepared from 4-bromophenyl ether in five steps. Polycondensations were performed in 1-methyl-2-pyrrolidinone at room temperature for 18 h, giving poly(amic acid)s (PAAs) with inherent viscosities up to 0.53 dl/g. PAAs were converted to corresponding poly(imide)s (PIs) by thermal treatment at 300 °C. New PIs showed good thermal stability (5% weight loss around 450 °C) and the low dielectric constant (2.74).     

Improved properties of scandia and yttria co-doped zirconia as a potential thermal barrier material for high temperature applications

Due to the limited temperature capability of current YSZ thermal barrier coating (TBC) material, considerable effort has been expended world-wide to research new candidates for TBC applications above 1200?°C. Our study suggested that Sc₂O₃ and Y₂O₃ co-doped ZrO₂ (ScYSZ) had excellent t’ phase stability even after annealed at 1500?°C for 336?h. The thermal expansion coefficient of ScYSZ was comparable to the value of YSZ. The thermal conductivity of fully dense ScYSZ was in the range of 2.13–1.91?W?m^?1?K^?1 (25–1300?°C), approximately 25% lower than that of YSZ. Although the fracture toughness of dense ScYSZ was slightly lower than YSZ, an evident decline in elastic modulus was found. Additionally, thermal cycling lifetime of plasma sprayed ScYSZ coating (914 cycles) at 1300?°C was about 2.6 times longer than its YSZ counterpart. The superior comprehensive properties confirm that ScYSZ is a prospective candidate material for high-temperature TBC application.     

New difunctional fluoro-epoxide monomers: synthesis, photopolymerization and characterization

New difunctional fluoro-epoxide monomers were synthesized starting by allylation of the corresponding fluorinated diols and subsequent epoxidation. The new products were photocured, in the presence of triphenylsulphonium hexafluoroantimonate as photoinitiator; a higher reactivity of the fluorinated monomers was evidenced with respect to a typical UV curable diepoxy resin, hexanedioldiglycidyl ether HDGE, and attributed to the lower nucleophihcity of the glycidyl oxygen in the presence of fluorinated structures. The fluorinated epoxides were also copolymerized with HDGE: completely hydrophobic surfaces were obtained with a selective enrichment of the fluorinated segments through the air surface.     

Polyimides with main-chain ethylene oxide units: synthesis and properties

This paper describes the synthesis of a new series of well-defined bis(ether anhydride)s based on ethylene oxide sequences of known length (up to six ethylene oxide units) with phthalic acid anhydride end-caps, and extends the synthesis of bisphenoxyamines based on ethylene oxide sequences to six ethylene oxide units. The dianhydrides and diamines were used in combination to synthesize a series of segmented polyimides of well-defined structures having defined sequences of from zero up to six ethylene oxide units separated by N-phenylphthalimide units. The polymers are characterized in terms of solubility, thermal stability, thermal transition behaviour, including crystal melting behaviour; glass-transition temperatures vary systematically with structure and composition. One polymer exhibits liquid crystallinity. The water absorption characteristics of the polymers, which vary from a hydrophobic aromatic polyimide to polymers containing up to 52.7 wt% ethylene oxide, were determined. The crystal melting behaviour is discussed in conjunction with data from other segmented polyimides containing similar structural units.     

Organosoluble semi-alicyclic polyimides derived from 3,4-dicarboxy-1,2,3,4-tetrahydro-6-tert-butyl-1-naphthalene succinic dianhydride and aromatic diamines: Synthesis,characterization and thermal degradation investigation

Semi-alicyclic polyimides (PI-1–PI-4) have been prepared from a newly-developed alicyclic dianhydride, 3,4-dicarboxy-1,2,3,4-tetrahydro-6-tert-butyl-1-naphthalene succinic dianhydride (TTDA) and various aromatic diamines. The asymmetrical alicyclic tetralin structure in TTDA endowed the derived PI resins good solubility in various organic solvents. They were soluble not only in polar aprotic solvents but also in solvents with low boiling points including dichloromethane and tetrahydrofuran. Flexible and tough PI films were successfully cast from their N-methyl-2-pyrrolidone (NMP) solution except PI-1 derived from TTDA and 4,4′-oxydianiline (ODA). The PI films exhibited good optical transparency in the ultraviolet and visible light region with optical transmittances around 80% at a wavelength of 400 nm. The PI films showed good thermal stability up to 400 °C in nitrogen, after that they thermally decomposed rapidly. The thermal degradation behaviors of the PIs were investigated by means of thermogravimetric analysis-mass spectrometry (TG-MS), thermogravimetric analysis-Fourier transform infrared spectrometry (TG-FTIR) and the real time FTIR spectra (RT-FTIR) analysis techniques. The results revealed that the decomposition of PI-1 (as a representative) took place mainly in the range of 450–550 °C. The major degradative fragments at 500 °C were clearly identified as the tert-butyl-substituted tetralin moiety and its dealkylation products. Therefore, it can be deduced that for the TTDA-PIs, the single bond bridge attached the maleic anhydride and tetralin anhydride units in TTDA moiety might break first during the heating; thus producing the tetralin fragments.     

Polyimides 6: Synthesis,characterization, and comparison of properties of novel fluorinated poly(ether imides)

Five new poly(ether imides) have been prepared on reaction with oxydiphthalic anhydride (ODA) with five different diamines: 1,4‐bis(p‐aminophenoxy‐2′‐trifluoromethyl benzyl) benzene, 4,4′‐bis(p‐aminophenoxy‐2′‐trifluoromethyl benzyl) benzene, 1,3‐bis(p‐aminophenoxy‐2′‐trifluoromethyl benzyl) benzene, 2,6‐bis(p‐aminophenoxy‐2′‐trifluoromethyl benzyl) pyridine, and 2,5‐bis(p‐aminophenoxy‐2′‐trifluoromethyl benzyl) thiophene. Synthesized polymers showed good solubility in different organic solvents. The polyimide films have low water absorption of 0.3–0.7%, low dielectric constants of 2.82–3.19 at 1 MHz, and high optical transparency at 500 nm (>73%). These polyimides showed very high thermal stability with decomposition temperatures (5% weight loss) up to 531°C in air and good isothermal stability; only 0.4% weight loss occurred at 315°C after 5 h. Transparent thin films of these polyimides exhibited tensile strength up to 147 MPa, a modulus of elasticity up to 2.51 GPa and elongation at break up to 30% depending upon the repeating unit structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 821–832, 2004     

First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural,mechanical and thermal characterization

First principles calculations were conducted on (HfTiWZr)B₂ high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B₂ HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB₂ structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10^?4 mm³, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C.     

Development and characterization of novel DOPO based phosphorus tetraglycidyl epoxy nanocomposites for aerospace applications

A study was made in the present investigation on the development and characterization of 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) based phosphorus tetraglycidyl epoxy nanocomposites and to find its suitability for use in aerospace and high performance applications. Phosphorus-containing diamine (DOPO-NH₂) was synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 4,4′-diaminobenzophenone (DABP), and this is utilized for the preparation of DOPO based phosphorus containing tetraglycidyl epoxy denoted as ‘D’. The synthesized resin was characterized by Fourier transform infrared spectra (FT-IR) and ¹H, ¹³C nuclear magnetic resonance (NMR) spectra. Nanoclay and polyhedral oligomeric silsesquioxane (POSS)-amine nano-reinforcements denoted as N1 and N2 were incorporated into the synthesized epoxy resin. Curing was done with diaminodiphenylmethane (DDM) and bis(3-aminophenyl) phenylphosphine oxide (BAPPO) curing agents denoted as X and Y respectively. Mechanical, thermal, flame retardant, water absorption behaviour and electrical properties of the epoxy nanocomposites were studied and the results are discussed.     

Effects of doping Al-metal powder on thermal,mechanical and dielectric properties of AlN ceramics

In this work, the influence of Al-metal powder addition upon that thermal, mechanical and dielectric properties of aluminium nitride (AlN) ceramic was studied. The findings show that adding Al-metal powder improves not only the mechanical and thermal properties of the AlN ceramic but also has no negative impact on its dielectric properties. Based on Y₂O₃ as sintering aid, the AlN ceramic with 1.0 wt% Al doping were 14.35% higher thermal conductivity, 11.73% higher flexural strength and 59.50% higher fracture toughness than those doped without Al, respectively. This study showed that the addition of Al-metal powder may favor the purifying of the AlN lattice and the formation of homogenous and isolated second phase, which would increase the AlN–AlN interfaces and improve the thermal conductivity. Furthermore, the grain boundaries of AlN ceramics might be strengthened by the isolated second phases due to the thermal mismatch between the second phases and AlN grains, thus strengthening and toughening the AlN ceramic doped with Al. However, the large additive amount of Al powder (>1.0 wt%) was not help the isolation and homogenization of the second phase, giving a deterioration in an AlN ceramic's mechanical and thermal properties. These results suggest that the introduction of an appropriate dose of aluminium metal powder is a simple method that can be used to improve the AlN ceramic's mechanical and thermal properties simultaneously.     

Dielectric and thermal properties of dicyclopentadiene containing bismaleimide and cyanate ester. Part IV

A novel bismaleimide (BMI), bis(4-maleimidophenoxy-3,5-dimethylphenyl)dicyclopentadiene (DCPDBMI), containing a large dicyclopentadiene (DCPD) and aryl ether linkage, was synthesized from bis(4-aminophenoxy-3,5-dimethylphenyl)dicyclopentadiene and maleic anhydride by the usual two-step procedure that included ring-opening addition to give bismaleamic acid, followed by cyclodehydration to bismaleimide. The monomers were characterized by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (NMR), elemental analyses (EA), and mass spectra (MS). A series of bismaleimide-triazine (BT) resins were prepared from synthesized bismaleimide (DCPDBMI) and then cured with 2,6-dimethyl phenol-dicyclopentadiene dicyanate ester (DCPDCY) at various molar ratios. Thermal properties of cured BT resins (DCPDBMI/DCPDCY) were studied using dielectric analyzer (DEA), dynamic mechanical analyzer (DMA) and thermalgravimetric analyzer (TGA). These data were compared with that of commercial bismaleimide (DDMBMI) cured with bisphenol A dicyanate ester (BADCY). The cured DCPDBMI/DCPDCY exhibits lower dielectric constant, dissipation factor and moisture absorption than those of DDMBMI/BADCY. The effects of blend composition on the glass transition temperatures and thermal stability are discussed.     

Synthesis and characterization of high strength polyimide/silicon nitride nanocomposites with enhanced thermal and hydrophobic properties

Polyimide (PI) composite films were synthesized incorporating amino modified silicon nitride (Si3N4) nanoparticles into PI matrix via in situ polymerization technique. The mechanical and thermal perfor-mances as well as the hydrophobic properties of the as prepared composite films were investigated with respect to the dosage of the filler in the PI matrix. According to Thermogravimetric (TGA) analysis, mean-ingful improvements were achieved in T5 (5％weight loss temperature) and T10 (10％weight loss temper-ature) up to 54.1 ℃ and 52.4 ℃, respectively when amino functionalized nano-Si3N4 particles were introduced into the PI matrix. The differential scanning calorimetry (DSC) results revealed that the glass transition temperature (Tg) of the composites was considerably enhanced up to 49.7 ℃ when amino func-tionalized Si3N4 nanoparticles were incorporated in the PI matrix. Compared to the neat PI, the PI/Si3N4 nanocomposites exhibited very high improvement in the tensile strength as well as Young's modulus up to 105.4％ and 138.3％, respectively. Compared to the neat PI, the composites demonstrated highly decreased water absorption behavior which showed about 68.1％ enhancement as the content of the nanoparticles was increased to 10 wt％. The SEM (Scanning electron microscope) images confirmed that the enhanced thermal, mechanical and water proof properties are essentially attributed to the improved compatibility of the filler with the matrix and hence, enhanced distribution inside the matrix because of the amino groups on the surface of Si3N4 nanoparticles obtained from surface functionalization.     

The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si₃N₄ ceramics was investigated in the temperature range from 900 °C to 1300 °C for 1 h, 5 h, and 24 h. The weight gain measured either at lower temperature (900 °C) for long time (24 h) or at higher temperature (1300 °C) for 1 h demonstrated that the porous Si₃N₄ ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO₂ was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO₂ and thermal expansion coefficient mismatch between SiO₂ and Si₃N₄.     

Advanced polyimide materials: Syntheses, physical properties and applications

Polyimides rank among the most heat-resistant polymers and are widely used in high temperature plastics, adhesives, dielectrics, photoresists, nonlinear optical materials, membrane materials for separation, and Langmuir-Blodgett (LB) films, among others. Additionally, polyimides are used in a diverse range of applications, including the fields of aerospace, defense, and opto-electronics; they are also used in liquid crystal alignments, composites, electroluminescent devices, electrochromic materials, polymer electrolyte fuel cells, polymer memories, fiber optics, etc. Polyimides derived from monomers with noncoplanar (kink, spiro, and cardo structures), cyclic aliphatic, bulky, fluorinated, hetero, carbazole, perylene, chiral, non-linear optical and unsymmetrical structures have been described. The syntheses of various monomers, including diamines and dianhydrides that have been used to make novel polyimides with unique properties, are reported in this review. Polyimides, with tailored functional groups and dendritic structures have allowed researchers to tune the properties and applications of this important family of high-temperature polymers. The synthesis, physical properties and applications of advanced polyimide materials are described.     

Mechanical and dielectric properties of silicon dioxide film fabricated from polyphenylcarbosilane

Abstract

Abstract

Silicon dioxide film derived from polyphenylcarbosilane (PPCS) is being investigated for electronic ceramics and applications to dielectric materials. Polyphenylcarbosilane was heat treated under oxygen atmosphere to fabricate SiO₂ film. The mechanical property and dielectric constant of the film were measured using a nanoindenter and a semiconductor parameter analyser with metal–insulator–semiconductor structure. The chemical behaviour and structure were analysed with an X-ray photoelectron spectrometer and a differential thermal analyser. From the results, the silicon oxide film was synthesised from PPCS due to proper heat treatment. Elastic modulus values of the fired PPCS film were from 8·9 to 27·9 GPa and hardness values of the fired PPCS film were from 0·47 to 2·29 GPa with increasing firing temperature. Change in mechanical properties is related to intermolecular binding strength in the film.     

Development of dimethylsiloxane based tetraglycidyl epoxy nanocomposites for high performance,aerospace and advanced engineering applications

A study was made in the present investigation on bis(p-aminophenoxy) dimethylsiloxane based tetraglycidyl epoxy nanocomposites in order to determine its suitability for use in high performance applications. The synthesis of the siloxane tetraglycidyl epoxy resin denoted as ‘F’ was done and it was characterized by Fourier transform infrared spectra (FT-IR) and ¹H, ¹³C nuclear magnetic resonance (NMR) spectra. Nanoclay and polyhedral oligomeric silsesquioxanes (POSS)-amine nanoreinforcements denoted as N1 and N2 were incorporated into the synthesized epoxy resin. Curing was done with diaminodiphenylmethane (DDM) and bis(3-aminophenyl) phenylphosphine oxide (BAPPO) curing agents denoted as X and Y respectively. The mechanical, thermal, flame retardant and water absorption behavior of the epoxy nanocomposites were studied and the results are discussed.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Synthesis and characterization of layer-aligned poly(vinyl alcohol)/graphene nanocomposites

Layer-aligned poly(vinyl alcohol)/graphene nanocomposites in the form of films are prepared by reducing graphite oxide in the polymer matrix in a simple solution processing. X-ray diffractions, scanning electron microscopy, Fourier-transform infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis are used to study the structure and properties of these nanocomposites. The results indicate that graphene is dispersed on a molecular scale and aligned in the poly(vinyl alcohol) (PVA) matrix and there exists strong interfacial interactions between both components mainly by hydrogen bonding, which are responsible for the change of the structures and properties of the PVA/graphene nanocomposites such as the increase in T_g and the decrease in the level of crystallization.     

Synthesis of itaconimide/nadimide-functionalized benzoxazine monomers: Structural and thermal characterization

A novel series of itaconimide/nadimide-functionalized benzoxazine monomers (I-Bz/N-Bz) from imide-terminated amines, paraformaldehyde and phenol/m-cresol/cardanol/4-(hydroxyphenyl) benzoxazole were synthesized and characterized by FT-IR, ¹H and ¹³C NMR and HRMS analyses. The I-Bzs exhibited lower curing temperatures compared to the N-Bzs, which may be attributable to the presence of the 1,1-disubstituted double bond in itaconimide. The poly(I-Bz)s exhibited higher glass-transition temperatures (210–216 °C) compared to the poly(N-Bz)s (151–178 °C). The itaconimide- and nadimide-functionalized benzoxazines showed comparable thermal stability, but differences in their phenolic structures affected their thermal behavior (char yield and T_max). The poly(I-Bz)s (19–47%) showed higher char yields compared to the poly(N-Bz)s (18–41%), depending on the backbone structure. The highest lap-shear strength at 200 °C was observed in I-Bz/N-Bz derived from m-cresol.