Performance of high-temperature thermosetting polyimide composites modified with thermoplastic polyimide

The preparation of polyimide (PI) resin with high heat resistance and toughness is a significant challenge. In this study, thermoplastic PI (TPI) was used to toughen thermosetting PIs, and toughened PI (TPI/PI) blends were prepared. The modified PI resin system exhibited good thermal stability, excellent heat resistance, and high toughness. The results indicated that the TPI/PI blends maintained the curing behavior and characteristics of the PI oligomer. The T_g of the cured TPI/PI blend exceeded 395 °C, and the T_5% values were in the range of 533–563 °C, suggesting excellent thermal stability and heat resistance. The maximum impact strength was increased by 46% compared with that of pure PI, indicating the excellent toughening effect of the TPI. Carbon fiber-reinforced PI composites were prepared using the toughening system as a matrix. The compression-after-impact values of the carbon fiber-reinforced PI composites were up to 190 MPa, indicating the excellent toughness of the materials.     

Dielectric relaxation and AC conductivity of modified polysulfones with chelating groups

The frequency-dependent dielectric properties and conductivity of partially quaternized polysulfones, quaternized polysulfones containing chelating groups, and chelated quaternized polysulfones with Cu²⁺ have been studied. The permittivity has low values and is dependent on the chemical characteristic of samples, in relation with the charge transfer complex and free volume and, consequently, with packing of the polymer chains and of the polarizable groups per volume units. At temperatures below 150 °C, all polysulfone films develop two relaxation processes, i.e., γ and β relaxation, involving different enthalpy and entropy contributions induced by their chemical structures. Frequency–temperature-dependent conductivity showed that conductivity increased with frequency, while the values of thermal activation energy of electrical conduction, lower that 1, suggest that a model based on energy bandgap representation could be suitable for explaining the temperature influence on AC conductivity for all samples. In addition, enhancement of mobility of the charge carrier upon complexation was observed for chelated quaternized polysulfones with Cu²⁺. All these typical semiconducting properties recommend the studied polymers as potential candidates for use in various applications in electrotechnical industry.     

A study of crystalline transitions in a thermoplastic polyimide

A new polyimide derived from 4,4′-isophthaloyldiphthalic anhydride (IDPA) and 1,3-bis(4-aminophenoxy-4′-benzoyl)benzene (1,3-BABB) having semicrystalline behavior was prepared at NASA Langley Research Center in 1987. The crystalline transitions of this thermoplastic polyimide have been studied. The differential scanning calorimetry (DSC) pattern of partially imidized film exhibited two distinct crystalline melt endotherms. For this study each crystalline phase was isolated and enhanced by controlled thermal treatment. A film containing approximately 50% of both phases and an amorphous film were also prepared. Evaluations of these films were performed by DSC, wide-angle x-ray scattering (WAXS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Three distinct crystalline morphologies; ellipsoid, cubic, and needlelike embedded in an amorphous matrix were observed as a function of various cure conditions by SEM.     

Curing of a network polyimide modified with a linear component

The curing and relaxation processes in polymeric composites based on blends of network and linear polyimides applied onto a filler were studied by dielectric spectroscopy. The sensitivity of dielectric spectroscopy to processes occurring in the course of curing was examined.     

Dielectric relaxation of a flexible molecule with an intramolecular interaction

The Laplace transform of the electric polarization with respect to time for a molecule having two equivalent interacting rotational groups has been expressed in terms of an infinite continued fraction. This modifies the Debye equation by considering the intramolecular interaction.     

Preparation of thermosetting polyurethane nanocomposites by montmorillonite modified with a novel intercalation agent

A novel thermosetting polyurethane (TSPU)/organic montmorillonite (OMMT) nanocomposite has been synthesized. N‐diamino octadecyl trimethyl ammonium chloride (DODTMAC) was used as an intercalation agent to treat Na⁺‐montmorillonite (MMT) and form a novel kind of OMMT. Fourier transform infrared spectroscopy (FT‐IR), wide angle X‐ray diffraction (WAXD), and thermogravimetric analysis (TGA) data indicated that the MMT was successfully intercalated by this intercalation agent, as evidenced by the fact that the basal spacing of MMT galleries was expanded from 1.5 to 3.2 nm. This OMMT was used to prepare the TSPU nanocomposites. Both the reinforcing and compatibilizing performance of the filler were investigated. Tensile tests showed that the tensile strength of TSPU/OMMT‐4 was the highest, and was about 3.62 times higher than that of the pure TSPU, and also the elongation at break showed an enhancement. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements illustrated that the glass transition temperature of the TSPU/OMMT‐4 nanocomposite was improved from 0.5 to 6.5 °C, which corresponded to the restriction of the soft segments of TSPU. The highest initial and center temperatures of TSPU/OMMT‐4 obtained from TGA were due to the highest retard effect of the TSPU molecular chains. WAXD studies showed that the formation of the nanocomposites in all the cases with the almost disappearance of the peaks corresponding to the basal spacing of MMT. SEM and TEM were used to investigate the morphologies of the TSPU/OMMT‐4 nanocomposite, and demonstrated that the nanocomposite was comprised of a well dispersion of a mixture of intercalated and exfoliated silicate layers throughout the matrix. It was proposed that the nano‐reinforcing effect caused by the well‐dispersed silicate layers might reduce the amount and size of voids and increase the length of the crack‐spreading path during tensile drawing. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 519–531, 2007.     

Synthesis of N-substituted bisitaconimide monomers for use as thermosetting polyimide resins

Two monomeric N-substituted bisitaconimides, N,N′-bisitaconimido-p,p′-diphenylmethane and N,N′-bis(itaconimido-p,p′-diphenyl ether), were synthesized from the corresponding diamines. The synthesis was accomplished by reaction of the diamine with itaconic anhydride and cyclocondensation of the resultant bisitaconamic acid. Attempts to use p,p′-diaminodiphenylsulfone as the diamine gave N,N′-biscitraconimido-p,p′-diphenylsulfone and N-citraconimido-N′-itaconimido-p,p′-diphenylsulfone instead of the bisitaconimide. The two bisitaconimides polymerize thermally at 180°C and 225°C, respectively, and yield tough polymers with very high thermal stability.     

Dielectric relaxation of a short molecule in a liquid-crystalline solvent

Abstract

Dielectric relaxation measurements of 5 mole % 4-n-hexyloxycyanobenzene (I) dissolved in 4-n-pentyloxyphenyl-trans-4-n-octylcyclohexylcarboxylate (II) were carried out from 1 kHz to 10 MHz in the nematic, smectic A and smectic B phases. The relaxation frequency of I parallel to the director is about 05 MHz in the S_Bphase and increases rapidly at the transition from S_B to S_A.     

Dielectric relaxation of a nematic eutectic mixture with a high glass temperature

Dielectric relaxation measurements in the frequency range from 2 Hz to 100 kHz have been carried out on an oriented binary nematic mixture of low molecular liquid crystals with a glass temperature of 307 K. For both relaxation frequencies, f_R‖ and f_R⊥, a glass behaviour according to the Vogel-Fulcher law could be detected. The Cole-Cole distribution parameter for reorientation around the long molecular axis increases strongly with decreasing temperature. This effect is discussed with regard to the optical biaxiality of the mixture.     

Dielectric relaxation of a short molecule in a liquid-crystalline solvent

Dielectric relaxation measurements of 5 mole % 4-n-hexyloxycyanobenzene (I) dissolved in 4-n-pentyloxyphenyl-trans-4-n-octylcyclohexylcarboxylate (II) were carried out from 1 kHz to 10 MHz in the nematic, smectic A and smectic B phases. The relaxation frequency of I parallel to the director is about 05 MHz in the S_Bphase and increases rapidly at the transition from S_B to S_A.     

Dielectric relaxation of polydimethylsiloxane

Dielectric studies were performed on crystallized and amorphous polydimethylsiloxane which had been characterized by differential thermal analysis and polarized light microscopy. The crystallized specimen displayed one relaxation near 160 K at 1 kHz while the amorphous specimen showed absorption peaks at 155 and 165 K. For the latter material the high-temperature peak was not due to a true relaxation but resulted from crystal nucleation at 160 K and the subsequent growth of spherulites. The low-temperature peak at 155 K resulted from the relaxation associated with the glass transition. A sharp decrease of dielectric constant was observed for both specimens at the melting point (235 K). For the dielectric relaxation associated with the glass transition in crystallized specimens, the values of the dispersion amplitude, the apparent activation energy at 160 K, and the half-width of the absorption curve are 0.43 and 29 kcal/mole, and 5.6 decades, respectively, which are in marked contrast to the corresponding values of 0.82, 18, and 2.2 for amorphous specimens.     

Dielectric behavior of some aromatic polyimide films

Aromatic polyimides were prepared by polycondensation reaction of two aromatic diamines, such as 4,4′-diaminodiphenylmethane (DDM) and 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDDM), with aromatic dianhydrides, such as 4,4′-isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), benzophenonetetracarboxylic dianhydride (BTDA) and hexafluoroisopropylidene-bis (phthalic anhydride) (6FDA). These polymers are soluble in polar aprotic solvents and can be cast into thin films from such solutions. The polyimides show high thermal stability, with decomposition temperature being above 430 °C in air, and high glass transition temperature being in the range of 200–287 °C. The free standing films, having the thickness of tens of micrometers, exhibited good mechanical and electrical insulating properties. The dielectric constant, molecular mobility and AC conductivity of thin films prepared from these polymers were investigated in detailed. The study of their dielectric behavior evidenced low dielectric constant values, in the range of 2.88–3.48 at 1 Hz at room temperature, and three relaxation processes (γ,β₁ and β₂) were observed at sub-glass temperatures for polyimides based on 6HDA and 6FDA and only two (γ and β) relaxations were detected for polyimides based on BTDA. The cooperativity of the molecular motions associated with the relaxation processes was discussed.     

Dielectric relaxation in carbosilane dendrimers with cyanobiphenyl end groups

Carbosilane dendrimers of generation 1 and 2 are functionalized with mesogenic end groups (cyanobiphenyl) via spacers of 5 and 11 carbons. The dielectric relaxation is measured over broad frequency (1 Hz–1 GHz) and temperature (170–470 K) ranges. Two relaxation regimes are observed and characterized as δ and β relaxation. The δ relaxation is nearly a single Debye process and varies strongly with temperature. The S_E to S_A transition observed for the dendrimers with long spacers causes a jump in the relaxation rate of the δ process. The β process displays an Arrhenius-type temperature dependence with an activation energy of 35 kJ/mol. The relaxation time depends on spacer length. The dielectric relaxation reflects the mutual distortion of the dendrimer scaffold and the smectic layers. Received: 9 June 1999 Accepted in revised form: 21 July 1999     

Design and characterization of thermosetting polyimide structural adhesive and composite matrix systems

Fully cyclized, organo soluble, phenylethynyl-terminated, ether-imide oligomers of 2–10,000 g/mol (M_n) were prepared by the reaction of 2,2′-bis[4-(3,4-dicarboxyphenoxy)phenyl]-propane dianhydride (bisphenol-A dianhydride, BPADA) with a stoichiometric excess of either para, meta, or isomeric mixtures of phenylene diamine and phenylethynylphthalic anhydride (4-PEPA) endcapper. High para-containing oligomers produced semicrystalline powders, but the all meta isomer was completely amorphous. The lower molecular weight oligomers displayed an attractive low viscosity melt and were cured to very high gel content networks at 350–380°C for 30–90 min. The cured 3000 g/mol oligomers showed a (DSC) glass transition temperature (T_g) of 267°C and produced tough, solvent-resistant films. Excellent adhesion to surface-treated titanium alloys was achieved, as judged by single-lap shear measurements. Resin infusion molding was conducted, which permitted low-void, graphite-fabric composite panels to be prepared. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2943–2954, 1997     

Dielectric relaxation processes in PVDF composite

The present study aims at the detailed elaboration of the dielectric relaxation behavior in PVDF composites using broadband dielectric spectroscopy and the Havriliak – Negami method. The composites with multi-wall nanotube carbon and zirconium dioxide in PVDF is fabricated using a simple melt mixing method. The polarization behavior in PVDF composites are investigated on the different frequency region with various temperature. The complex dielectric constants are calculated with the aid of the Havriliak – Negami equation. The characteristic parameters in Havriliak – Negami equation were in excellent agreement with the experimental complex dielectric constants. The results of utilizing these calculated parameters to analyze the origination of the polarization relaxation are given. The purposes of this work expect to give a deeper insight into the impact of different fillers on the dielectric relaxation behavior, and it could provide the technique for the discrepancy with the dipolar for interfacial polarization and the filler effect on the dielectric relaxation.     

Dielectric relaxation in some polyacrylate solutions

The dielectric constant ?′ and loss factor ?″ of poly(butyl acrylate), poly(butyl methacrylate), and poly(isobutyl methacrylate) solutions are reported in the frequency region of 1 kHz to 24.42 GHz at four different temperatures of 27, 40, 50, and 60°C. Cole–Cole plots are plotted to obtain the distribution parameter and relaxation time. The activation energies are evaluated assuming dielectric relaxation to be a rate process in these solutions. A possible relaxation mechanism is discussed.     

Molecular level study of the crystallization of a thermoplastic polyimide by infrared spectroscopy

The crystallization process of the thermoplastic polyimide derived from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 2,2-dimethyl-1,3,-(4-aminophenoxy) propane (DMDA) is studied at the molecular level by utilizing infrared spectroscopy's sensitivity to local molecular environment. Assignments of the unique spectral features appearing upon crystallization are made with additional verification from crystalline model imide compounds. A direct comparison of an annealed polyimide film studied by both infrared spectroscopy and x-ray diffraction shows fairly close agreement in the amount of crystallinity obtained. The unique crystalline infrared spectral features provide insight into the molecular mechanism involved with the crystallization process. Spectral shifts occurring upon crystallization reflect intramolecular rotations toward increased coplanarity as well as intermolecular chain packing of certain molecular groups at specific sites of the chain backbone. The site specific information is then used to study the relative kinetic behavior of the intramolecular and intermolecular effects occurring during crystallization. © 1994 John Wiley & Sons, Inc.