Polyimides

Phenylated aromatic polyimides have been prepared in two steps by the Diels-Alder polymerization of biscyclopentadienones with dimaleimides followed by dehydrogenation and in one step by the polymerization of phenylated bis(phthalic anhydrides) with diamines. Although the polymers prepared by both routes were soluble in chlorinated-hydrocarbon solvents, the latter afforded considerably higher molecular weight materials. Polyimides obtained in this manner had intrinsic viscosities as high as 4.0 dl/g and glass transition temperatures that ranged from 238 to 466°C. Thermogravimetric analysis thermograms of the polymers in air and nitrogen atmospheres showed no weight loss until near 530°C. Two new ethynyl-substituted diamines, 4(2,4-diaminophenoxy)phenylacetylene (1) and 3,5-diaminodiphenylacetylene (2), have been synthesized and polymerized with a phenylated bis(phthalic anhydride). The polymer prepared with diamine 1 underwent cross-linking at 250°C without the evolution of volatile byproducts, while the polymer prepared with diamine 2 underwent an analogous cure at 350°C. A phenylated bis(phthalic anhydride) has also been copolymerized with pyromellitic dianhydride (PMDA) and 4,4’-diaminodiphenyl ether to afford soluble copolymers that contain as high as 50 mole % PMDA.

The development of water soluble polyimide resin precursors is described in terms of the synthesis and the chemistry of the resulting polymers. Selected properties of films and composites fabricated from the water soluble polyamic acids are presented and compared with organic solvent based resins where applicable. The advantages and differences found in processing the water soluble polymers are described.

An acetylene-substituted isomer of a polyimide was prepared and thermally isomerized into acetylene-substituted polyimide. Subsequent cure through the acetylene end groups yielded resin with mechanical and thermal properties similar to “Thermid 600” the first of the acetylenic polyimides to be marketed. Advantages of the isomer designated “IP-600” over the polyimide HR-600 are its lower melting range, high solubility in common solvents, and longer gel time. These advantages lead to markedly improved processing characteristics.

Three new silicon-containing dianhydrides, 4,4-bis(3,4-dicarboxyphenyldimethylsilyl)biphenyl 12, 1,3-bis(3,4-dicarboxyphenyl)-1,1,3,3,-tetramethyldisiloxane dianhydride, 18, and l,5-bis(3,4-dicarboxyphenyl)-1,13,3,5,5-hexamethyltrisiloxane dianhydride, 20 were synthesized by multistep synthesis. Dianhydrides, containing dimethylsiloxy groups, were polymerized with m,m and p,pdiaminodiphenylmethane to poly(amic acids) which were converted to polyimides. The objective of the present investigation was to study the effects of increasing the distance between and varying the isomeric positions of diamines/dianhydrides on glass transition temperatures of polymers.

Copolymerization of a variety of dianhydrides with molar quantities of organic diamines and/or amino-functional di- and/or polysiloxanes is discussed.

V-378A is a modified bismaleimide matrix resin which exhibits significant advantages over previously available bismaleimides and 350°F curing epoxy resins. The Tg of this new resin is in excess of 700°F. It has excellent tack at ambient temperatures and is available at prepreg resin solids contents as low as 30% for “net” resin (i.e. no bleed) cures. High modulus graphite unidirectional tapes based on V-378A exhibit excellent uncured or wet transverse strength. V-378A can be cured under standard low autoclave pressure of 100 psi and temperatures of 350°F similar to current 350°F curing epoxy systems. Unrestrained post cure at temperatures of 475 – 500°F are required to maximize elevated tem-perature strength retention. Cured V-378A/high modulus graphite composites exhibit strength retention at temperatures up to 600°F, excellent resistance to conditions of high humidity, no microcracking after repeated thermal spikes, and excellent tensile, compressive and shear strength. V-378A offers easy, epoxy-like processing characteristics with no volatile condensation products given off during cure with significantly su-perior high temperature strength retention after severe humidity aging.

A polymer system has been prepared which has the excellent thermoplastic properties generally associated with polysulfones, and the solvent resistance and thermal stability of aromatic polyimides. This material, with improved processability over the base polyimide, can be processed in the 260–325°C range in such a manner as to yield high quality, tough unfilled moldings; strong, high-temperature-resistant adhesive bonds; and well consolidated, graphite-fiber-reinforced moldings (composites). The unfilled moldings have physical properties that are similar to aromatic polysulfones which demonstrates the potential as an engineering thermoplastic. The adhesive bonds exhibit excellent retention of initial strength levels even after thermal aging for 5000 hours at 232°C. The graphite-fiber-reinforced moldings have mechanical properties which makes this polymer attractive for the fabrication of structural composites.

A melt processable polyimide, BDSDA/APB, which contains sulfur and oxygen bridges between the aromatic rings was synthesized and characterized. Its physical, mechanical, thermal and flow properties were determined as was its resistance to some of the more commonly used solvents. The melt flow properties were measured for the temperature range 250°C–350°C and under the stress/ strain conditions encountered in commercial processes.

The development of a broad line of 232°–280°C service adhesives which are easily processed is described. The first generation film adhesives, EA9655 and EA9655.1 typically perform best up to about 232°–260°C. Adhesive properties using various substrates including aluminum, titanium and graphite composites are described. EA9351 and EA9367 are one-component pastes with excellent adhesive strength to 260°C. LR 100-581 is a sprayable 232°–260°C service primer which complements these products. Preliminary data for a second generation film adhesive called LR 100-573 indicates that its service capabilities are in the range 260°–280°C. In all cases, the processing of each adhesive is very simple. Typical cure cycles are one hour at 177°C with minimum autoclave pressure (0.14 to 0.3MPa) followed by an unrestrained post-cure for two hours at 246°C.

Polyetherimide is a new resin introduced in 1982 by General Electric Company under the Tradename ULTEM®. An amorphous, high performance engineering thermoplastic polyetherimide is based on the regular repeating units of ether and imide linkages. The aromatic imide units provide the high performance properties while the flexible ether linkages allow for good melt flow characteristics and easy processability.

A series of polyamic-acid polymers with varying molecular weight have been prepared by condensation of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) in N-methylpyrrolidone (NMP). Polyamic-acid solutions between 10 – 20 wt% total solids content were prepared under highly controlled conditions.

The kinetics of acylation of aromatic diamines by dianhydrides of aromatic tetracarboxylic acids in amide solvents have been investigated.

A series of tin-containing polyimide films derived from either 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride or pyromellitic dianhydride and 4,4′-oxydianiline have been synthesized and their electrical properties examined. Highest quality materials (i.e. homogeneous, smooth surface, flexible) with the best electrical properties were doped with either SnC1₂·2H₂O or (n-Bu)2SnC1₂. In all cases, extensive reactivity of the tin dopant with water, air or polyamic acid during imidization is observed. Lowered electrical surface resistivities appear to be correlatable with the presence of surface tin oxide on the film surface.

Dynamic dielectric measurements of the imidizing LARC-160 resin have been made at a series of frequencies from 5 to 5 × 106 Hz. Capacitance (C), Dissipation (D), and Resistance (R) were recorded. These results have been correlated with the observed cure chemistry of the LARC-160 system. Changes in these measurements with both resin age and resin composition have been monitored. The cross-linking reaction has also been followed.

A fundamental understanding of the structure of the cured polyimide and the precursor polyamic acid from the condensation of pyromellitic dianhydride and 4,4′-diaminodiphenylether has been obtained through the use of several dilute solution techniques on samples prepared and cured under controlled conditions. The results obtained by these methods are compared with similar measurements on commercially available materials. Measurement of the concentrated solution viscosity as a means to identify changes in the molecular weight of the precursor polyamic acid is found to be inadequate because of its sensitivity to changes unrelated to molecular weight, i.e., strong polymer-polymer interactions. In this study we have augmented the concentrated solution viscosities with both light scattering and dilute solution viscometry to separate molecular weight changes from other effects. Unusual behavior in both dilute solution viscosity and light scattering measurements is attributable to polyelectrolyte effects arising from base impurities. We have also been able to measure directly the weight average molecular weight of the cured polyimide by low angle light scattering in concentrated sulfuric acid, and compare these molecular weights with those of the uncured precursor. These results have demonstrated that little change in molecular weight or chain dimensions occurs on curing, i.e., no evidence of branching or cross-linking is observed for this system.

A technique was developed for monitoring the conversion of polyamic acid to polyimide. The technique was used to characterize the cure reaction and to assist in understanding the relationship between degree of cure and etchability of films by a wet alkaline method. Interpretation of infrared spectra using a band ratio method gave consistent degree of cure determinations. Dynamic monitoring of films undergoing cure demonstrated that the rate of reaction is dependent on temperature and film thickness. Thicker films or thicker regions of nonuniform films were found to cure to a greater extent than thinner films or regions with identical thermal processing. Etchability was shown to have a high linear correlation with degree of cure. The study indicated that for optimal etch results film thickness uniformity and thermal treatment must be closely controlled.

The kinetics of the imidization reaction of various polyimides used in microelectronic applications have previously been studied by infrared spectroscopy. This paper reports the use of a new dielectric technique, called Microdielectrometry, to monitor the imidization reaction of PMDA-ODA polyamic acid films deposited on the surface of an integrated circuit. Dielectric permittivity and loss factor in the frequency range 1–1000 Hz were measured with Microdielectrometry probes during imidizations at temperatures between 100 and 160°C. At temperatures below 130°C, the permittivity remains dispersive even after five hours; whereas at higher temperatures, the permittivity becomes constant in less than two hours. The frequency dependence of the loss factor indicates that the film exhibits a weakly dispersiv bulk conductivity that varies between 3×10−10 and 5×10 4(Ohm cm)−1, increasing with temperature but decreasing with degree of imidization. At temperatures below 130°C, the conductivity remains relatively high even after five hours; whereas above 130°C, the conductivity decreases much more rapidly to low values, although conductivity changes are still detectable in the films at 2.5 hours. Results show a significant change in rate of reaction between 120 and 140°C, but with clearly observable changes still taking place well after the nominal one hour cure time.

The imidization reaction of various polyamic acids having different chemical structures has been followed by measuring the weight losses that occurred during dehydro-cyclization. From these studies it was found that when polyamic acids were heated rapidly to a given temperature, the imidization reaction proceeded during the temperature rise but slowed down very markedly after the given temperature was reached. The temperature at which the imidization reaction ended was closely related to the glass transition temperature of the resulting polyimide. Based on these observations, it is concluded that the imidization reaction slows down markedly because the glass transition temperature of the polymer rises as the reaction proceeds, and molecular motion is frozen. In other words, the free rotation of amide bonds in the main chain is frozen. As a result, suitable conformation for imidization cannot take place any more.

Isothermal reaction kinetics are reported for the thermal cyclization of the poly(amide-acid) based on benzophenone tetra-carboxylic acid dianhydride reacted with 3,3′-diaminobenzophenone. Isothermal solvent loss kinetics are also reported. The data suggest a strong interdependence between the rate of imidation and the rate of solvent loss. Additional reactions may occur for isothermal reaction temperatures above about 180°C. Infrared analysis showed the presence of moisture even at elevated temperatures; hence, hydrolysis is one possible side reaction. Infrared spectra of films exposed to elevated temperatures further suggest that complete solvent removal is difficult. The infrared band assignments are proposed. The kinetic data were fitted to a modified absolute reaction rate theory equation where the time dependence of the reaction rate constant may be explained by entropic changes which follow a continuous function of time. The reaction kinetics yielded reasonable values for the frequency of vibration of the activated complex transition state and the activation energy. Low values for the activation entropy may be explained by solvent effects. The solvent loss kinetic model yields an estimate of the polymer-solvent interactions.

Polyamic acids with different chemical structure and molecular weights have been used to analyze the kinetic relationships of their transformation into polyimides. Models describing the cyclization of polyamic acids under both isothermal and non-stationary conditions are developed. Cyclization is regarded as a two-stage physico-chemical process in which both the molecular mobility of the polymer matrix and the physical state of intrachain groups transformed into the imide rings play an important role. The nature of non-equivalent physical states of amic acid groups is discussed.

The dynamic viscoelastic properties of PMR-15 polyimide resin during thermal curing have been studied by the parallel-plate technique. The stoichiometry and molecular weight distribution of the imidized prepolymer are shown to have a strong influence on flow behavior during cure. Hydrolysis of polyamide acid intermediates and preferential reactivity of monomeric ingredients are discussed as factors contributing to the broad molecular weight distribution of the imidized prepolymer. Low molecular weight oligomers were prepared, and their effect on cure rheology studied. The presence of the lowest molecular weight oligomer was found to be critical to PMR-15 flow properties during cure.

Graphite reinforced polyimide matrix composite materials are attractive because of their outstanding specific strengths, specific stiffnesses, and thermal oxidative stability. Processing problems have, however, curtailed their application in aerospace hardware. The key to better understanding and control of the processing of such materials is their rheological behavior. Addition type polyimide resins and prepregs were obtained or synthesized and characterized. Rheological studies were carried out on neat resin systems. Problems were encountered because of outgassing during cure. A pressure cell transducer failed to solve these problems. A staging technique was developed to successfully handle polyimide samples. Techniques were developed to generate rheological curves on the graphite reinforced prepreg. Rheological prepreg data correlated well with the processing characteristics of these materials. Several commercial graphite/polyimide systems were studied and compared, including PMR 15, LARC 160 (AP-22), LARC 160 (Curithane 103), and V378A.

The fabrication of patterns in partially cured polyimide films may be accomplished by wet etching with potassium hydroxide. In practice, problems encountered in obtaining uniform etching result from thickness variations in coatings and temperature variations in typical process ovens. The relationship between coating thickness, thermal exposure, degree of cyclization, and etchability was studied. The degree of cyclization was determined by Fourier Transform Infrared Spectroscopy and etch rates were measured by monitoring coating thickness. It was found that the level of imidization was directly influenced by coating thickness as well as thermal exposure. For films exposed to the same thermal conditions, the etch rate decreased as the thickness increased beyond 15 pm. However, when films of various thicknesses were brought to the same level of imidization, a constant etch rate was obtained. A significant improvement in the etch rate can be achieved by reducing the level of imidization, increasing the etchant temperature, or enhancing the solubility of the carboxylate salt of the polyamic acid produced during etching.

A laser-enhanced chemical process has been developed for the removal of material from polyimide to produce vias, grooves, and cavities on the order of tens of pm in size. The process takes advantage of the highly absorbant nature of polyimide to blue light. The focused light of an argon laser is used to heat the polyimide locally in the presence of a caustic bath such as NaOH. The heating speeds the chemical reaction of the polyimide with the NaOH solution to form soluble compounds which are then dissolved. The effects of varying concentration of etchant have been investigated; removal rates have been measured for NaOH and other solutions; and etching of copper/polyimide laminates has been accomplished.

The Tg of polyimides is lowered by a decrease in the rigidity of the chain through the introduction of flexibilizing linkages, or by a reduction of interchain interactions. The interchain interactions in polyimides are charge transfer complexes whose strength is dependent on the electron affinity of the dianhydride and the ionization potential of the diamine. Charge transfer complex formation can be minimized, most effectively, by the introduction of “separator” groups in the dianhydride which reduce its electron affinity, or by the use of meta diamines which distort the polyimide chain and interfere with chain packing.

Softening temperatures T_s and melting temperatures T_m of a great number of aromatic polyimides and their chemical derivatives have been studied. The results obtained are generalized. The analysis is based on the chemical classification of polyimides taking into account the presence and location of “hinge” atoms and atom groups making polymer chain flexible. If there are such “hinges” in both components of a monomer unit then T_s and T are distinctly revealed. For these polyimides the dependence of T_s and T_m on the number of “hinges” in a monomer unit cân be súccessfully explained by the well-known physical theories of transition temperatures of polymers. Other groups of polyimides have no distinct T_s and T_m and are usually considered as “infusible”. Calculations and experiments have shown that it is because in these groups the values of T_s and T_m are close to or higher than the value of the polyimide thermal degradation temperati e. The maximum values of T_s and T_m can be as high as ≃ 1000 and ≃ 1300°C, respectively. The influence of chemical structure on T_s and T_m for the whole class of aromatic polyimides can be understood if strong intermolecular interaction of benzimide rings and internal degrees of freedom in monomer units are taken into consideration. Simple models are used in this work for this purpose. The conclusions made here are thought to be valid both for aromatic polyimides and other related classes of polyheteroarylenes.

Thermal behavior of polymers has long been recognized as a useful guide in designing materials for specialized applications and for predicting their end-use properties. As a general rule, polymer structural units can be purposely varied in synthesis to produce specific compositions with the desired properties. This paper is concerned with the application of thermal methods for unravelling the composition-property relationship for a newer class of polymers called poly-silicone imides (PSI), resulting from modification of aromatic polyimides by specially equilibrated silicone blocks. These polymers are novel from a synthetic point of view and unique in their applicability in a wide range of service temperature, high thermal stability, significant mechanical strength and good adhesive and electrical properties. Although thermoplastic in nature, they are able to cross-link mechanistically through a silicone block and exhibit thermoset properties. Thus, PSI materials have opened up new possibilities in polymer applications not known for typical polyimides. Thermal studies show these properties to be directly related to the compositional variations of both the aromatic and the silicone moieties, which are discussed in some detail here.

An investigation was conducted to determine the effect on selected polyimide properties when pendant alkyl groups were attached to the polymer backbone. A series of polymers were prepared using benzophenone tetracarboxylic acid dianhydride (BTDA) and seven different p-alkyl-m,p′-diaminobenzophenone monomers. The alkyl groups varied in length from C₁ (methyl) to C₉ (nonyl). The polyimide prepared from BTDA and m,p′-diaminobenzophenone was included as a control.

Polyimides possess high thermal stability, good barrier properties and resistance to solvents. Because of these properties these polymers are becoming attractive in electronic devices. This work describes the study of the curing of a commercial polyamic acid after thermal treatment, by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Infra-red spectroscopy to monitor the curing reaction leading to polyimide formation.

Gravimetric sorption and desorption data are reported for water and anhydrous ammonia in Kapton® at 30°C. The data for water at low activity are described well by Fick’s Law with a concentration dependent diffusion coefficient. At high vapor activity, evidence of small extents of chemical reaction of the water with imide groups was discovered. Even after exposure to a relative humidity of 81.4% for two weeks, however, only 0.31% of the imide structures were affected. Anhydrous ammonia, on the other hand, at rather low relative saturations (0.012 to 0.032) interacted strongly with some of the imide structures of Kapton®. Infrared and gravimetric sorption/desorption measurements were used to characterize the locus and extent of reaction. It was found that after exposure of a 0.5 mil sample to an ammonia relative saturation of 0.0316 for 16 days, roughly 17% of the imide structures were disturbed. Analysis of the coupled diffusion and chemical reaction suggests that only a fraction of the total number of imide groups (~20%) appear to become “activated” or “labilized” during the solid state curing step in which stresses may arise. Stress relief by the limited ammonolysis reaction presumably deactivates the remaining imide groups. The possiblity of using Kapton® as a combined protective barrier and scavenger coating or film is explored briefly.

X-ray scattering measurements were made on Kapton®1 films with thicknesses of 8, 25 and 50µm. These measurements indicate that the atomic scale structure is basically the same for the three Kapton films. However, the measurements reveal in-plane anisotropy of polymer chain alignment with preferred orientation parallel to an optical extinction direction. Anisotropy of x-ray scattering and therefore of polymer chain alignment within the polyimide film plane is greatest for the thinnest film, 8µm, and least for the thickest film, 50µm. The degree of anisotropy in tensile strength shows a similar dependence on film thickness. These appear to be the first x-ray scattering measurements demonstrating in-plane anisotropy for Kapton films. Furthermore, our measurements agree with previous measurements which have shown these polymer chains align in-plane rather than out-of-plane.

Thermally stimulated discharge current (TSDC) technique has been applied to investigate the polarization mechanism due to corona discharge in as-received and heat-treated Kapton® samples. The samples were charged on the upper free surface (non-metallized surface) by exposure to a negative corona discharge in air. Three peaks (termed as P₁, P₂ and P₃) were ob-served in the TSDC spectra. Tfie peaks P₁ and P₂ appear at 46° and 60°C, respectively and are in the normal current direction opposite to that of charging current. The heat treatment causes a decrease in the peak current which may be due to a reduction in the number of traps due to high temperature aging. The peak P₃ appears at 249°C in as-received Kapton samples and is in the normal current direction. For heat-treated samples this peak appears around 225°C and is in the abnormal current direction. For iodine doped samples the peaks P₁ and P₂ appear at 38° and 50°C, respectively and are in the normal current direction. The peak current is nearly 10 times that in as-received samples which may be due to an increase in the carrier mobility because of“handing-on”of carriers as a result of diffusion of 12 ions in the polymer matrix. The peaks P₁? P₂ have been attributed to shallow trap levels and peak P 3 to deep trap levels present in Kapton.

The photoelectret state in Kapton® polypyro-mellitimide has been investigated as a function of polarizing temperature (30° to 250°C), field 85–214 kV/cm), time (5–25 minutes) and illumination intensity (5000 lux). The optimum values of polarizing temperature and field were obtained for photo-electret charge. With the help of these optimized parameters, the effect of intensity of illumination on photo-polarization was studied using visible light and ultraviolet radiations. Experiments were conducted with heat-treated as well as as-received Kapton samples. An increase in photo-electret charge with polarizing field and temperature followed by a saturation has been observed which is a characteristic property of photoelectrets. Increase in polarizing temperature results in an increase in photo-polarization but at high temperatures(above 200°C) the photoelectret charge starts decreasing; this may be due to an increase in thermal recombination of the charges. The photoelectret charge is observed to increase with the intensity of illumination followed by a saturation which is in accordance with the reciprocity law. Ultraviolet radiation of 254 nm wavelength was found to be more efficient for photo-polarization than visible light. Heat treatment causes a reduction in photoelectret charge which may be due to the decrease in the number of traps due to high temperature aging. The nature of dark depolarization characteristics shows the presence of shallow and deep trap levels in Kapton. Tie total volume trap density was estimated to be 3 x 10 traps/cm. The autophotoelectret state, i.e., obtaining of polarization by illumination alone (without applying any electric field) has also been investigated in Kapton. The photoelectromotive force is seen to increase with temperature as well as with illumination intensity. The conducting glass-Kapton-Aluminum combination gives a photoelectromotive force of 1.15 volts at a temperature of 200°C.

A series of polyimide films containing pyridine coordinating groups doped with cobalt or nickel ions has been examined. Using 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride, films were prepared in combination with 4,4′-oxydianiline, 4,4′-dianiline sulfide, 3,3′-bis(aminopyridine)sulfide and 2,2′dioxyphenelenebis(5,5′-aminopyridine). Room temperature volume resistivities have been found to decrease slightly with sulfur and pyridine containing diamines relative to diamines which do not contain these functionalities. Doping with cobalt and nickel additives results in further lowering of room temperature volume resistivity. Preliminary temperature profiling experiments have shown that volume resistivity decreases as T_g is approached.

Experiments have been conducted to study the effect of electrode materials (Cu, Al, Pb & Ag) on various relaxation processes in Kapton® film using thermally stimulated discharge current technique. Thermoelectrets were prepared at 80°C(E_p =133.3 kV/cm) and 200°C(E_p =133.3 kV/cm and 280.6 kV/cm). Samples were depolarized from room temperature to 300°C at a linear heating rate of 2°C/min. TSDC spectra consist of three peaks, termed as β, α and ρ. The peak which appears around 108°C is seen to be unaffected by the nature of electrode material and therefore has been attributed to the dipole orientation polarization, associated with the residual reactive groups in polypyromellitimide. The α and ρ peaks appearing around 200°C and 250°C have been attributed to the transference of intrinsic space charge and to the surface charge injected from the electrodes, respectively. For both α and ρ peaks, the peak temperature and peak current is seen to be dependent on the work function of the electrode material. The values of activation energy and relaxation times were also calculated for these peaks. Further, analysis of these relaxation processes has been done by means of M₁-I-M₂ structure.

PMDA-ODA polyimide gives rise to three major carbon is ESCA peaks: 285.0eV peak assigned to aromatic carbons not joined to nitrogen or oxygen, 286.1eV peak assigned to aromatic carbons joined to nitrogen or oxygen, and 288.8eV peak assigned to carbonyl carbons. The observed intensity of the 286.1eV peak is too high (about 2.5 fold) to be accounted for by the assignment given above. An alternative assignment is that the aromatic carbons in the PMDA ring are all rendered sufficiently electron deficient by the attached carbonyls that their carbon 1s peaks are shifted to the higher binding energy (286.1eV). This assignment has been confirmed by model compound studies and very good agreement with observed intensities was obtained.

The polymer-polymer compatibility of polyamic acids having different chemical compositions has been determined by ultrasonic and viscometric techniques. The variation of ultrasonic velocity and viscosity with compositions is indicative of limited compatibility and strong polymer-polymer interactions in the blends of structurally different polyamic acids. The compatibility data are expected to facilitate the development of the polyimide blends possessing tailor-made properties.

Photoacoustic, dielectric and surface-morphological properties of Kapton* were studied after exposing sheets (25 μm thick) separately to fast reactor neutrons (E>0.1 MeV) and to 250 keV protons up to doses 1.2×1018 (maximum)and 7.0×1015 cm−2, respectively. The photoacoustic spectrum (PAS) of the maximum neutron dose irradiated sample shows the red-shift of the absorption peak from 450 nm to 470 nm, whereas in the proton irradiated (7×1015 cm−2) sample, a new flattened absorption peak appears at λ ≃ 515 nm. There is a net enhancement in the D.C. dielectric constant of about 13% after the maximum neutron dose. It is attributed to the enhanced water absorptivity of the material due to neutron induced radiation damage.

By means of a two-component dilution technique we have evaluated a series of primary and tertiary aminosilanes as adhesion promoters for polyimides and native SiO₂ in order to study the effect of chain-scission during covalent bond formation between the adhesion promoters and the polyimide. A rapid leveling-off of the increase in adhesion with an increase in the concentration of the reactive component may indicate that the scission of the polymer chains at the interface is detrimental to adhesion. An attempt to use a secondary aminosilane to obtain bonding without scission was unsuccessful. The reaction was not quantitative, as judged by model compounds, and the diamide product decomposed at elevated temperatures yielding a thermally stable imide via chain scission.

Upon the addition of amines or ammonia, to solutions of different polyamic acids (PAA) it was noticed that sometimes precipitates were formed. This is particularly the case with the PAA based on the condensation of pyromellitic acid dianhydride with 4,4 -diaminodiphenylether, (PAA(I)), and with primary amines or ammonia. It is assumed that the addition of amines to polyamic acids causes the polymer to complex with itself; normally polymer complexation involves two different polymers.