Pyromellitic Dianhydride-4,4′-oxydianiline Research Articles

Radiation Effect of Vacuum Ultraviolet on the Performance of Polyimide Films

Polyimide (PI) films equipped on the surface of spacecraft are susceptible to ultraviolet damage. Herein, the performance of two kinds of polyimide films i.e. 3,3′,4,4′-biphenyl dianhydride-p-phenylenediamine (BPDA-PDA) and pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) under vacuum ultraviolet (VUV, 172 nm) irradiation was studied. The failure behavior of the PI films was studied by tensile tests, thermogravimetry (TGA), thermomechanical analysis (TMA), electric breakdown, UV–visible transmittance and scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the molecular structure. The results indicated that the tensile strength of BPDA-PDA PI and PMDA-ODA PI films decreased by 10.4 and 7.4% under VUV irradiation, respectively; the breakdown strength and UV-transmittance also deteriorated. Besides, the surface morphology of the PI films showed a large number of microcracks after VUV irradiation, which we ascribe to the breakage of imide rings (C-N-C) and ether bonds (C-O-C) of the PI films. However, the thermal stability (residual weight) of both PI films remained constant compared to the initial level; this can be explained by the combined effect of bond breaking and cross-linking of the polyimide films.

Molecular dynamics study on the mechanism of poly(amic acid) chemical structure on thermal imidization process and polyimide architecture

In this work, the thermal imidization of 3,3′, 4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PDA), pyromellitic dianhydride/4,4′-oxydianiline (PMDA/ODA), 4,4′-oxydiphthalic anhydride/4,4′-oxydianiline (ODPA/ODA), and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride/2,2′-bis(trifluoromethyl)benzidine (6FDA/TFMB) polyimides (PIs) were investigated based on molecular dynamics (MD) simulation. It was found that two kinds of imide bonds were formed, i.e., inter-molecular and intra-molecular imide bonds, which results in differences in the final architecture of the target PI systems. The BPDA/PDA and PMDA/ODA PIs present crosslinking network structure, while the ODPA/ODA PI shows a hybrid structure composed of crosslinking and linear fragments, and the 6FDA/ODA PI displays linear and branched structures. Compared with the previous view of linear PI molecular chains, these results agree well and better account for the thermoset of PMDA/ODA and BPDA/PDA, thermoplasticity of ODPA/ODA, and solubility of 6FDA/TFMB. The mechanism causing the above differences was studied from radial distribution function (RDF), coordination number, fractional free volume (FFV), and density field analysis. By comparing the experimental and simulated values of glass transition temperature and modulus of PI models, the rationality of inter-molecular imide bonds is proved. In addition, through the PAA/DMF compound models, it was found that the increase of solvent content hindered the formation of inter-molecular imide bonds. This study provides new insights into the thermal imidization process and the final architecture of PIs derived from poly(amic acid).

Supramolecular Salts for Additive Manufacturing of Polyimides.

Recent advances in vat photopolymerization (VP) additive manufacturing of fully aromatic polyimides employed photoreactive high-molecular-weight precursors dissolved at modest loadings (<20 wt %) in organic solvent. These earlier efforts revealed high isotropic shrinkage, approaching 52% on a linear basis while converting to the desired polyimide. To increase the polyimide precursor concentration and decrease shrinkage during VP processing of high-performance polyimides, photoreactive fully aromatic polyimide and thermoplastic polyetherimide (PEI) supramolecular salt precursors now serve as versatile alternatives. Both pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) and 4,4'-(4,4'-isopropylidene-diphenoxy)diphthalic anhydride-meta phenylene diamine (BPADA-mPD) supramolecular dicarboxylate-diammonium salts, termed polysalts, provided prerequisite rheological performance and photoreactivity for VP. Solutions (50 wt %) of both photoactive polysalts exhibited viscosities more than two orders of magnitude lower than previously reported polyimide precursor solutions for VP. In addition, VP of 50 wt % polysalt solutions yielded high resolution, self-supporting organogel structures. During thermal postprocessing to the desired fully aromatic polyimide and PEI, photocrosslinked polysalt organogels exhibited retention of part shape in concert with linear isotropic shrinkage of only 26%, the lowest reported value using organogel strategies for VP of fully aromatic polyimides. Furthermore, the imidized structures exhibited comparable thermal and mechanical properties to analogous polyimides synthesized using classical methodologies for 2D films. The combination of facile synthesis and increased precursor concentrations designates polysalt polyimide precursors as a versatile platform for additive manufacturing of well-defined 3D polyimide structures.

Effect of photoreactivity of polyimide on the molecular orientation of liquid crystals on photoreactive polymer/polyimide blends

We prepared blend alignment layers from polymethacrylate with coumarin side chains (PMA-g-coumarin) and polyimides for the orientation of liquid crystals (LCs) using linearly polarized ultraviolet (UV) irradiation. We used two different polyimides, namely 4,4′-(hexafluoro-isopropylidene) diphthalic anhydride-3,5-diamino-benzoic acid (6FDA-DBA) and pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA). It was found that the molecular orientation of the LC depended on the type of polyimide in the blend alignment layer. The thermal stability of the LC orientation was enhanced regardless of the type of polyimide, while the direction of LC orientation was different for each type of polyimide. The photoreactivity of the polyimide was a very important factor in determining the molecular orientation of the LC on the blend alignment layer. This may be attributed to the different mechanisms of LC orientation on PMA-g-coumarin and polyimide induced by the polarized UV irradiation. The direction of the LC orientation could be changed by controlling the photoreaction of the polyimides using the appropriate UV filter for the polarized UV irradiation.

Forward recoil spectrometry study of the diffusion of PMDA/ODA-based poly(amic ethyl esters)

The poly(amic ethyl esters) (PAEs) of pyromellitic dianhydride/4,4′-oxydianiline (PMDA/ODA) were prepared in three isomeric forms, with meta-linkages, para-linkages and randomly alternating meta- and para-linkages (referred to as mixed PAEs). In addition to the normal PAEs (which contained only hydrogen and which were cast as base layers for diffusion studies), each of these isomeric PAE was synthesized in two other forms: (1) labelled with deuterium on the ODA segments, and (2) labelled with deuterium on the ethyl ester moiety to allow determination of the imide fraction, f. Forward recoil spectrometry (FRES) was utilized to provide information about the polyimide—polyimide interface by measuring the depth profile of hydrogen and deuterium for bilayer samples consisting of a thick non-deuterated base layer, and a thin deuterated tracer layer. FRES measurements indicated that the interdiffusion distance, w, decreased markedly with increasing imidization temperature, T i. For base layers prepared from N-methylpyrrolidone, w for the para- and mixed PAE base layers decreased more rapidly with T i than with w for the meta-PAE base layers. However, these differences were found to result from the differences in imidization kinetics of the various isomers with f increasing more rapidly with T i, for para- and mixed PAE than for the meta-PAE. Nevertheless, when w was plotted as a function of f, all of the isomers exhibited similar behaviour, i.e. w decreased steadily as the imide fraction f approached 1. Base layers cast from dimethyl sulfoxide did not show differences in either f or w after imidization at a given T i.

Ramp rate effect on polyimide film properties and morphology

AbstractThe stress in films of semirigid and rigid polyimides (PIs) on silicon (Si) substrate has been measured in situ during curing using a newly modified bending beam apparatus. By using the apparatus, the onset of residual stress in the initially solvent‐rich films has also been investigated. The stress characteristics of the PI films are strongly ramp rate dependent. Different ramp rates result in markedly different stress patterns, thermo‐mechanical properties, and film morphologies. The residual stress at room temperature after curing is scattered around 23–31 MPa for the films of pyromellitic dianhydride‐4,4′‐oxydianiline (PMDA‐ODA). For pyromellitic dianhydride‐p‐phenylenediamine (PMDA‐PDA), it systematically increases from −6 to 28 MPa for ramp rate increasing from 0.5 to 10°C/min. The residual stress is very low in the slowly cured PMDA‐PDA films. However, these films have gone through a very high‐stress transient state during curing. The maximum transient stress is comparatively higher in the films of PMDA‐PDA than in PMDA‐ODA. Slowly cured PMDA‐PDA films exhibit high structural ordering, high in‐plane anisotropy, and low coefficient of thermal expansion. ©1995 John Wiley &amp; Sons, Inc.

Temperature effect on solvent diffusion in rigid and semirigid polyimide films

AbstractThe diffusion of NMP (n‐methyl‐pyrrolidinone) solvent in a semirigid rod‐like PMDA‐ODA (pyromellitic dianhydride‐4,4′‐oxydianiline) film coated on silicon is found to be case I diffusion at temperatures ranging from 30 to 90°C by the use of a bending beam technique. The diffusion constant increases for the 7.4 μm PMDA‐ODA film (which was cured at 300°C) from 3.3 to 318 × 10−11 cm2/s as the diffusion temperature increases from 30 to 90°C. The corresponding hygroscopic stress in the direction parallel to the film decreases with the increase of temperature, possibly due to the softening of the film at elevated temperatures. The diffusion mechanism, however, changes from case II to case I in a rigid rod‐like PMDA‐PDA (pyromellitic dianhydride‐p‐phenylenediamine) film when the diffusion temperature increases. The change in the diffusion mechanism occurs at a higher temperature for thinner films, presumably due to higher ordering and/or orientation in the films. The activation energy for NMP diffusion in the PMDA‐ODA films markedly decreases from 93 to 59 kJ/mole as the film thickness increases from 2.2 μm to 11.3 μm. This may also be attributed to decreased ordering in thicker films. © 1994 John Wiley &amp; Sons, Inc.

Valence-band structure of PMDA-ODA polyimide studied by x-ray photoelectron spectroscopy and theoretical calculations*

The valence-band structure of pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) film was investigated experimentally and theoretically. High-resolution valence band spectra were obtained with an x-ray photoelectron spectroscopy (XPS) instrument equipped with a monochromatic high-power Al Kα x-ray source. Several new valence-band structures were observed. Electron energy loss spectra showed the presence of an inelastic peak at about 6.5 eV loss energy. A photoelectron background was synthesized from the electron energy loss spectrum by a convolution procedure. This background gave a good understanding of the valence band spectrum. Valence effective Hamiltonian and extended Hückel calculations were performed to study the XPS intensities of the valence-band structures. The intensities were approximated by utilizing the XPS intensity model with net atomic populations. The total intensities were partitioned into intensity contributions from various functional groups of the polyimide. Three energetically separated regions with dominating contributions from the imide nitrogens, the ODA carbons, and the imide oxygens, respectively, were found.

Effects of the imidization temperature of poly(imide-siloxane) on the adhesion strength of epoxy resin/poly(imide-siloxane) joints

The effects of the imidization temperature of poly(imide-siloxane) on the adhesion strength of epoxy resin/poly(imide-siloxane) joints were studied. The effects of the content and molecular weight of polydimethylsiloxane in poly(imide-siloxane) on the adhesion strength were also examined with samples of pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) containing various amounts and molecular weights of aminopropyl-terminated polydimethylsiloxane (SDA). The adhesion strength increased with increasing imidization temperature in the air environment and was found to be sensitive to the content and molecular weight of SDA. The adhesion mechanism of epoxy resin/poly(imide-siloxane) joints was investigated using various analytical techniques. The failure mode of the joint was found to be cohesive in the poly(imide-siloxane) layer very close to the interface.

Thermoplastic character of polyimide blends

AbstractPolyimide blends consisting of pyromellitic dianhydride/4,4′‐oxydianiline (PMDA/ODA) and biphenyl‐tetracarboxylic dianhydride/p‐phenylene diamine (BPDA/PDA) show a distinct glass transition behavior at temperatures lower than each component does. Disruption of molecular packing by blending of polymers having dissimilar interaction sites leads to a significant increase in molecular mobility at much lower temperatures. This is examined by laminating two pieces of film cast from the blend and measuring the adhesive strength at the interface. A strong adhesion, 11.5 N/cm (6.6 lbf/in) by 180° peel test, was achieved indicating interpenetration of polyimide molecules. It was also found that the polyimide blends can be converted into highly ordered states by mechanical deformation of the blends above their glass transition temperatures (Tgs).

Effects of imide structure/thickness/curing on solvent diffusions in polyimides

It is found that the diffusion of NMP solvent in PMDA-ODA (pyromellitic dianhydride-4,4'-oxydianiline) imide films belongs to case I diffusion, while that in PMDA-PDA (pyromellitic dianhydride-p-phenylenediamine) belongs to case II. The diffusion mechanisms do not vary with thickness ranging from 0.5 to 1.0 μm or curing temperature from 200 to 400°C, while the diffusion processes apparently become faster with increasing the thickness and decreasing the curing temperature. The latter phenomena can be correlated to the film ordering or/and orientation that decrease with increasing the film thickness and decreasing the curing temperature

Coating thickness effect on the orientation and thermal expansion coefficient of polyimide films

From X-ray diffraction experiments, it is observed that on-substrate cured polyimide films are so oriented that the imide chains are preferentially aligned in the plane direction of the films. For films with rigid-rod polyimide, film orientation is especially prominent and decreases significantly as the film thickness increases. The effect of coating thickness on the orientation and ordering of polyimide films is most pronounced in pure pyromellitic dianhydride- p-phenylenediamine (PMDA-PDA) films, slightly less in films with 50% pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) and 50% PMDA-PDA, and relatively insignificant in those with 100% PMDA-ODA. According to the C=O and C-N stretching absorption bands from Fourier-transform infra-red-attenuated total reflectance spectroscopic experiments, imide molecules located near the centre of the film exhibit poorer structure ordering than those near the surface. This reveals why the average film orientation decreases and the corresponding thermal expansion coefficient increases with increase of film thickness.

Bending Beam Measurement of Moisture Diffusion in Polyimide Films on Silicon Substrate

A theoretical model, correlating the bending curvature variation ratio of a layered structure caused by moisture-induced-swelling in its polymer overcoat with time under Case I diffusion, has been developed. This model has considered the pre-existing intrinsic strain in the polymer film upon deposition. According to this model, any specimen to be studied does not need to be bending or strain free. Furthermore, an useful and relatively simplified formula has been derived therefrom and used in identifying the diffusions of moisture in the films of two rod-like and one semi-flexible polyimides, their blends, and a random copolyimide. In PMDA-ODA (pyromellitic dianhydride-4,4′-oxydianiline), the diffusion constant, D, is 4.64×10−9 cm2 s−1. In PMDA-PDA (p-phenylenediamine), D=1.25×10−9, and in PMDA-B (-benzidine) 0.10×10−9 cm2s−1. The diffusion fastest in PMDA-ODA can be attributed to its amorphous structure, and slowest in PMDA-B to its small interchain spacing. The diffusion in a random copolymer with 50 wt% PMDA-PDA and 50 wt% PMDA-B is faster than in the blend with 60 wt% PMDA-PDA and 40 wt% PMDA-B. This can be attributed that the random copolyimide film is more amorphous according to an X-ray diffractometry study.

Studies of adhesion of metal films to polyimide

Bonding of Cu/Cr films to several polyimides has been studied as a function of polymer surface modification by ion beam and chemical pretreatment. The effects of the metal deposition method and parameters have also been examined. The materials of interest include a low thermal expansion polyimide derived from 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride-p-phenylene diamine (BPDA-PDA) polyamic acid, and pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) polyimide, formed from polyamic acid or polyamide ethyl ester precursors. The metal/polyimide adhesive strength was determined by the 90° peel test, while the interfacial regions were examined using x-ray photoelectron spectroscopy and ellipsometry. It is found that for PMDA-ODA systems, exposure to low energy Ar+ and/or O+2 ions improves adhesion of the metal overlayer, while for BPDA-PDA polyimide, the role of O+2 is more significant. The fracture location is found to lie 20–300 Å within the polymer, depending upon the ion beam dose and the specific polyimide employed.

Metal–polymer chemistry. II. Chromium–polyimide interface reactions and related organometallic chemistry

We have studied the reactions occurring for the deposition of chromium onto pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) polyimide for a wide range of metal coverages, using x-ray photoelectron spectroscopy as well as solution studies of model compounds. A multistep, coverage-dependent mechanism is proposed which includes an initial electron transfer from Cr to PMDA-ODA, followed by formation of Cr–O and Cr–N polymer-bound intermediates. At higher metal coverage, Cr–oxides and Cr–nitrides are formed which may activate the polymer for subsequent formation of Cr–carbide. In the course of this work we have shown that similarities exist between the chemistry of metal atoms in condensed phases and the reactions occurring between metal atoms and surfaces.

Chromium-Polyimide Interface Chemistry

ABSTRACTWe have studied the reactions occurring for the deposition of chromium onto pyromellitic dianhydride -4,4′-oxydianiline (PMDA-ODA) polyimide for a wide range of metal coverages, using X-ray photoelectron spectroscopy as well as solution studies involving model compounds. A multi-step, coverage-dependent, mechanism is proposed which includes an initial electron transfer from Cr to PMDA-ODA. followed by formation of Cr-O and and Cr-N polymer-bound intermediates, At higher metal coverage, Cr-oxides and Cr-nitrides are formed which may activate the polymer for subsequent formation of Cr-carbide. No evidence was obtained in support of a model involving initial formation of a π-arene complex between chromium atoms and PMDA-ODA. In the course of this work we have shown that similarities exist between the chemistry of metal atoms in condensed phases and the reactions occurring between metal atoms and surfaces.