505,013 research outputs found
Polyimidazoles via aromatic nucleophilic displacement
Polyimidazoles (PI) are prepared by the aromatic nucleophilic displacement reaction of di(hydroxyphenyl) imidazole monomers with activated aromatic dihalides or activated aromatic dinitro compounds. The reactions are carried out in polar aprotic solvents such as N,N-dimethyl acetamide, sulfolane, N-methylpyrrolidinone, dimethylsulfoxide, or diphenylsulfone using alkali metal bases such as potassium carbonate at elevated temperatures under nitrogen. The di(hydroxyphenyl) imidazole monomers are prepared by reacting an aromatic aldehyde with a dimethoxybenzil or by reacting an aromatic dialdehyde with a methoxybenzil in the presence of ammonium acetate. The di(methoxyphenyl) imidazole is subsequently treated with aqueous hydrobromic acid to give the di(hydroxphenyl) imidazole monomer. This synthetic route has provided high molecular weight PI of new chemical structure, is economically and synthetically more favorable than other routes, and allows for facile chemical structure variation due to the availability of a large variety of activated aromatic dihalides and dinitro compounds
Process for lowering the dielectric constant of polyimides using diamic acid additives
Linear aromatic polyimides with low dielectric constants are produced by adding a diamic acid additive to the polyamic acid resin formed by the condensation of an aromatic dianhydride with an aromatic diamine. The resulting modified polyimide is a better electrical insulator than state-of-the-art commercially available polyimides
Methyl substituted polyimides containing carbonyl and ether connecting groups
Polyimides were prepared from the reaction of aromatic dianhydrides with novel aromatic diamines having carbonyl and ether groups connecting aromatic rings containing pendant methyl groups. The methyl substituent polyimides exhibit good solubility and form tough, strong films. Upon exposure to ultraviolet irradiation and/or heat, the methyl substituted polyimides crosslink to become insoluble
On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
Although it is generally accepted that the so-called "unidentified" infrared
emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are
characteristic of the stretching and bending vibrations of aromatic hydrocarbon
materials, the exact nature of their carriers remains unknown: whether they are
free-flying, predominantly aromatic gas-phase molecules, or amorphous solids
with a mixed aromatic/aliphatic composition are being debated. Recently, the
3.3 and 3.4 micrometer features which are commonly respectively attributed to
aromatic and aliphatic C-H stretches have been used to place an upper limit of
~2\% on the aliphatic fraction of the UIE carriers (i.e. the number of C atoms
in aliphatic chains to that in aromatic rings). Here we further explore the
aliphatic versus aromatic content of the UIE carriers by examining the ratio of
the observed intensity of the 6.2 micrometer aromatic C-C feature (I6.2) to
that of the 6.85 micrometer aliphatic C-H deformation feature (I6.85). To
derive the intrinsic oscillator strengths of the 6.2 micrometer stretch (A6.2)
and the 6.85 micrometer deformation (A6.85), we employ density functional
theory to compute the vibrational spectra of seven methylated polycyclic
aromatic hydrocarbon molecules and their cations. By comparing I6.85/I6.2 with
A6.85/A6.2, we derive the fraction of C atoms in methyl(ene) aliphatic form to
be at most ~10\%, confirming the earlier finding that the UIE emitters are
predominantly aromatic. We have also computed the intrinsic strength of the
7.25 micrometer feature (A7.25), another aliphatic C-H deformation band. We
find that A6.85 appreciably exceeds A7.25. This explains why the 6.85
micrometer feature is more frequently detected in space than the 7.25
micrometer feature.Comment: 18 pages, 10 figures, 3 tables; accepted for publication in MNRA
On the Origin of the 3.3 Micron Unidentified Infrared Emission Feature
The 3.3 m unidentified infrared emission feature is commonly attributed
to C-H stretching band of aromatic molecules. Astronomical observations have
shown that this feature is composed of two separate bands at 3.28 and 3.30
m and the origin of these two bands is unclear. In this paper, we perform
vibrational analyses based on quantum mechanical calculations of 153 organic
molecules, including both pure aromatic molecules and molecules with mixed
aromatic/olefinic/aliphatic hydridizations. We find that many of the C-H
stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules
are coupled. Even considering the un-coupled modes only, the correlation
between the band intensity ratios and the structure of the PAH molecule is not
observed and the 3.28 and 3.30 m features cannot be directly interpreted
in the PAH model. Based on these results, the possible aromatic, olefinic and
aliphatic origins of the 3.3 m feature are discussed. We suggest that the
3.28 m feature is assigned to aromatic C-H stretch whereas the 3.30 m
feature is olefinic. From the ratio of these two features, the relative
olefinic to aromatic content of the carrier can be determined.Comment: 33 pages, 14 figures. Accepted for publication in Ap
Poly(1,3,4-oxadiazoles) via aromatic nucleophilic displacement
Poly(1,3,4-oxadiazoles) (POX) are prepared by the aromatic nucleophilic displacement reaction of di(hydroxyphenyl) 1,3,4-oxadiazole monomers with activated aromatic dihalides or activated aromatic dinitro compounds. The polymerizations are carried out in polar aprotic solvents such as sulfolane or diphenylsulfone using alkali metal bases such as potassium carbonate at elevated temperatures under nitrogen. The di(hydroxyphenyl) 1,3,4-oxadiazole monomers are synthesized by reacting 4-hydroxybenzoic hydrazide with phenyl 4-hydrobenzoate in the melt and also by reacting aromatic dihydrazides with two moles of phenyl 4-hydroxybenzoate in the melt. This synthetic route has provided high molecular weight POX of new chemical structure, is economically and synthetically more favorable than other routes, and allows for facile chemical structure variation due to the large variety of activated aromatic dihalides which are available
Aromatic cyclotriphosphazenes
Four-Aminophenoxy cyclotriphosphazenes are reacted with maleic anhydride to produce maleamic acids which are converted to the maleimides. The maleimides are polymerized. By selection of starting materials (e.g., hexakis amino or trisaminophenoxy trisphenoxy cyclotrisphosphazenes), selection of molar porportions of reactants, use of mixtures of anhydrides and use of dianhydrides as bridging groups a variety of maleimides and polymers are produced. The polymers have high limiting oxygen indices, high char yields and other useful heat and fire resistant properties making them useful as, for example, impregnants of fabrics
Polyimide adhesives
A process was developed for preparing aromatic polyamide acids for use as adhesives by reacting an aromatic dianhydride to an approximately equimolar amount of an aromatic diamine in a water or lower alkanol miscible ether solvent. The polyamide acids are converted to polyimides by heating to the temperature range of 200 - 300 C. The polyimides are thermally stable and insoluble in ethers and other organic solvents
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