Co-crystallization of solvents with polymers: The x-ray diffraction behavior of solvent-containing and solvent-free polyacrylonitrile

Differences in the x-ray diffraction pattern of polyacrylonitrile (PAN) have been observed, both here, as well as in the work reported in the literature. Various workers have cited grossly differing unit cells for PAN and this has never been explained. In this work, it is shown that in dry samples, the diffraction pattern contains only two equatorial peaks, d₁ ≈ 5.3 Å and d₂ ≈ 3.0 Å. This is consistent with a hexagonal packing of rod-like chains of PAN with no chain-axis order. However, in solvent-plasticized films, the diffraction pattern is altered. It is proposed that certain solvents cause a hexagonal to orthohombic transition. However, the orthorhombic polymorph appears to be a solvated form, where the solvent has co-crystallized with the polymer. After removal of solvent, the structure reverts to hexagonal packing. Hence, it is suggested that the discrepancy in cell parameters cited in the literature, particularly when large cell dimensions are quoted, may be due to the inadvertent presence of solvent which could have been introduced during sample preparation. Similarities with solvated crystls in other polymeric systems are also briefly discussed. © 1994 John Wiley & Sons, Inc.     

NMR and x-ray studies on the morphology of thermoreversible polyacrylonitrile gels

The morphology of thermoreversible polyacrylonitrile–propylene carbonate (PAN-PC) gels was examined using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, “cross-linked” by regions of rigid polymer. The mobile regions of the gels showed solution-type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crysttallites detected previously by x-ray diffraction. Wide-angle x-ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites. © 1993 John Wiley & Sons, Inc.     

Titrimetric determination of chloramine-T and some aldoses by amplification reactions

A titrimetric method with amplification has been worked out for the determination of chloramine-T and certain aldoses. It is based on Leipert determination of iodide produced in the case of chloramine-T by reduction of the determinand with excess of iodide, and extraction and subsequent reduction of the iodine liberated. In the other case, the aldoses are oxidized with iodine, the surplus oxidant is extracted, and the residual iodide is determined. The method is applicable to chloramine-T in the range 0.01-3 mg, with a relative error between -3.0 and -0.4% and a relative standard deviation of 0.6-0.8%, depending on the amount present, and to 0.05-1 mg of glucose, galactose or arabinose, or 0.1-2 mg of lactose or maltose, with a relative standard deviation of 1.4% for > 0.5 mg of aldose.     

Photometric determination of nitrite

A new sensitive colour reaction for nitrite determination is presented. The method is based on the reaction of nitrite with p-aminobenzoic acid to form a diazonium ion, which is coupled with 8-hydroxyquinoline, in alkaline medium, to form a reddish-orange water-soluble azo dye. Diazotization and coupling are very fast and control of temperature is unnecessary. Moreover, the colour is stable and its intensity is in direct proportion to nitrite concentration over a wide range.     

Spectrophotometric determination of aniline in aqueous solution by azo-dye formation with diazotized p-nitroaniline

A simple, sensitive, and rapid Spectrophotometric method for the trace determination of aniline in aqueous solution is described. The method is based on coupling aniline with diazotizedp-nitroaniline in the presence of ethanol and sodium hydroxide to form an intense red water-soluble azo-dye which shows maximum absorption at 505 nm and greater stability and sensitivity in the presence of cetyltrimethylammonium bromide. Beer's law is obeyed over the aniline concentration range 0.2–2.4g/ml in the solution measured; the molar absorptivity is 3.46×10⁴l· mole^–1·cm^–1, and the relative standard deviation 1–2%, depending on the aniline concentration.     

Microdetermination of acetone in aqueous solution

Diazotized anthranilic acid has been found to be a possible reagent for the spectrophotometric microdetermination of acetone in aqueous solution. The determination range is 10–130 μg/ml. The coefficient of variation does not usually exceed 12% for 60–130 μg of acetone but increases to 2.8% at the 10-μg level. The average relative error for five determinations ranges from ?1.5 to 2%. The molar absorptivity is 9.28 × 10³ liter · mol^?1 · cm^?1 in the presence of starch as a surfactant. Possible reaction path mechanism has been suggested for the colored body formation.     

Indirect spectrophotometric determination of cerium(IV)

A simple and very fast photometric method for the determination of trace levels of cerium(IV) has been developed. Use has been made of the reaction in which iron (II), on oxidation with cerium(IV), gives iron(III) which is complexed with tiron, in acidic medium, to form a stable blue color, the intensity of which is in direct proportion to the amount of cerium(IV) originally present in the sample solution. In addition, the method has a wide range of determination and reasonable sensitivity, and it avoids both temperature control and the tedious extraction step.     

A cluster of many small holes with negative imaginary surface impedances may generate a negative refraction index

We deal with the scattering of an acoustic medium modeled by an index of refraction n varying in a bounded region Ω of and equal to unity outside Ω. This region is perforated with an extremely large number of small holes D_m's of maximum radius a, a << 1, modeled by surface impedance functions. Precisely, we are in the regime described by the number of holes of the order M:=O(a^{β ? 2}), the minimum distance between the holes is d ～ a^t, and the surface impedance functions of the form λ_m～λ_m,0a^?β with β > 0 and λ_m,0 being constants and eventually complex numbers. Under some natural conditions on the parameters β,t, and λ_m,0, we characterize the equivalent medium generating approximately the same scattered waves as the original perforated acoustic medium. We give an explicit error estimate between the scattered waves generated by the perforated medium and the equivalent one, respectively, as a→0. As applications of these results, we discuss the following findings:

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