Kinetic compensation effect in the thermal degradation of polymers

The kinetic study of thermal degradation takes into account the validity of the Arrhenius equation. From TG data, the activation energy,E _a and pre-exponential factor,A, are evaluated. These results are interpreted by using the kinetic compensation effect as basis. A linear correlation between In(A) andE _a is obtained in all cases studied. However, in a plot of the logarithm of the rate constant as a function of reciprocal temperature for the same series of reactions, the thermal oxidative degradations of Nylon-6 and PVC display a point of concurrence and one isokinetic temperature, whereas those of HIPS and PC do not. Therefore, in the thermal oxidative degradations of Nylon-6 and PVC a true compensation effect occurs, which could be related to the bulk properties of metal oxides, such as different valence states, whereas for other polymers it displays only an apparent compensation effect. This means that degradation is largely independent of the bulk properties of oxides, but may be related to the distribution of different kinds of active links in the polymer surface having different activation energies.     

Liquid phase benzylation of toluene using benzyl alcohol over iron promoted sulfated zirconia

Concentrated (0.2 M) aqueous solutions of HP-acids, such as H_3+x+mPV^IV _mV^V _x-mMo_12-xO₄₀ and their analogues with an excess VO²⁺ cation, are oxidized by dioxygen at 343 K and atmospheric pressure through intermediate active complexes (IAC) [H_x+m-1PV^IV _mV^V _x-mMo_12-xO₄₀ ^{4 -}] · [VO²⁺]_y · O₂, where m + y ≥ 3. The electron transfer to the coordinated O₂ molecule inside AC is the limiting stage at high m. At low m, the formation of IAC becomes the limiting stage that results in a sharp decrease in the oxidation rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Trimethyl phosphate-acetylene interaction: a matrix-isolation infrared and ab initio study

Trimethyl phosphate (TMP) and acetylene were codeposited in nitrogen and argon matrices and adducts of these species were identified using infrared spectroscopy. Formation of the adducts was evidenced by shifts in the vibrational frequencies of the modes involving the TMP and acetylene submolecules. The structures of these adducts, energies and the vibrational frequencies were computed at the HF/6-31G** level. Both the experimental and computational studies indicated that two types of TMP-acetylene complexes were formed; one in which the hydrogen in acetylene was bonded to the phosphoryl oxygen and another in which the bonding was at the alkoxy oxygen of the phosphate. In addition to the primary hydrogen bonded interaction at the phosphoryl oxygen, this complex, also appeared to be stablilized by a secondary and weaker interaction involving a methyl hydrogen in TMP and the pi cloud in acetylene--a case of a H...pi interaction. The computed vibrational frequencies in the adducts agreed well with the observed frequencies for the modes involving the TMP submolecule, while the agreement was relatively poor for the modes involving the acetylene submolecule. The stabilization energies of these adducts, corrected for both zero-point energies and basis set superposition errors, were approximately 3 kcal/mol for the phosphoryl complex and, approximately 1 kcal/mol for the alkoxy complex.     

Ion-chromatographic determination of iodide in sea water with UV detection

A highly sensitive and selective ion-chromatographic procedure with UV detection at 227 nm was developed for the determination of trace amounts of iodide. The procedure utilizes 0.1M NaOH as eluent, which enables the determination of 1–100 g I^–1 I^– with a precision of ca. 6%. 1000-fold amounts of other anions such as fluoride, chloride, phosphate, sulphate, bromide, nitrite and nitrate do not interfere. The method has been applied to determine iodide in sea water samples from the Indian coast.     

New processable polyaromatic amides curable by Diels-Alder cycloaddition

New processable polyaromatic amides were prepared from the acid chloride of bis-m-carboxyphenyl acetylene (V), the acid chloride of 1,4-bis-m-carboxyphenyl-1,3-butadiene (VI), and several aromatic diamines. The polyamides that contained acetylene units were cured by Diels-Alder cycloaddition reaction with 1,4-diphenyl-1,3-butadiene, whereas the polyamides with 1,3-butadiene units were cured with N-phenyl maleimide. Cured polyamides showed an increase in t_g, thermal, and heat stabilities. The polyamides can be cast into films and produce good glass-fiber laminates.     

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.     

Effect of Metal Oxides on the Thermal Degradation of High Impact Polystyrene

The kinetic parameters for the thermal degradation of high impact polystyrene (HIPS) in presence of some metal oxides exhibit reaction rate compensation effect. In thermal degradation of HIPS in presence of transition metal oxides different active centers act simultaneously as reaction sites and macroradicals are formed through random chain scission, disproportion or cyclization. Some oxides retard the polymer degradation through crosslinking and cyclization by the interaction of macroradicals with the double bond in butadiene. This revised version was published online in July 2006 with corrections to the Cover Date.