The transport properties of polyimide isomers containing hexafluoroisopropylidene in the diamine residue

The effect of modification of the central moiety of the dianhydride residue and isomerism on the gas transport and physical properties were compared for six polyimides containing the hexafluoroisopropylidene group in the diamine residue. Substitution of bulkier groups within the dianhydride residue resulted in disruption of chain packing and slight increases in resistance to chain motions which led to an increase in permeability with little loss in selectivity. The permeabilities and diffusivities in the meta connected polyimide isomers were considerably lower than in the para connected polyimide isomers. Similarly, the permselectivities in the meta connected isomers were consistently higher than in the para connected isomers. These lower permeabilities and higher permselectivities were a result of the more dense packing and a significant suppression of small scale motions in the meta connected isomers. The suppression of segmental mobility in the meta connected isomers was indicated by an increase in the sub Tg transition temperatures in these materials relative to the para connected isomers. The differences in transport properties for these polyimides were attributed to contributions by several factors, including: (1) total free volume (2) distribution of free volume (3) intersegmental resistance to chain motions, and (4) intrasegmental resistance to chain motions. © 1994 John Wiley & Sons, Inc.     

Identification of the IR frequencies of the interstitial carbon atom in some M6C (M = Fe,Ru) carbonyl clusters by13 C-enrichment

¹³C-enriched carbido carbonyl clusters of the type [M₆C(CO) _n ] ^z? (M=Fe,n=16,z=2; M=Ru,n=16 ifz=2 andn=17 ifz=0) have been prepared and their IR spectra observed in the 900?600 cm^?1 region; the band shifts occurring upon¹³ C substitution enable the assignment to the stretching modes of the interstitial C atom. Band patterns reflect the symmetry of the M₆C core, but the frequencies seem dependent on other molecular features.     

The kinetics and thermochemistry of S2F10 pyrolysis

Data on the kinetics of S₂F₁₀ pyrolysis, which gives SF₄ + SF₆, have been reinterpreted to give a value for the equilibrium constant of S₂F₁₀ ? SF₄ + SF₆. This, together with statistical estimates of the entropy and heat capacity of S₂F₁₀, can be used to give for this reaction values of ΔH = 19.7 ± 1.0 kcal/mole and ΔS = 47.6 ± 2 gibbs/mole. ΔH(S₂F₁₀) = –494 kcal/mole. A compatible mechanism is shown to be S₂F₁₀ ? 2SF₅ (fast); 2SF₅ ? SF₆ + SF₄ (slow) with step 2 rate-determining. The overall, best first order rate constant is proposed as k_meas = 10^{17.42–43.0/θ} sec^?1 = K₁k₂, where θ = 2.303RT in kcal/mole. Independent measurements of δH and S° for the SF₅ radical, permits the evaluation of the equilibrium constant K₁ = 10^{8.92–(27.1 ± 6)/θ} l./mole-sec and yields k₂ = 10^{8.50–15.9/θ} l./mole-sec. The observed homogeneous catalysis by NO and CHCl ? CHCl can be explained in terms of a direct abstraction of F from S₂F₁₀ : C + S₂F₁₀ → CF + S₂F₉, followed by S₂F₉ → SF₅ + SF₄ and SF₅ + CF ? SF₆ + C (C ? NO or C₂H₂Cl₂).     

ET cetera

Experimental Techniques -     

On the sign of the ρ-ω mixing charge asymmetric nn potential

The sign of the single-particle exchange nucleon force due to ρ°-ω mixing is shown on experimental and theoretical grounds to make the resulting charge asymmetric potential, V_nn ? V_pp in the ¹S₀ state, attractive. The sign agrees quite well with the binding energy difference in the A = 3 mirror nuclei and is not inconsistent with present knowledge of NN scattering lengths, predicting $\text{0 < ¦}\text{a}{}_{\mathrm{nn}}\text{¦ ? ¦a}{}_{\mathrm{<mtext>pp</mtext>}}\text{¦ ? 1}\text{fm}$.     

Self-Reaction of HO2 and DO2: Negative temperature dependence and pressure effects

The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO₂ are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO₂ self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high-pressure limit of the self-reaction of HO₂.