The evolution of the viscoelastic retardation spectrum during the development of an epoxy resin network

The evolution of the viscoelastic behavior of an epoxy resin at various stages of curing has been followed with the changes in the retardation spectrum. The creep J(t) and recoverable creep compliance J_r(t) curves of the neat epoxy resin Epon l00lF (Shell) were determined at temperatures between 30 and 77°C. The viscosity decreased over 8 orders of magnitude as the temperature was increased. Specimens with eight stages of network development were prepared by reacting all of the epoxy resin's oxirane rings with amine hydrogens from varying ratios of a monofunctional amine (methyl aniline) and a tetrafunctional amine 4,4′-diamino diphenyl sulfone (DDS). Preparations in which 25, 35, and 40% DDS were used did not result in a molecular network, so they were viscoelastic liquids. With 45% DDS, the product had a nascent network and was judged to be just beyond the point of incipient gelation. The remaining preparations from 0.50, 0.60, 0.70, and 1.0 DDS yielded tighter less compliant molecular networks. The creep and recoverable compliance curves were measured over a range of temperatures above the glass transition temperature, T_g. They were reduced to T_g, and retardation spectra L(ln τ) were calculated.     

Physical aging of a polyetherimide: Creep and DSC measurements

Creep and differential scanning calorimetry (DSC) measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures ranged from 160°C to T_g with aging times ranging from 10 min to 8 days. The only measurable effect of physical aging on the short-time creep curves is a shift of the creep compliance to longer times. Andrade plots of the compliance versus the cube root of time are linear at short times with the slope β decreasing with increasing aging time to a constant value once equilibrium is reached. Log β³ is related directly to the degree to which the creep curves shift to longer times with physical aging, and is used in this work as a measure of physical aging. A reduced curve of log β³ versus log aging time is obtained for the aging temperatures investigated by appropriate vertical and horizontal shifts. The enthalpy change during aging increases linearly with the logarithm of the aging time, t_a, leveling off at equilibrium at values which increase with decreasing aging temperature. Hence, both nonequilibrium and equilibrium temperature shift factors can be calculated from the DSC data. Good agreement is observed between the equilibrium temperature shift factors obtained from the creep and DSC data. The temperature dependence of the nonequilibrium temperature shift factors is found to be an order of magnitude smaller than that of the equilibrium shift factors. The time scales to reach equilibrium for enthalpy and for mechanical measurements are found to be the same within experimental error. © 1995 John Wiley & Sons, Inc.     

Consistency of decision processes

If a statistical or a voting decision procedure is used by several subpopulations and if each reaches an identical conclusion, then one might expect this conclusion to be the outcome for the full group. It is shown that this property fails to hold for large classes of decision procedures. The geometric reasons why the consistency does not hold are described. A general theorem is given to characterize the procedures that satisfy this property of weak consistency.This research was supported in part by NSF Grant IRI-8803505 and a Guggenheim Fellowship. Also, the author thanks a referee for some comments that stimulated certain revisions.     

Quantum relativistic action at a distance

A well-known relativistic action at a distance interaction of two unequal masses is altered so as to yield purely Newtonian radial forces with fixed particle rest masses in the system center-of-momentum inertial frame. Although particle masses experience no kinematic mass increase in this frame, speeds are naturally restricted to less than the speed of light. We derive a relation between the center-of-momentum frame total Newtonian energy and the composite rest mass. In a new proper time quantum formalism, we obtain an L²(R⁴ R⁴, C) Hilbert space by varying individual particle rest masses. We propose the use of density operators, recognizing that the auxiliary proper time parameter is not an observable. The quantum formalism is applied to our altered version of the relativistic harmonic oscillator. Our generalized coherent states yield four-dimensional wave packets which follow the correct classical world lines. Appendices contain reviews of classical Hamiltonian reparametrization (incorporating our notion of manifest covariance), and a comparison of this work with the literature.     

Reactions of tetrahydro-as-triazine-3(2H)-thiories with α, β-difunctional compounds

The results of allowing tetrahydro-as-triazine-3(2H)-thiones to react with various α,β-diiunctional compounds, such as, α-bromoethyl p-toluenesulfonate, chloroacetaldehyde, α-bromophenylacet-uldehyde, phenaeyl bromide, chloroacetonitrile, α-bromophenylaeelonitrile, and α-cyanobenzyl p-toluenesulfonate are discussed. These condensations give either a 5H-thiazolo[3,2-b]-as-triazine or a 2H-thiazolo[2,3-c]-as-triazine.     

Ion-pair formation in aqueous solutions of silver nitrate: A Raman and infrared spectral study

A detailed study of the Raman and infrared spectral line shapes and line parameters of aqueous solutions (both H₂O and D₂O) of AgNO₃ is interpreted in terms of an equilibrium between “free” ions and the ion pair Ag⁺NO ₃ ^? . An association constant of 0.1M ^?1 was obtained from both the 717 cm^?1 and 1047 cm^?1 line intensities. Spectral features suggest a significant degree of covalence in the interaction. A C_s(σ_V) model with Ag⁺ above the plane of NO ₃ ^? is consistent with the data.     

Germination, growth rates, and electron microscope analysis of tomato seeds flown on the LDEF

The purpose of the experiment was to determine cosmic rays long-term effects on living tissue. A batch of tomato seeds were flown in orbit aboard the Long Duration Exposure Facility (LDEF) for almost 6 y. During this time, the seeds received an abundant exposure to cosmic radiation. Upon the return of the LDEF to Earth, the seeds were distributed throughout the United States and 30 foreign countries for analysis. Our university analysis included germination and growth rates as well as scanning electron microscopy (SEM) and X-ray analysis of the control as well as space exposed tomato seeds.

In analyzing the seeds under the electron microscope, usual observations were performed on the nutritional and epidermis layer of the seed. These layers appeared to be more porous in the space exposed seeds than on Earth-based control seeds. This unusual characteristic may explain the increases in the space seeds growth pattern. (Several test results showed that the space-exposed seeds germinated sooner than Earth-based seeds. Also, the space-exposed seeds grew at a faster rate.) The porous nutritional region may allow the seeds to receive necessary nutrients and liquids more readily, thus enabling the plant to grow at a faster rate.

Roots, leaves and stems were cut into small sections and mounted. After sputter coating the specimens with argon/gold palladium plasma, they were viewed under the electron microscope. Many micrographs were taken. The X-ray analysis displayed possible identifications of calcium, potassium, chlorine, copper, aluminum, silicon, phosphate, carbon, and sometimes sulfur and iron. The highest concentrations were shown in potassium and calcium. As a result of the electron interaction and X-ray production within the open seeds, the traditional layers of the space-exposed seed gave peaks of Mg, P and S, while the Earth seed gave an iron peak, which was not detected in the space-exposed seed because of electron beam positioning difference. The space-exposed seed and the Earth-control seed specimens displayed high concentrations of copper.     

Comparison of diatomics-in-molecules and simple valence-bond potential surfaces for FH2

The method of diatomics-in-molecules (DIM) is applied to the FH₂ system. With spin—orbit interaction neglected, all elements of the 24 × 24 hamiltonian matrix are tabulated as analytic functions of the six diatomic fragment potential curves. It is found that neglect of off-diagonal 8 × 8 blocks in the DIM hamiltonian matrix leads to an energy expression for the ground 1 ²A′ level which is identical to the valence-bond formula used by Blais and Truhlar in dynamical studies of the F + D₂ reaction. The ²A″ excited level from DIM theory is identical to the result derived by Blais and Truhlar, without neglect of the 8 × 8 off-diagonal blocks. The DIM and simple valence-bond energies are compared numerically for noncollinear geometries.     

A.C. Cyclic voltammetry: A digital simulation study of the slow scan limit condition for a reversible electrode process

Rate laws presented to date for analysis of a.c. cyclic voltammetric data have invoked the so-called “slow scan limit approximation” which requires that ΔEω ? v, where Δ E and ω are the applied a.c. potential amplitude and angular frequency, respectively, and v is the d.c. potential scan rate. To provide a more useful guideline for the experimentalist than this qualitative condition, a pure digital simulation approach has been used to compute the a.c. cyclic time domain waveform for a reversible process under small amplitude conditions. The a.c. content of this waveform is extracted by the digital FFT alogirthm. Results of this study are presented here. Among the conclusions reached are more quantitative limitations for the slow scan limit rate laws describing the fundamental and second harmonic responses (approximately 128 a.c. cycles/d.c. cyclic sweep and 512 a.c. cycles/d.c. cyclic sweep, respectively) and an interesting prediction that the latter limitations can be relaxed further by a current waveform subtraction strategy, to as low as about 16 a.c. cycles/d.c. cyclic sweep for the fundamental and second harmonics. The cycles/sweep values assume one triangular wave potential scan of ±200 mV is encompassed.