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INTRAMOLECULAR VIBRATIONAL-ENERGY TRANSFER IN METHYL FORMATE
No full textIntramolecular vibrational energy transfer in isolated gas phase molecules of methyl formate has been studied by spectrally resolving the IR laser induced fluorescence from C???H stretch fundamentals, combination bands, and overtones in the region from 2900 to 3500 cm???1. By comparing the experimentally observed relative energy content of the IR active modes with that calculated from a statistical model it is concluded that complete randomization of the energy initially deposited in the zeroth order optically excited state occurs. However, due to the finite density of vibrations, that statistical distribution differs clearly from a thermal distribution. Both the model and experiment agree that different optically excited states deposit different amounts of energy in the carbonyl stretching mode at 1754 cm???1. We also observe a rotational state dependence of vibrational energy transfer in the 2939 cm???1 band. The experiments are performed by exciting a molecule cooled in a nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra are recorded with a cryogenic Michelson interferometer.
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VISCOELASTIC PROPERTIES OF SULFONATED ETHYLENE PROPYLENE TERPOLYMER NEUTRALIZED WITH ZINC CATION
No full textStress???relaxation measurements in uniaxial extension, in the terminal zone, were made of sulfonated ethylene???propylene terpolymer (EPDM), neutralized with zinc cation. The sulfonation levels were 15 to 20 meq/100 g polymer (0.47 and 0.62 mol %), and for each level the effect of the ionic plasticizer, zinc stearate, was investigated. For all the polymers, the decay of the modulus with elapsed time was gradual and featureless, reflecting a broad distribution of relaxation times. Sensitivity of the ionomers' structures to thermal history before an experiment was suggested by irreproducibility in the magnitudes, but not the distribution, of relaxation times at a given temperature. For polymers containing zinc stearate measured at 80??C, the amount of permanent set was low for relaxation up to about 20h, but increased rapidly with additional hours of stretch. Time???temperature superposition did not apply. Composite curves were constructed by matching the stress???relaxation response at short times; moduli measured at long times fell above the composite curve. The shapes of the composite curves were similar, regardless of the level of EPDM sulfonation or the presence of zinc stearate. The apparent Arrhenius activation energy for short???time relaxation at similar levels of the modulus was about 35 kcal/mol at 140??C for the unplasticized polymers and about 70 kcal/mol at 90??C for the polymers containing zinc stearate, in contrast to the usual effect of plasticizer on the rate of relaxation of amorphous polymers except near the glass transition temperature
Water-soluble copolymers. III. Dextran-g-poly(acrylamides) control of grafting sites and molecular weight by Ce(IV)-induced initiation in homogeneous solutions
No full textModel graft copolymers were synthesized by grafting acrylamide onto dextran (Mw = 500,000) utilizing an initiation method in which a Ce(IV)/HNO3 solution was added to the dextran solution in order to allow coplexation prior to monomer addition. Three synthetic reaction parameters were optimized on the basis of conversion and solution viscosity: monomer concentration, dextran concentration, and nitric acid concentration. Molar ratios of [Ce(IV)]/[dextran] were changed systematically to affect the number and length of the acrylamide grafts. The number of grafting sites and graft chain lengths, determined by selective hydrolysis of the carbohydrate backbone, were in good agreement with those theortically predicted from knowledge of initiation efficiency and monomer conversion. Rheological studies of the model graft copolymers were conducted in aqueous solutions as a function of temperature, added salt, and copolymer concentration.clos
Water-soluble copolymers. I. Synthesis of model dextran-g-polyacrylamides by Fe(II)/H2O2 initiation and characterization by aqueous size exclusion chromatography
No full textModel graft copolymers were synthesized by grafting acrylamide onto dextran (M̄w = 500,000) utilizing the Fe(II)/H2O2 initiation system. Aqueous size exclusion chromatography (SEC) was used to determine the effects of changing reaction parameters on hydrodynamic dimensions of the resulting graft copolymers. It was also possible to optimize reaction conditions yielding the highest viscosity graft copolymer with the least amount of homopolyacrylamide and unreacted substrate. The molecular structures of the graft copolymers were determined by elemental analysis, SEC, and solution viscometry. Selective hydrolysis of the dextran backbone allowed determination of average molecular weight of acrylamide grafts, number of grafting sites, and average molecular weight of the graft copolymers. Rheological studies indicated viscosity and pseudoplastic behavior were largely related to the graft length of the polyacrylamide side chains.clos
STRESS-RELAXATION AND DYNAMIC VISCOELASTIC PROPERTIES OF END-LINKED POLY(DIMETHYL SILOXANE) NETWORKS CONTAINING UNATTACHED POLY(DIMETHYL SILOXANE)
No full textStress relaxation in uniaxial extension and dynamic shear moduli G??? and G??? have been studied in networks of vinyl???terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights (M n from 1800 to 29,200) crosslinked with cis???dichlorobis (diethyl sulfide) platinum (II) and containing 10 and 15 wt % of two samples of high???molecular???weight unattached linear hydroxyl???terminated PDMS (M w 700,000 and 950,000). The M w /M n ratio of both the network prepolymers and the unattached linear species was approximately 2. In stress relaxation the stretch ratio was 1.25 or less and the shear relaxation modulus was calculated from the neo???Hookean stress???strain relation. In the dynamic measurements, the strain amplitude was 15% or less; after conversion to the timedependent shear relaxation modulus G (t ) the two sets of measurements were combined and the contribution of the unattached species G 1(t ) was calculated by difference. After multiplication by (1 ??? v urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-1)???1G urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-2/G e, where v 2 is the volume fraction of network, G urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-3 is the plateau modulus of the uncrosslinked polymer, and G e is the equilibrium modulus of the network containing unattached molecules, G 1(t ) was compared with G 11(t ), the relaxation modulus was essentially the same in both environments. The relaxation was slower in the networks than in the uncrosslinked polymer by 1 to 2 orders of magnitude, and it increased gradually with increasing G e, which is a measure of total to pological obstacles represented by crosslinks plus trapped entanglements. A similar but less striking difference between relaxation in a network and in the homologous environment of a linear polymer was previously observed in end???linked polybutadiene networks and the butadiene phase of a styrene???butadiene???styrene block copolymer. It appears that, in these systems where the topology of the obstacles is fixed, the reptation is severely restricted or else alternative modes of configurational rearrangement which contribute to relaxation in the uncrosslinked polymer are suppressed
Scattering of matter waves from various optical reflection gratings under grazing incident conditions
No full textHandy mirrors and gratings for atoms and molecules would open new frontiers in matter-wave optics
with neutral particles. The realization of such components for atoms and molecules, however, has
required sophisticated electromagnetic fields, nano-fabrication, or particle cooling, because of their
inherently short wavelength and strong interaction with a surface. We demonstrate that reflective
gratings designed for photons can work as a grating or a mirror for atoms and molecules of a thermal
energy when used under grazing incidence conditions. The matter-waves of He atoms and D2
molecules are reflected from various gratings that are easily fabricated or commercially available. We
find out the critical factors affecting diffraction peak widths and efficiencies. Our analyses imply that
quantum and multiple-diffraction reflections occurring at the grazing incidence conditions are the
underlying physics allowing optical square-wave and blazed gratings to function as a grating or a
mirror for those particles