Broken symmetries and pattern formation in two-frequency forced faraday waves

We exploit the approximate (broken) symmetries of time translation, time reversal, and Hamiltonian structure to obtain general scaling laws governing the process of pattern formation in weakly damped Faraday waves. Using explicit parameter symmetries we determine, for the case of two-frequency forcing, how the strength of observed three-wave interactions depends on the frequency ratio and on the relative phase of the two driving terms. These symmetry-based predictions are verified for numerically calculated coefficients, and help explain the results of recent experiments.     

Excited states in the photochemistry of chromium(III) complexes

Experimental data for chromium(III) complex ions on the spectroscopic properties, photoreactions, and intermolecular energy transfer are analyzed in terms of the primary photophysical and photochemical processes. It is suggested that the lowest excited quartet state,⁴T_2g inO _h symmetry, is the immediate precursor to photoaquation and that the lowest doublet state,²E, is substitution inert except via the⁴ T _2g state reached by back intersystem crossing.

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Zusammenfassung Experimentelle Daten von Chrom-(III)-Komplex-Ionen in bezug auf spektroskopische Eigenschaften, Photoreaktionen und intermolekularen Energieübertragung wurden mit dem Begriffssystem der photophysikalischen und photochemischen Primärprozesse analysiert. Und zwar wird vorgeschlagen, daß der tiefste angeregte Quartettzustand,⁴ T _2g inO _h -Symmetrie, der unmittelbare Vorläufer der Photoaquatisierung ist und daß der tiefste Doublettzustand,²E, inert gegenüber Substitutionen ist, außer über den⁴ T _2g Zustand, wenn er durch Re-intersystem crossing erreicht wird.

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Dedicated to the memory of Professor Hans-Ludwig Schläfer.     

Steady-state and dynamic rheology of poly(1-olefin) melts

The steady-state and dynamic melt rheology for a series of poly(1-olefins) has been investigated. The series includes poly(1-butene), poly(1-hexene), poly(1-heptene), poly(1-octene), Poly(1-undecene), poly(1-tridecene), poly(1-hexadecene), and poly(1-octadecene). The flow behavior was investigated by use of a Weissenberg rheogoniometer. Measurements on poly(1-butene) were also made using an Instron capillary rheometer. The empirical relationship developed by Cox and Merz was obeyed for the entire series of poly(1-olefins) at all temperatures investigated. Graessley's theory was used to calculate the flow curves for the poly(1-olefins) from the measured molecular weight distributions. The purpose was to investigate the effect of polymer composition on the shear rate dependence of viscosity. It was found that all experimental flow curves except those for poly(1-hexene) can be fitted with the calculated curves from the individual molecular weight distributions. The conclusion is made that flow curves of poly(1-olefins) depend predominately on molecular weight distribution and are essentially independent of side-chain length even for poly(1-olefins) with pendant groups as long as 16 carbon atoms. The low-shear limiting Newtonian viscosity η₀ for all poly(1-olefins) was expressed by, η₀ = KM? or by η₀ = K′P? where M?_w is the weight-average molecular weight and P?_w is the weight-average degree of polymerization. The K and K′ values obtained decrease systematically as the side chain is increased.     

The effect of anhydrous ammonia on the crystalline-state deformation of nylons 6 and 6,6

The aliphatic polyamides nylon 6 and 6,6 have been cold drawn by using the new concept of reversible plasticization with NH₃. Two aspects were investigated: the effect of NH₃ plasticization on the stress-strain behavior of nylon 6,6, and the extent of NH₃ absorption as determined by x-ray analysis for the structurally simpler nylon 6. X-ray analysis shows that ammonia is absorbed preferentially by the amorphous phase and that the plasticization has a significant effect on the unit-cell orientation, the small-angle void scattering, and the interplanar spacings between sheets of hydrogen-bonded chains. The effect of plasticization was seen in the lower yield stress, the higher draw ratio (up to 8X) and tensile modulus (10 GPa) not attainable with unplasticized specimens.     

Drawing of poly(p-phenylene sulfide) by solid-state coextrusion

The effects of drawing temperature on the physical and mechanical properties of poly(p-phenylene sulfide) have been studied. A melt-quenched film was drawn by solid-state coextrusion both below (75°C) and above (95 and 110°C) the glass transition temperature T_g (85°C) of PPS. The maximum extrusion draw ratio (EDR_max) increased from 3.4 to 5.6 with increasing extrusion temperature T_e from 75 to 110°C. It was found that extrusion drawing just above the T_g of PPS (95°C) produced more stress-induced crystals. A high efficiency of draw in the amorphous region was achieved by extrusion at T_e-75°C. The tensile modulus at EDR_max decreased from 5.1 to 3.5 GPa with increasing T_e from 75 to 110°C. The low efficiency of draw for the samples extruded at 110°C is explained in terms of disentanglement and chain slippage during drawing due to a less effective network.     

Chain orientation in polystyrene/poly(2,6-dimethyl-1,4-phenylene oxide) blends

The differential orientation of polymer chains has been measured in polystyrene (PS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) compatible blends. Density measurements are reported as a function of binary blend composition at 23°C. Drawing was performed by solid-state coextrusion. PS/PPO blend compositions of 90/10 and 75/25 were drawn within sandwiches of polyethylene at 145°C and isotactic polypropylene at 155°C, i.e. at ca. 25°C above the glass transition temperatures of the two blends. The change in Fourier-transform infrared dichroisms on drawing these blends was measured at 906 and 1190 cm^?1, corresponding to predominantly PS and PPO, respectively. The orientation of PS and PPO was observed as a function of draw ratio λ in the range 1–5; orientations increased with λ for both PS and PPO in both blends but to different degrees. Both polymers decreased in orientation with increasing PPO content. Annealing with fixed ends showed that the PPO chains disorient more slowly than those of PS. All binary systems were found to be amorphous and compatible.