The effect of low-temperature crystallization on the mechanical behavior of rubber

In cold climates the correct performance of rubber components such as seismic isolators depends on them maintaining their elastic properties when exposed to prolonged periods at low temperatures. The high damping compounds developed for seismic isolation are normally especially prone to crystallization when exposed to subzero temperatures for periods of a few weeks. The effect of low-temperature crystallization on the mechanical stiffening of natural rubber is evaluated. The relationship between the shear modulus and amount of crystallization is measured using a technique in which the dimensional change and stiffness are monitored simultaneously. The relationship is found to be approximately independent of the crosslink density and the temperature of crystallization. It appears not to be realistically modeled by considering the crystals to behave as rigid filler particles but good qualitative agreement with experiment was obtained by modeling the crystals as a network of threads. Partially crystalline rubbers are found to yield under the application of a large stress like other partially crystalline polymers. Mechanisms for suppressing crystallization in rubber are discussed and the low-temperature stiffening of specially formulated rubber compounds for seismic isolation is presented. These results show that carefully formulated high damping natural rubber compounds can give adequate performance at low temperatures. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2181–2190, 2004     

Shear-banding structure orientated in the vorticity direction observed for equimolar micellar solution

 The non-monotonic shear flow of a viscoelastic equimolar aqueous surfactant solution (cetylpyridinium chloride-sodium salicylate) is investigated rheologically and optically in a transparent strain-controlled Taylor Couette flow cell. As reported before, this particular wormlike micellar solution exhibits first a shear thinning and then a pronounced shear-thickening behavior. Once this shear-thickening regime is reached, a transient phase separation/shear banding of the solution into turbid and clear ring-like patterns orientated perpendicular to the vorticity axis, i.e., stacked like pancakes, is observed (Wheeler et al. 1998; Fischer 2000). The solution exhibit several unique features as no induction period of the shear induced phase, no structural build-up at the inner rotating cylinder, jumping pancake structure of clear and turbid ringlike phases, and oscillating shear stresses appear once the pancake structure is present. According to our analysis this flow phenomenon is not purely a mechanical or rheological driven hydrodynamic instability but one has to take into account structural changes of the oriented micellar aggregates (flow induced non-equilibrium phase transition) as proposed by several authors. Although this particular flow behavior and the underlying mixture of shear induced phases and mechanical instabilities is not fully understood yet, some classification characteristics based on a recent theoretical approach by Schmitt et al. (1995) and Porte et al. (1997) where a strong coupling between the flow instability (non-homogeneous flow profile due to the bands) and the structural changes causes the observed transient phenomena can be derived. In reference to the presented model the observed orientation of the rings is typical for complex fluids that undergo a spinodal phase separation coupled with a thermodynamic flow instability. In contrast to other shear banding phenomena, this one is observed in parallel plate, cone-plate, and Couette flow cell as well as under controlled stress and controlled rate conditions. Therefore, it adds an additional aspect to the present discussion on shear banding phenomena, i.e., the coupling of hydrodynamics and phase transition of rheological complex fluids. Received: 8 January 2001 Accepted: 15 May 2001     

Stress tensor measurement using birefringence in oblique transmission

A method for measuring the stress tensor of liquids obeying the stress-optical rule is presented. In particular, the procedure makes it possible to determine the shear stress, and the first and second normal stress differences for rheometric flows. This technique is an extension of the procedure recently described by Burghardt and coworkers (Brown et al., 1995) wherein light is sent obliquely through a sample sheared between transparent plates. However, in the present development, the light is transmitted in the plane containing the velocity gradient and neutral directions, thereby reducing the necessary optical measurements by one. A polystyrene-tricresyl phosphate (TCP) solution is used as the test sample. The first and second normal stress differences in steady shear flow measured by this method show good agreement with the mechanical results measured by Madga et al. (1993) using a modified cone and plate rheometer. The transient behavior of the first and second normal stress differences following the start-up of shear flow is also presented.     

Über das System: Benzylzellulose-Benzol—Äthanol. II

Colloid and Polymer Science - Zum Schluß danken wir nochmals der Deutschen Forschungsgemeinschaft, der Industrie und besonders Herrn Prof. Wo. Ostwald für seine freundliche Hilfe.     

Ensemble expectations for structures on two-dimensional lattices

Expressions are obtained for the limiting behavior of ensemble expectations, as functions of coverage, of the number of simultaneous occurrences of various structures when indistinguishable single particles are arranged on a two-dimensional lattice. For the general expressions obtained no restrictions are placed on the geometrical nature of the lattice. Averages for specific geometrical arrays, such as rectangular and hexagonal arrays, may be calculated directly from the general results.     

Microwave spectrum and rotational isomerism of n-propyl isocyanide

The microwave spectrum of n-propyl isocyanide has revealed the existence of two rotational isomers, trans (methyl trans to the isocyanide substituent), and gauche. Plausible structures have been fitted to the data, giving the gauche dihedral angle as 119° ± 2° from the trans position. Stark effect measurements have yielded dipole moments for the two rotamers: μ_trans = 4.16 ± 0.02 D and μ_gauche = 4.10 ± 0.09 D. The rotational constants of the trans form are A = 23 700 ± 100, B = 2407.632 ± 0.020, and C = 2278.853 ± 0.030 MHz, and those of the gauche form are A = 10 208.983 ± 0.030, B = 3479.219 ± 0.015, and C = 2859.981 ± 0.015 MHz. It has been found from relative intensity measurements that the gauche ground state is slightly more stable than the trans ground state, with an energy difference of 99 ± 45 cm^?1. Several vibrationally excited states have been assigned to the torsional motion around the central carbon-carbon bond, the CNC bending motion, and the methyl internal rotation. The torsional vibration frequency is 114 ± 20 cm^?1 in the trans form and 123 ± 20 cm^?1 in the gauche form. A four-term potential function for internal rotation about the central CC bond has been determined.     

Anodic acetamidoselenation of alkenes

Anodic oxidation of diphenyldiselenide in acetonitrile in the presence of an alkene gives acetamidoselenides.