Amorphous polyarylene ether ketones were examined in the glassy state by positron annihilation lifetime spectroscopy (PALS) and in the melt by standard rheological techniques. Specimens were well‐characterized fractions of two isomeric structures. PALS clearly shows that the polymer with meta linkages in its backbone contains larger voids (> 0.25 nm radius). Thus despite their similar bulk densities, the two materials must pack very differently on a local scale. On the other hand, the free volumes inferred from the WLF treatment of melt viscosity data are practically identical in both materials ca. 4% at Tg. The comparison between techniques sheds some light on the distribution of free volume. © 1992 John Wiley & Sons, Inc.

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Results are reported for the density and temperature dependencies of the annihilation rates of o-Ps in ethane gas. The pickoff quenching rate of o-Ps in ethane is observed to vary with temperature at low densities of the gas. The observed o-Ps annihilation behavior with density and temperature of ethane is interpreted in terms of density fluctuations in the gas.

Characteristics of a liquid crystal system, comprised of a shear sensitive cholesteric monomer liquid crystal thin-film coated on a liquid crystal polymer substrate, are described. The system provides stable Grandjean texture, a desirable feature for shear stress measurements using selective reflection from the monomer liquid crystal helix structure. Impingement of gas or air flow on the monomer liquid crystal free surface changes the wavelength of the selective reflection for an incident white light from red towards blue with increase in the rate of gas flow. The contrast of the selectively reflected light improves considerably by providing a thin (∼5 μm) black coating at the monomer-polymer interface. The coating thickness is such that the steric interactions are still sufficiently strong to maintain Grandjean texture. For a small angle of incidence (∼15°) of a monochromatic light, the measurement of the reflected light intensity normal to the monomer-polymer liquid crystal interface enables us to determine the wavelength (λs) for selective reflection as a function of the gas flow differential pressure (Δp) applied in the plane of the interface. In the range of the Δp (0–100 mmHg) used in the present studies, the variation of λs with Δp is linear with a slope ∼2 nm/mmHg. Furthermore, the shear stress effects are reversible unlike for monomer liquid crystal−metal systems used so far for flow visualization on wind-tunnel model surfaces. Thus the present system offers a suitable method for direct on-line measurement of shear stress field from measurement of λs for an incident white light.