Abe Eftekhari

A shear sensitive monomer‐polymer liquid crystal system for wind tunnel applications

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 deter...

Microstructural Characterization of Semi-Interpenetrating Polymer Networks by Positron Lifetime Spectroscopy

Abstract Thermoset and thermoplastic polyimides have complementary physical/mechanical properties. Whereas thermoset polyimides are brittle and generally easier to process, thermoplastic polyimides are tough but harder to process. It is expected that a combination of these two types of polyimides may help produce polymers more suitable for aerospace applications. Semi-Interpenetrating Polymer Networks (S-IPNs) of thermoset LaRC™-RP46 and thermoplastic LaRC™-IA polyimides were prepared in weight percent ratios ranging from 100:0 to 0:100. Positron lifetime measurements were made in these samples to correlate their free volume features with physical/mechanical properties. As expected, positronium atoms are not formed in these samples. The second life time component has been used to infer the positron trap dimensions. The “free volume” goes through a minimum at about 50:50 ratio, suggesting that S-IPN samples are not merely solid solutions of the two polymers. These data and related structural properties of the S-IPN samples have been discussed in this paper.

A low energy positron flux generator for microstructural characterization of thin polymer films

A low energy positron flux generator using well‐annealed polycrystalline moderators and a Na22 positron source has been developed for microstructural characterization of thin polymer films. A 200 μc Na22 source, deposited on a thin (2.54 μm) aluminized mylar film, is sandwiched between two 0.0127 cm×2.54 cm×2.54 cm tungsten strips. Two identical test polymer films, whose thicknesses may range from 0.001 to 0.01 cm, insulate the two tungsten moderator strips from the aluminized source film. A potential difference of 10–100 volts, depending on the test film thickness, is applied between the tungsten strips and the aluminized source film. Thermalized positrons diffusing out of the moderator strips are attracted to, or repelled from, the source foil depending on the polarity of the potential difference between the moderator strips and the source foil. Thus, more positrons are expected to stop/annihilate in the test films when the source is at a negative potential than when it is at a positive potential. The d...