Jag J. Singh

Moisture dependence of positron annihilation spectra in nylon-6

Abstract Positron annihilation time spectra have been measured in nylon-6 specimens as a function of their moisture content. The measured average longlife component lifetime values are: 1722 ± 47 ps (dry), 1676 ± 40 ps (14.6% saturation value), 1719 ± 26 ps (29.3% saturation value), 1720 ± 35 ps (50% of saturation value), 1857 ± 35 ps (78.1% of saturation value) and 1936 ± 57 ps (saturated). It is noted that the longlife component lifetime at first decreases and then increases with increasing moisture content in the specimens. This behavior is quite different from that observed in earlier studies of various epoxy, polyamide, and polyimide materials, where the longlife components lifetime decreased linearly with increasing moisture content. The longlife component intensity on the other hand, decreases steadily as the moisture content increases from 0 to 100% of the saturation value. A possible explanation for these anomalous features is discussed.

Free volume model for molecular weights of polymers

Abstract A free volume model has been developed for determining molecular weights of linear polymers. It is based on the size of free volume cells in two geometries of poly (arylene ether ketone)s. Free volume cell sizes in test samples were measured using positron lifetime spectroscopy. The molecular weights computed from free volume cell sizes are in good agreement with the values measured by gel permeation chromatography, with a low angle laser light scattering photometer as the detector. The model has been further tested on two atactic polystyrene samples, where it predicted the ratio of their molecular weights with reasonable accuracy.

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...

Partially exposed polymer dispersed liquid crystals for boundary layer investigations

A new configuration termed partially exposed polymer dispersed liquid crystal in which the liquid crystal microdroplets dispersed in a rigid polymer matrix are partially entrapped on the free surface of the thin film deposited on a glass substrate is reported. Optical transmission characteristics of the partially exposed polymer dispersed liquid crystal thin film in response to an air flow induced shear stress field reveal its potential as a sensor for gas flow and boundary layer investigations.

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...

Effects of transition metal ions on positron annihilation characteristics in epoxies

Abstract Effects of several 3d-transition metal ions on positron annihilation characteristics in MY-720 epoxy resin have been investigated. All metal complexes were added at the mole ratio of 1 metal complex for every 10 MY-720 repeat units. In all cases, it appears that the 3d magnetic electrons are localized on their respective ions. Cr 3+ -ions make the strongest impact — both on the probability of formation of positronium atoms as well as their subsequent decay. The effects of Co 2+ and Co 3+ ions are not greatly different, indicating that the final oxidation states of Co x + -ions in the host epoxy may be the same, namely 2 + . Ni 2+ -ions are unique because their coordination geometry remains unchanged when Ni(acac) 2 is introduced in the MY-720 epoxy. In this respect, Ni(acac) 2 is similar to Co(aeac) 2 and both have comparable impacts on positron annihilation characteristics. The effects of Fe 3+ -ions are consistent with coordination symmetry change from sp 3 d 2 to d 1 sp 3 when Fe(acac) 3 is introduced in the epoxy.

Microstructural characterization of thin polymer films using the Langley low energy positron flux generator

Abstract Thin, high-performance polymer films are finding widespread applications in industry, medicine, and aerospace research. Since the microstructure of the films determines how well they meet their theoretical promise, it is necessary to develop efficient techniques for producing reasonable fluxes of low energy positrons needed for investigating thin polymer films. We have developed a highly efficient scheme for generating high fluxes of slow positrons. These positrons have been successfully used to measure lifetimes in the thin test films. The lifetime data have been used to develop two structure-property models for the test films. The first model relates the free volume cell size ( V f ) with the molecular weight of the polymer repeat unit ( M ) by an expression of the form V f = AM B , where A and B are structural constants. The second model relates the free volume fraction (ƒ) with the dielectric constant of the polymer film (ϵ) by an expression of the form 1 ϵ = (1 − ƒ) ϵ R + f ϵ vac where ϵ R is the dielectric constant of the microvoid-free film.

Free volume model for dielectric constant of polymer films

A slow positron flux generator reported in another paper at this conference was used to measure positron lifetime in a series of especially developed fluorine containing thin polyimide films. The positron lifetime spectra was analyzed into 2‐components using a standard least square routine. No evidence for positronium formation was observed in any of test films studied. The trapped positron lifetimes were used to calculate the radii of the shallow trap sites. Equating the total volume occupied by the traps with the saturation of the shallow trap sites. Equating the total volume occupied by the traps with the saturation moisture content of Kapton (reference) films, free volume fractions (f) were calculated in all the samples. These free volume fractions affect the dielectric constants (e) of the test films as follows: 1/e= (1−f)/eR+f(1−d)/eAir+fd/eWater Where, eR is the dielectric constant of the trap‐free medium, eAir is the dielectric constant of air, eWater is the dielectric constant of water, and d...

Microstructural characterization of earth and space processed polymers with positrons

Positrons provide a versatile probe for monitoring microstructural features of molecular solids. In this paper, we report on positron lifetime measurements in two different types of polymers. The first group comprises polyacrylates processed on earth and in space. The second group includes fully-compatible and totally-incompatible Semi-Interpenetrating polymer networks of thermosetting and thermoplastic polyimides. On the basis of lifetime measurements, it is concluded that free volumes are a direct reflection of physical/electromagnetic properties of the host polymers.