Low energy positron scattering from atoms in and on solids observed with low energy positron diffraction and reemission microscopy

The present status of low energy positron diffraction (LEPD) and positron reemission microscopy (PRM) is reviewed in the context of unanswered questions regarding the elastic scattering of positrons in the 1–300 eV energy range from atoms in solids and at solid surfaces. Recent LEPD studies yield an agreement between theoretical and experimental diffraction intensities for semiconductors that has never been equalled in electron diffraction studies. This situation is discussed in terms of the repulsive interaction between the positron and the embedded atomic potential and the lack of exchange with the nonspherically distributed valence electrons. The scattering of re-emitted positrons in PRM from atoms chemisorbed or physisorbed on the re-emitting surface has not yet received the same theoretical attention as scattering from embedded atoms in LEPD. Possible ways in which positron scattering from overlying atomic structures manifests itself in PRM as well as positron re-emission holography are discussed, both from the practical viewpoint of observing these structures and in the context of fundamental questions regarding the positron re-emission process itself.     

Variational approach to unsteady flow and heat transfer in a channel

The transient thermal response of a Newtonian fluid to several assumed forms of pressure pulse in a channel is analyzed by the semi-direct variational method of Kantorovich. The temperature distribution in unsteady laminar flow in a channel is investigated when the pressure gradient is an arbitrary function of time. Results are obtained for the linearly varying as well as the suddenly applied pressure gradient. Received on 1 July 1997     

Viscoelasticity of a butadiene–styrene block copolymer

A commercial elastomeric block copolymer of butadiene (B) with styrene (A) is studied. A single chain of the material has the formula A-B-A. Differential thermal analysis studies show the presence of two transitions. The lower transformation temperature corresponds to the T_g of the butadiene chain segments, and the upper transformation temperature corresponds to the T_g of the styrene chain segments. The upper transition of the material is also studied by following the variation of the torsional modulus with temperature. This transition is found to be quite unusual. Our experiments show that the upper transformation of unstressed block copolymer samples is broad. The transition sharpens for samples which, prior to the modulus–temperature experiments, are stress-relaxed at high elongations. These observations (and those of the literature) suggest that the styrene and butadiene chain segments in the block copolymer aggregate in the solid state and give rise to two distinct transition phenomena. Our studies of the upper transformation suggest that stretching of the bulk material enhances the aggregation of the styrene chain segments. Pure polystyrene (A) blocks of the material are recovered by selective cleavage and fractionation experiments. The T_g of the pure polystyrene blocks is found to be similar to the value of the upper transition temperature of the copolymer. The ABA blocks copolymer is found also to undergo a stress-softening phenomenon analogous to that of reinforced rubber.     

Physical Gels of a Syndiotactic Polystyrene Derivative with a Large Side-Chain Group

Summary: Gelation of syndiotactic poly(p-tert-butylstyrene) (sPTBS), a syndiotactic polystyrene (sPS) derivative having a larger side-chain group, was first examined with several solvents. The temperature-concentration phase diagram of sPTBS/trans-decalin gel clearly exhibited that sPTBS formed a polymer-solvent molecular compound with a ratio of 2.7 trans-decalin per one monomer unit. Our polarized fluorescence technique demonstrated that there appeared to exist more spacious free volume among sPTBS chains than sPS in a gel state. A cause determining the morphology of sPTBS with organic solvents was discussed in the present paper.     

Evidence for a ramsauer minimum in positron-argon collisions

The lifetime spectra are obtained for positrons annihilating in argon gas at room temperature and at lower temperatures of the order of 100 K. By suitably normalising and subtracting room temperature from low temperature lifetime spectra, certain features of the spectra which are determined by the momentum transfer cross-section for positron-argon collision are greatly enhanced. As a result, it is found that the cross-section must possess a well defined minimum below 3 eV in order to account for the observed effects.