Vibrational dynamics of solid poly(ethylene oxide)

Molecular dynamics (MD) simulations of crystalline poly(ethylene oxide) (PEO) have been carried out in order to study its vibrational properties. The vibrational density of states has been calculated using a normal mode analysis (NMA) and also through the velocity autocorrelation function of the atoms. Results agree well with experimental spectroscopic data. System size effects in the crystalline state, studied through a comparison between results for 16 unit cells and that for one unit cell has shown important differences in the features below 100 cm^-1. Effects of interchain interactions are examined by a comparison of the spectra in the condensed state to that obtained for an isolated oligomer of ethylene oxide. Calculations of the local character of the modes indicate the presence of collective excitations for frequencies lower than 100 cm^-1, in which around 8 to 12 successive atoms of the polymer backbone participate. The backbone twisting of helical chains about their long axes is dominant in these low frequency modes.Comment: 19 pages, 7 figures (Phys.Rev.B submitted on 28.11.2002) Revised versio

Poverty and Productivity in Female-Headed Households in Zimbabwe

A household survey conducted in rural Zimbabwe in 2001 is used to compare the position of de facto and de jure female-headed households to those with a male head. These households are characterised by different forms of poverty that impinge on their ability to improve agricultural productivity. However, once inputs are accounted for, it is only for growing cotton that female-headed households' productivity is lower than that found for male-headed households. General poverty alleviation policies will benefit the female-headed household but specific interventions via extension services and access to marketing consortia are also indicated

Solid-Fuel Ramjet Assisted Gun-Launched Projectiles

The principles of construction and operation of a solid-fuel ramjet assisted gun- launched projectile are briefly explained. A concise global-survey of the projects on solid-fuel ramjet powered missiles is presented. Pseudovacuum trajectory is a ballistic trajectory in air of a powered projectile where the thrust always balances the drag. Easy and accurate predictability and insensitiveness to external disturbances are the two major advantages of the pseudovacuum trajectory. This trajectory can be easily achieved for gun-launched projectiles by the use of solid fuel ramjets. A preliminary-sizing procedure for solid fuel ramjet powered gun launched projectile is presented. Supersonic spillage and its momentum, bypass-air momentum, real time variations of stagnation pressure losses at the two rearward steps (one at the inlet to and the other at the exit of the combustion chamber), heat addition losses, and combustion efficiency are included in the procedure. Also, presented are the ramjet-control requirements for a typical 155-mm gun launched projectile. The control requirements are minimal, demonstrating the "self throttling characteristics" of solid fuel ramjets. For the typical 155-mm gun launched projectiles, following pseudovacuum trajectories using solid fuel ramjets, the maximum range is found to be in excess of 40 km

A Distributed Scheduling Algorithm to Provide Quality-of-Service in Multihop Wireless Networks

Control of multihop Wireless networks in a distributed manner while providing end-to-end delay requirements for different flows, is a challenging problem. Using the notions of Draining Time and Discrete Review from the theory of fluid limits of queues, an algorithm that meets delay requirements to various flows in a network is constructed. The algorithm involves an optimization which is implemented in a cyclic distributed manner across nodes by using the technique of iterative gradient ascent, with minimal information exchange between nodes. The algorithm uses time varying weights to give priority to flows. The performance of the algorithm is studied in a network with interference modelled by independent sets