Towards maximal cell density predictions for polymeric foams

Self-consistent field theory is used to make direct predictions for the maximum possible cell densities for model polymer foam systems without recourse to classical nucleation theory or activation barrier kinetic arguments. Maximum possible cell density predictions are also made subject to constraining the systems to have maximal possible internal interface and to have well formed bubbles (no deviation from bulk conditions on the interior of the bubble). This last condition is found to be the most restrictive on possible cell densities. Comparison is made with classical nucleation theory and it is found that the surface tension is not an important independent consideration for predicting conditions consistent with high cell density polymeric foams or achieving the smallest possible bubble sizes. Instead, the volume free energy density, often labelled as a pressure difference, is the dominant factor for both cell densities and cell sizes.     

Maximal cell density predictions for compressible polymer foams

Thermodynamic upper bounds for polymer foam cell densities are predicted using compressible self-consistent field theory. It is found that the incompressible limit always gives the highest, and therefore ultimate, upper bound. Qualitative comparisons between the compressible and incompressible cases agree, indicating that low temperatures and high blowing agent content should be used to achieve high cell densities. The inhomogeneous bubble structure reveals deviations from the expected homogeneous Sanchez–Lacombe equation of state, consistent with some experimental results. A generalized Sanchez–Lacombe equation of state is discussed in the context of its suitability as a simple alternative to the Simha–Somcynsky equation of state.     

A New Adaptive Transmission Scheme for Wireless Communication Systems Utilizing Frequency Diversity

In this paper, we propose a new adaptive transmission scheme for wirelesscommunication systems utilizing frequency diversity combining technique. Thecapacity of Rayleigh fading chanels under the proposed adaptive transmissionscheme is analytically evaluated and compared with the capacity under theconventional adaptive transmission scheme. The results show that the channelcapacity under the new adaptive scheme exceeds that under the conventionalone, and this capacity increment becomes more apparent especially for loweraverage carrier-to-noise ratio (CNR) and larger number of diversity branches.Also, the capacity with the new scheme isshown to exceed even the capacity ofAWGN channels for some small values of CNR and large number of diversitybranches.