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[en] The specific features of the implementation of the positron annihilation technique, positron non-destructive testing, in studying polymers with an abnormally high free volume (polyvinyltrimethylsilane, polytrimethylsilylpropyne, and polyphenylene oxide) as materials for nonporous gas-separating membranes and porous ultra-and microfilters were considered. The possibilities and advantages of describing the positron lifetime spectrum using three or four components were analyzed. The role of triplet positronium and of its bound state on pore walls as annihilation channels was considered in connection with the extremely long-lived (for polymers) positronium states that were observed, for example, in the porous polyphenylene oxide (τ = 26.2±3.7 ns). Structural transformations due to the open-quotes agingclose quotes of polytrimethylsilylpropyne were also studied

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[en] The present not is concerned with how the ecosystem maintains its energy and matter processes, and how those processes change throughout ecological and geological time, or how the constituent biota of an ecosystem maintain their life, and how ecological (species) succession and biological evolution proceed within an ecosystem. To advance further Tansky's (1976) approach to ecosystem organization, which investigated the characteristic properties of the developmental process of a model ecosystem, by applying Margalef's (1968) maximum maturity principle to derive its long term change, we seek a course for deriving the macroscopic trends along the organization process of an ecosystem as a consequence of the interactions among its biotic components and their modification of ecological traits. Using a simple ecosystem model consisting of four aggregated components (open-quotes compartmentsclose quotes) connected by nutrient flows, we investigate how a change in the value of a parameter alters the network pattern of flows and stocks, even causing a change in the value of another parameter, which in turn brings about further change in the network pattern and values of some (possible original) parameters. The continuation of this chain reaction involving feedbacks constitutes a possible mechanism for the open-quotes coevolutionclose quotes or open-quotes matchingclose quotes among flows, stocks, and parameters

[en] In this work it is reviewed a few known types of transition to turbulence, as the cascade of period doubling and the intermittent transition. This happens in dynamical systems with a few degrees of freedom, as modelled by the iteration of non linear maps. Then it is presented specific transitions for systems with many degrees of freedom. It is condidered first the occurence of a low frequency broadband noise in large cells at the onset of Rayleigh-Benard convection; then the transition by intermittent bursts in parallel flows. In this last case, one is concerned with localized and finite amplitude perturbations. Simple geometric arguments show that these fluctuations, when they are isolated and with a well definite relative speed, exist for a single value of the Reynolds number only

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[en] A new formulation of the membrane theory is presented in this paper. The assumptions which allow the Budiansky-Sanders' model or the membrane theory to be deduced from the three-dimensional case are pointed out