Improving glass transition temperature of unsaturated polyester thermosets: Conventional unsaturated polyester resins

Highly crosslinkable unsaturated polyester resins (UPR) have attracted many interests in the application as reinforced matrix materials. Here, we present a systematical study of the influence of different curing conditions and styrene concentrations on resin viscosity and dynamic-mechanical properties of the thermoset. The pure maleic Palapreg® P18-03 was selected as model UPR because of its broad industrial use. By applying newly developed thermal curing profiles (without thermal initiators) and by raising the styrene content, the T_g of the network could be increased up to 206/215°C (1/10 Hz). For the first time, a fast curable UPR based on propylene glycol and neopentyl glycol with a T_g of up to 215°C is described. A partial substitution of problematic styrene with methylmethacrylate, tert-butylacrylate, and maleic anhydride (MA) was studied as well. MA leads to significantly improved resin reactivity. A resin containing 42 wt% styrene and 8 wt% MA yields thermosets with remarkably improved mechanical properties and with a narrower glass transition range compared to the original P18-03.     

Impact of Spatial Channel Reuse and Adaptive Modulation on Wireless Mesh Network Capacity

The capacity analysis based on the concept of collision domains may be applied to wireless networks with mesh topology, e.g. wireless backhaul networks for rural areas, sensor networks, etc. The strength of the method is the fact that it may be easily fitted to any MAC protocol, routing algorithm, channel allocation method, and, therefore, is a valuable tool for verification and performance analysis of such algorithms. However, simplifications assumed by some authors may lead to erroneous results, e.g. significantly underestimated capacity of the WMN. Therefore, phenomena specific to wireless networks, such as spatial channel reuse, and features specific to wireless standards, such as link adaptivity, should be included in the analysis, as presented in this paper.     

Big plans for housing in the GDR

The German Democratic Republic recently adopted a massive housing programme for 1976-90. How such a programme is organised and financed in a centrally-planned economy is described here by the chief reporter of “Deutsche Architektur”, who is also vicepresident of the GDR architects' group.     

Using research to guide culturally appropriate drug abuse prevention.

Several ways in which research can be an important tool in creating culturally appropriate prevention programs are suggested. To illustrate cultural vulnerability, the authors present data from a study of 5th-grade African-American and European-American students. These data describe rates of early initiation, levels of exposure to identified risk factors, and degree to which identified risk factors predict substances initiated. Results indicate that European-American Ss have higher rates of tobacco and alcohol initiation, that the same risk factors predict the variety of substances initiated for African-American and European-American Ss, and that several differences exist in the level of exposure to risk factors between the 2 groups. Implications of the findings for developing culturally appropriate drug abuse prevention are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A sustainable genetic algorithm for satellite resource allocation

A hybrid genetic algorithm (GA) is used to schedule tasks for a satellite, which can be modeled as a robot whose task is to retrieve objects from a two-dimensional field. The objective is to find a schedule that maximizes the value of objects retrieved. Typical of the real-world tasks to which this corresponds is the scheduling of ground contacts for a communications satellite. An important feature of our application is that the amount of time available for running the scheduler is not necessarily known in advance. This requires that the scheduler produce reasonably good results after a short period but that it also continue to improve its results if allowed to run for a longer period. We satisfy this requirement by developing what we call a sustainable GA.     

Growth criteria for solvent crazes

Single methanol crazes are grown from sharp cracks in polymethylmethacrylate. Double exposure holographic interferometry is used to determine the sequential strain energy release rates G and opening displacement profile of the craze from the initiation of growth to its cessation. The craze stress profile is determined at various points in its growth from the opening displacement profiles using a Fourier transform method. The rapid increase in G observed just before the craze ceases to grow demonstrates that craze growth criteria based on the concept of a constant critical total strain energy release rate cannot be correct. Similarly, the large stress concentration which develops just behind the craze tip at growth cessation is incompatible with the assumption that the craze grows to a length that just eliminates a stress singularity at its tip (Dugdale model). This feature, however, would be expected if sufficient methanol cannot reach the fibrils just behind the tip of the craze to plasticize them fully.