Compatibility studies of polystyrene–polybutadiene blends by thermal analysis

In order to examine the behavior of incompatible blends of polystyrene and polybutadiene, the glass transition temperature, the melting point, and the specific heat increment at the glass transition temperature for atactic polystyrene (a-PS), isotactic polystyrene (i-PS), polybutadiene (PBD), and blends of a-PS/PBD and i-PS/PBD were determined by use of a differential scanning calorimeter. Blends were prepared by solution casting, freeze-drying, and milling. Weight fractions of polystyrene in the blends ranged from 0.95 to 0.05. The glass transition temperature of polystyrene changed with weight fraction in the blends, and with blending preparation methods; the glass transition temperature of polybutadiene remained essentially unchanged. The specific heat increment at the glass transition temperature of PBD decreases linearly with increasing proportions of PS in the PS/PBD blend for the broad and narrow molecular weight distribution polybutadience polymers, whereas the specific heat increment for PS did not decrease with increasing proportions of PBD in the PS/PBD blend. These results suggest that the polybutadiene dissolves more in the polystyrene phase than does the polystyrene in the polybutadiene phase.     

Spatial and seasonal variability of dissolved methylmercury in two stream basins in the eastern United States

We assessed methylmercury (MeHg) concentrations across multiple ecological scales in the Edisto (South Carolina) and Upper Hudson (New York) River basins. Out-of-channel wetland/floodplain environments were primary sources of filtered MeHg (F-MeHg) to the stream habitat in both systems. Shallow, open-water areas in both basins exhibited low F-MeHg concentrations and decreasing F-MeHg mass flux. Downstream increases in out-of-channel wetlands/floodplains and the absence of impoundments result in high MeHg throughout the Edisto. Despite substantial wetlands coverage and elevated F-MeHg concentrations at the headwater margins, numerous impoundments on primary stream channels favor spatial variability and lower F-MeHg concentrations in the Upper Hudson. The results indicated that, even in geographically, climatically, and ecologically diverse streams, production in wetland/floodplain areas, hydrologic transport to the stream aquatic environment, and conservative/nonconservative attenuation processes in open water areas are fundamental controls on dissolved MeHg concentrations and, by extension, MeHg availability for potential biotic uptake.     

Modeling and experimental investigation of a diode-pumped Tm:YAlO/sub 3/ laser with a- and b-cut crystal orientations

The Tm:YAlO/sub 3/ laser performance for two crystal orientations in diode-end-pumped configurations is presented in this paper. An advanced numerical model, which included reabsorption, temperature profile in the medium, and pump and laser beam spatial distribution, was used to predict laser performance. An experimental investigation comparing a- and b-oriented Tm:YAlO/sub 3/ crystals laser performance is presented and discussed in detail. The novel a-cut orientation produced superior performance over the traditionally used b-cut orientation. The modeling results were found to be in a good agreement with the measured data.     

An entity-relationship programming language

The syntax for an integrated E-R programming language is presented. The problems that arise when a query language is embedded in a general-purpose programming language are discussed. Other E-R languages are also discussed. The requirements for the language and a syntax for an E-R model in which entity sets are mutually disjoint and each entity type has a unique, perhaps multiattribute, key are presented. The syntax for a more limited model restricted to binary relationships between entity types and without attributes is presented. Some implementation considerations are discussed     

Regime Theory,State Government,and a Takeover of Urban Education

Abstract State government possesses the resources and authority to directly shape urban education policy but regime theorists understate the roles governors, state legislatures, and other state actors play as members of urban education regimes. This article examines the state takeover of schools in Newark, New Jersey to demonstrate why and how a state government leads an urban education regime. The Newark case illustrates how politics and structural conditions motivated state government to change the nature of the education regime and directly shape education policy at the local level. It highlights the role state government played in reshaping an educator‐centered coalition that operated a poorly performing school district. Despite the existence of a new regime, Newark students’ achievement scores have not significantly improved, and in some instances they have declined under the state‐led regime. This article encourages scholars of city politics to continue to investigate state government's role in urban governing coalitions because state political players maintain the capacity and motivation to join urban regimes.     

SBS suppression using a depolarized source for high power fiberapplications

A depolarized source for fiber applications is demonstrated using a traveling wave LiNbO₃ phase modulator. The source suppresses stimulated Brillouin scattering (SBS) in the fiber when operated at high drive frequencies, enabling more power to be delivered to a remote location. The threshold for SBS is examined in detail using optical powers up to 150 mW and drive frequencies up to 4 GHz. A resonant enhancement technique is demonstrated which reduces the drive power requirement for frequencies <2 GHz by up to 7.6 dB     

Genomic characterization of the coding region of the human type II 5''-deiodinase gene

The conserved residue Asp477 in yeast transketolase is located in the substrate channel of the enzyme and forms a hydrogen bond with the C2-hydroxyl group of the acceptor substrate. The significance of this interaction for the recognition of the preferred acceptor substrates, D-alpha-hydroxyaldehydes was investigated by site-directed mutagenesis. In the wild-type enzyme the kcat/KM values are by three to four orders of magnitude lower for 2-deoxyaldoses or substrates with L-configuration at the C2-atom. In the Asp477 Ala mutant, the kcat/KM values for D-alpha-hydroxyaldehydes are decreased by a thousandfold, while the kcat/KM values for substrates with L-configuration or 2-deoxyaldoses are similar to wild-type enzyme. These results indicate that Asp477 is involved in determining the enantioselectivity of transketolase.