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Polymer brushes   总被引:1,自引:0,他引:1  
Polymers attached by one end to an interface at relatively high coverage stretch away from the interface to avoid overlapping, forming a polymer "brush." This simple picture may serve as the basis for models in diverse interfacial systems in polymer science, such as polymeric surfactants, stabilized suspensions of colloidal particles, and structures formed by block copolymers. The structure and dynamics of polymer brushes have been the subject of considerable theoretical and experimental activity in recent years. An account is given of recent advances in theoretical understanding of stretched polymers at interfaces, and the diverse experimental probes of systems modeled by brushes are briefly reviewed.  相似文献   
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It is becoming increasingly apparent that chemistry at surfaces, whether it be heterogeneous catalysis, semiconductors etching, or chemical vapor deposition, is controlled by much more than the nature and structure of the surface. Recent experiments that principally make use of molecular beam techniques have revealed that the energy at which an incident molecule collides with a surface can be the key factor in determining its reactivity with or on the surface. In addition, the collision energy of an incident particle has proven essential to the finding of new mechanisms for reaction or desorption of molecules at surfaces, collision-induced activation and collision-induced desorption. These phenomena are often responsible for the different surface chemistry observed under conditions of high reactant pressure, such as those present during a heterogeneous catalytic reaction, and of low pressure of reactants (< 10(-4) torr), such as those present in an ultrahigh vacuum surface science experiment. This knowledge of the microscopic origins of the effect of pressure on the chemistry at surfaces has allowed the development of a scheme to bypass the high-pressure requirement. Reactions that are normally observed only at high reactant pressures, and which are the ones most often of practical importance, can now be carried out in low-pressure, ultrahigh vacuum environments.  相似文献   
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A soilborne disease of lettuce, associated with necrosis and dieback, has been found with increasing frequency in California and Arizona over the last 10 years. An isometric virus, serologically related to Tomato bushy stunt virus (TBSV), was consistently isolated from lettuce plants with these disease symptoms. Back-inoculation to healthy lettuce plants and subsequent reisolation of the virus from symptomatic lettuce leaves suggested that this virus was the causal agent of this disease. A tombusvirus was also associated with a necrosis disease of greenhouse-grown tomatoes in Colorado and New Mexico. Complementary DNA representing the 3' end of viral genomic RNAs recovered from diseased lettuce and tomato plants had identical nucleotide sequences. However, these sequences were divergent (12.2 to 17.1%) from sequences of the previously described strains of TBSV, Petunia asteroid mosaic virus (PAMV), Artichoke mottled crinkle virus, and Carnation Italian ringspot virus. Additional tombusvirus isolates were recovered from diseased lettuce and tomato plants and these were most closely related to the TBSV-cherry strain (synonymous with PAMV) and to Cucumber necrosis virus based on comparison of 3'-end sequences (0.1 to 0.6% and 4.8 to 5.1% divergence, respectively). Western blot analysis revealed that the new tombusvirus isolated from diseased lettuce and tomato plants in the western United States is serologically distinct from previously described tombusvirus species and strains. Based on genomic and serological properties, we propose to classify this virus as a new tombusvirus species and name it Lettuce necrotic stunt virus.  相似文献   
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Many dogs with chronic illness have serum biochemical abnormalities consistent with hyperadrenocorticism (HAC). Lymphoma (LSA) is a chronic disease of dogs. The purpose of this study was to evaluate adrenocortical screening test results in dogs with LSA to evaluate their specificity. Criteria for inclusion in the study included a diagnosis of LSA, an expected survival time of 16-56 weeks, no glucocorticoid treatment beyond 4 weeks after the initiation of chemotherapy, no evidence of HAC, and owner consent. Post-ACTH stimulation plasma cortisol concentrations (PACs), urine cortisol : creatinine (UC : Cr) ratios, and maximal left adrenal width measurements were performed at the time of LSA diagnosis before the initiation of chemotherapy and at 16, 24, 32, 40, and 52 weeks or until the loss of remission or the development of another disease. Ten dogs met the criteria for inclusion. Forty-two PACs were performed; 1 abnormal, 2 borderline, and 39 normal values were detected. Thirty-five maximal left adrenal width measurements were obtained; 0 abnormal, 5 borderline, and 30 normal measurements were detected. Thirty-six UC : Cr ratios were obtained, with 26 abnormal, 4 borderline, and 6 normal values detected and 9 of 10 dogs having at least 1 abnormal value. These data suggest that in dogs with LSA, the UC : Cr ratio frequently is abnormal and may not be a specific test for HAC, or it may be the most sensitive test for increases in cortisol secretion due to chronic illness. Maximal left adrenal width measurements and PACs were almost always normal and may be more specific for HAC or less sensitive for demonstrating chronic increases in cortisol secretion.  相似文献   
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The safety and efficacy of 2% moxidectin/12.5% praziquantel oral gel administered at a rate of 0.4 mg moxidectin and 2.5 mg praziquantel/kg was studied in client-owned horses under field use conditions. Four hundred horses (300 treated with moxidectin/praziquantel oral gel and 100 treated with vehicle) were enrolled, feces were collected, and eggs were counted. Investigators as well as horse owners were masked to treatment assignment. No adverse reactions to treatment were observed in any horses. Moxidectin/praziquantel gel reduced Anoplocephala spp by more than 99% and provided a significant (P <.05) reduction (> 98%) in the strongyle egg count of treated horses.  相似文献   
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