A new polymer dissolution model was developed by incorporating the polymer chain disentanglement mechanism into the relevant transport equations. The disentanglement time was used as a dissolution characteristic time controlling the moving position of the solvent-polymer boundary. A dimensionless dissolution number was defined as the ratio of the characteristic polymer disentanglement time to the characteristic solvent diffusion time. The dissolution number was shown to be proportional to the square of the gel layer thickness. Scaling law expressions for the dependence of the gel layer thickness and the polymer dissolution rate on polymer molecular weight were also derived. Solution of the model for one-dimensional dissolution showed three distinct dissolution stages and confirmed the proposed scaling law relations for the gel layer thickness and the dissolution rate. Experimental studies of dissolution of polystyrene and poly(methyl methacrylate) in methyl ethyl ketone were used to verify the model, and two types of polymer dissolution behavior were observed. For dissolution of polystyrene in MEK, the solvent diffusion behavior was Fickian and a constant gel layer thickness was observed during the stationary dissolution stage. The effect of polymer molecular weight on the gel layer thickness was investigated for nine monodisperse samples, with Mn ranging from 28 000 to 2 830 000. The experimental results showed that the dependence of the gel layer thickness on molecular weight is more prominent in the high molecular weight region. The polystyrene data verified the new dissolution model. The dissolution of PMMA in MEK was controlled by crack propagation as no significant gel layer was observed.
The effects of rice plants treated with various pesticides (jingganmycin, bisultap and methamidophos) on feeding, survival rates and population growth of Nilaparvata lugens Stål (Homoptera: Delphacidae), susceptibility of the treated rice plants and amounts of free amino acids and sucrose were studied. Experiments indicated that the effects of the tested pesticides were dependent on nymphal age, pesticide and their dose and time after application. Jingganmycin at 75 g a.i. ha−1 significantly increased the N. lugens population. Both jingganmycin and bisultap increased the survival rate of N. lugens nymphs. The feeding rate of the insects was also affected by the pesticide application, but the effect varied between nymphal age and time after application and lasted no longer than 15 d. Results clearly indicated that pesticide application increased the susceptibility of rice plants to N. lugens. Although the free amino acids in rice plants did not change with the pesticide treatments, the concentration of sucrose significantly decreased 5 d after application and the C/N ratio significantly decreased in jingganmycin treated plants 5 d and 10 d after application.
Planthoppers are serious rice pests in Asia. Their population resurgence was first reported in the early 1960s, caused mainly by insecticides that indiscriminately killed beneficial arthropods and target pests. The subsequent resurgence involved two mechanisms, the loss of beneficial insects and insecticide-enhanced planthopper reproduction. In this review, we identify two forms of resurgence, acute and chronic. Acute resurgence is caused by traditional insecticides with rapid resurgence in the F1 generation. Chronic resurgence follows application of modern pesticides, including fungicides and herbicides, with low natural enemy toxicity, coupled with stimulated planthopper reproduction. The chemical-driven syndrome of changes leads to later resurgence in the F2 or later generations. Chronic resurgence poses new threats to global rice production. We review findings on the physiological and molecular mechanisms of chronic planthopper resurgence and suggest research directions that may help manage these new threats.
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