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111.
Effects of solution pH on the physical properties of a semidilute aqueous alginate solution without cross-linker agent and during gelation via the Ugi multicomponent condensation reaction at a fixed cross-linker concentration have been investigated. Both rheology and turbidity results on the alginate solution without cross-linker revealed enhanced associations at low pH. In the course of the cross-linker reaction, the time of gelation is shortest at pH = 3.5 and at pH values above 3.8 no gel is formed but only a viscosification of the solution is observed. The turbidity during the cross-linking reaction rises as the pH increases from 3.5 to 5. Furthermore, the initial change of the turbidity in the course of the cross-linking process is more pronounced at higher pH. The dynamic light scattering (DLS) results of the reaction mixture at pH = 4.0 (ergodic features at this condition) show that the chain relaxation is slowed down as the reaction proceeds. The effect of pH on the kinetics of the Ugi reaction is discussed. 相似文献
112.
Freddy RommOleg Figovsky 《Journal of solid state chemistry》2002,164(2):237-245
A theoretical (thermodynamic) method for the estimation of mechanical characteristics of polymeric systems is proposed. This method uses the statistical polymer method for modeling of branched/cross-linked structures. The weak interaction between macromolecules is modeled in the approach of their mutual interpenetration. The proposed method is used for the estimation of mechanical resistance and stability of microporous polymeric materials. An engineer method for the evaluation of mechanical stability and resistance of polymeric materials is derived. 相似文献
113.
叶绿体延迟荧光中730nm成分产生机理的光谱学研究 总被引:1,自引:0,他引:1
叶绿体685 nm延迟荧光成分被认为源于PSⅡ作用中心的电荷复合。利用多种光谱学测量手段研究了叶绿体延迟荧光光谱中730 nm峰的产生机制。不同浓度下叶绿体延迟荧光光谱实验结果表明:初始随浓度的增加,延迟荧光光谱中685和730 nm成分强度均增强;当浓度增加到7.8 μg·mL-1时,685 nm成分强度达最大,730 nm成分强度继续上升;当浓度增加到31.2 μg·mL-1时,延迟荧光光谱中730 nm成分强度达最大,而685 nm成分已明显下降。吸收光谱实验结果表明:A685/A730在叶绿体浓度增加的过程中几乎不变。叶绿体730 nm荧光成分的激发光谱实验结果表明:685 nm光对730 nm荧光有较高的激发效率。上述实验结果表明叶绿体延迟荧光光谱中730 nm峰是由PSⅡ所发685 nm成分激发PSⅠ所产生的荧光。同一浓度下叶绿体延迟荧光光谱波形随延迟时间(1~9 s)的不变性进一步证明了这一结论。 相似文献
114.
交联阳离子淀粉的制备及其脱色性能 总被引:12,自引:0,他引:12
在碱催化剂6%氢氧化锂水溶液存在下,以N-(2,3-环氧丙基)三甲基氯化铵(GTA)为阳离子化试剂,制备了交联高取代度季铵型阳离子淀粉。考察了6%氢氧化锂水溶液、水含量、反应温度和反应时间对取代度和反应效率的影响。当淀粉用量为5.8g,GTA用量为3.0g时,最佳反应条件为:6%氢氧化锂水溶液2.0g、反应温度70℃、反应时间2h、反应体系含水量24.6%。在此条件下,取代度可达0.5,反应效率为83.3%。对制得阳离子淀粉的脱色性能研究结果表明,交联取代度阳离子淀粉对活性染料具有优异的脱色效果。当投加量为105mg/L时,对质量浓度为100mg/L活性红X-3B、活性黄KN-6G、活性蓝X-BR溶液的脱色率分别为99.1%、88.0%、95.5%。 相似文献
115.
116.
The permanence and global attractivity of positive equilibria are obtained for some multi-species Kolmogorov competition models with delay by embedding the system into a larger cooperative system with delay and then appealing to the theory of monotone dynamical systems.
117.
118.
Both coherence resonance (CR) and stochastic resonance (SR) in a delayed bistable system driven by additive and multiplicative white noises and a weak harmonic excitation are studied by using the theory of two-state model. For the weak noise intensity and delayed feedback, the analytic expressions of power-spectrum and linear-spectrum amplification are derived to quantify the CR and the SR, respectively. The study shows that the peak in the power spectrum at the frequency corresponding to the time delay attains the maximum for an appropriate amount of additive noise intensity and the CR manifests. The feedback gain plays an important role in the SR. For example, the positive feedback gain enhances the SR, but the negative feedback gain suppresses the system output and makes the SR disappear. Moreover, the system also exhibits the frequency SR. 相似文献
119.
Tincy Lis ThomasA.K. Mishra 《European Polymer Journal》2002,38(9):1805-1810
A method based on monitoring the fluorescence intensity of 1-anilinonaphthalene-8-sulphonate (ANS) has been introduced to follow the cross-linking polymerization of acrylamide initiated by ammonium persulfate-TEMED redox initiator system. There is about a 20-fold increase in the fluorescence intensity of ANS accompanying polymerization. Preferential location of ANS molecules near the hydrophobic regions of the polymer surface appears to explain the fluorescence intensity enhancement. The method is found to be consistent with the conventional dilatometric method. 相似文献
120.
Khaled A. Alhazza Mohammed F. Daqaq Ali H. Nayfeh Daniel J. Inman 《International Journal of Non》2008,43(8):801-812
Non-linear feedback control provides an effective methodology for vibration mitigation in non-linear dynamic systems. However, within digital circuits, actuation mechanisms, filters, and controller processing time, intrinsic time-delays unavoidably bring an unacceptable and possibly detrimental delay period between the controller input and real-time system actuation. If not well-studied, these inherent and compounding delays may inadvertently channel energy into or out of a system at incorrect time intervals, producing instabilities and rendering controllers’ performance ineffective. In this work, we present a comprehensive investigation of the effect of time delays on the non-linear control of parametrically excited cantilever beams. More specifically, we examine three non-linear cubic delayed-feedback control methodologies: position, velocity, and acceleration delayed feedback. Utilizing the method of multiple scales, we derive the modulation equations that govern the non-linear dynamics of the beam. These equations are then utilized to investigate the effect of time delays on the stability, amplitude, and frequency–response behavior. We show that, when manifested in the feedback, even the minute amount of delays can completely alter the behavior and stability of the parametrically excited beam, leading to unexpected behavior and responses that could puzzle researchers if not well-understood and documented. 相似文献