Effect of thickening agents on the penicillin fermentation

The effects of thickening agents (polyethylene glycol [PEG] 20000 and carboxymethylcellulose [CMC]) on the penicillin fermentation byPenicillium chrysogenum were investigated. By adding the thickening agents to the fermentation medium, the growth form of the mold can be manipulated. Depending on the amount of thicking agent added, the change in morphology is from compact smooth pellets to various intermediate forms, and finally to filamentous mycelia. It was found that better penicillin production was obtained when the mold was in small, fluffy, loose pellets. The penicillin fermentation is not only affected by the thickening agents, but also the status of inoculum and agitation. Under the condition that the mold will otherwise grow in large pellets (e.g., under a low level of spore inoculum), the enhancement in the penicillin production through addition of the thickening agents may be more significant. In tank fermentation, the thickening agent was introduced in the stage of preculture, rather than main culture. The increase in the broth viscosity caused by addition of the thickening agent resulted in a decrease in dissolved oxygen level, which could be compensated in the case of PEG 20000.     

剪切增稠液的力学性能与机理

剪切增稠液(shear thickening fluids,STFs)是一种新型功能材料,论文对其最新进展进行综述和评价.首先介绍了高性能STF的研制,分析了STF在不同外加载荷下的力学行为;然后从实验和理论等方面讨论了STF机理的研究现状;最后对STF的实际应用进行了总结,并对STF的发展前景进行了展望.     

Dynamic thickening investigation of the gelation process of PAM/PEI system at high temperature and high pressure

The gelation process of organically polymer gel was investigated by dynamic thickening measurements. Rheological measurements were used to evaluate the viscosity of the gel. During the gelation process, high temperature resulted in higher rate of crosslinking. Rigid and stable gel was formed in neutral and alkaline media, and the higher of the pH value, the faster of the gelation process. However, gel could not be formed in acid condition. Moreover, the rate of crosslinking increased with the increase of concentration of polymer and crosslinker. The addition of NH₄Cl elongated the gelation time significantly, but played a negative role in the gel strength, while a rigid gel was formed in the presence of Sodium acetate or trisodium citrate dehydrate. This paper summarizes the results and discusses how various parameters affect the gelation process of the gel.     

Heat Storage Performance of Disodium Hydrogen Phosphate Dodecahydrate： Prevention of Phase Separation by Thickening and Gelling Methods

Disodium hydrogen phosphate dodecahydrate （Na2HPO4·12H2O） is an attractive candidate for phase change materials. The main problem for its practical use comes from incongruent melting character during thermal cycling. Experimentally, heat of fusion of the pure salt decreased from 200 to 25 jog 1 in a four-run freeze-thaw cycling. Additives such as thickening agent or in-situ synthesized polyacrylate sodium in the molten salt can prevent its phase separation to some extent. In the test, sodium alginate 3.0%-5.0% （w/w） thickened mixture containing Na2HPOn·12H2O and some water showed constant heat storage capacities. Polyacrylate sodium gelled salt was synthesized through polymerizing sodium acrylate in the melt of Na2HPOn·12H2O and some extra water at 50 ℃. Optimum conditions composed of sodium acrylate 3.0%-5.0% （w/w）, cross-linking agent N,N-methylenebis-acrylamide 0.10%-0.20% （w/w）, K2S208 and Na2SO3 （mass ratio 1 ; 1） 0.06%-0.12% （w/w）. As opposed to normal large crystals of pure Na2HPOn·12H2O in solid state, the gelled salt existed in a large number of tiny particles dispersed in the gel network at room temperature, commonly less than 2 mm. But only those sample particles with sizes less than 0.2 mm may have relatively stable thermal storage property. A problem encountered was the poor reproducibility of the synthesis method： heat storage capacity of the product was often very different even though the synthesis was carried out in the same conditions. An alternative gelling method by sodium alginate grafted sodium acrylate was tried and it showed a fairly good effect. Heat capacities and heat of fusion of Na2HPO4·12H2O were measured by an adiabatic calorimeter.     

Non-linear Viscoelastic Rheological Properties of PCC/PEG Suspensions

在不同的恒定频率下,研究了PCC(沉淀碳酸钙)/PEG200(聚乙二醇,平均分子量为200)分散体系的剪切变稀和剪切增稠的流变性能．随着动态剪切应力的升高,复数粘度开始不随应力而改变,而后出现明显的剪切变稀,当复数粘度达到最小值后出现强烈的剪切增稠．在固定频率5 rad/s下研究了模量以及滞后角的正切与应力幅度的关系,在低的应力下储能模量开始几乎与应力幅度无关,继而快速下降．滞后角的正切从0.6升高到92,表明体系发生了从弹性到粘性的转变,尤其在发生剪切增稠后滞后角的正切快速增大．从Lissajous图分别探索了在剪切变稀区和剪切增稠区的能量与最大应变的关系．应变周期内的消耗的能量与最大应变幅度指数关系得到了确认．在剪切变稀区指数为1.91,而在剪切增稠区由于形成了粒子簇激增至3.97．以上现象运用\粒子簇"理论得到了定性的解释．     

ABSENCE OF ISOTHERMAL LAMELLAR THICKENING IN A LINEAR POLYETHYLENE BLEND

Melting and lamellar morphology of a polyethylene blend were studied. Two linear polyethylene (LPE) samples were used. A commercial LPE and a low molecular weight LPE fraction (M _n ≈ 2015) were solution blended. The pure LPE and a blend (30% commercial LPE content) were held in the melt at 126° C for up to 48 h, above the equilibrium melting point of the fraction, but below the crystallization temperature of the commercial LPE. The melting behavior of both materials as a function of storage time was studied using differential scanning calorimetry (DSC), in addition to transmission electron microscopy (TEM) of chlorosulfonated samples. Results showed that, although the LPE lamellae grew at the same temperature, those in the pure LPE were thicker than in the blend. Correspondingly, isothermally grown lamellae in pure LPE melt at higher temperatures.     

分散介质和温度对纳米二氧化硅胶体剪切增稠行为的影响

对分散介质和温度对纳米二氧化硅胶体剪切增稠行为的影响进行了系统研究. 用四种液体分散介质(乙二醇, 聚乙二醇400, 丙二醇, 聚丙二醇400)制备的纳米二氧化硅胶体表现出不同的连续剪切增稠或者跳变剪切增稠行为. 温度上升降低了分散介质的黏度, 进而降低了胶体的表观黏度. 剪切增稠的临界黏度与温度的关系符合“Arrhenius”公式的描述. 胶体黏度与分散介质黏度的比值用来归一化不同温度下的稳态剪切流变曲线. 在低剪切速率的剪切变稀阶段, 剪切变稀现象与分散介质黏度没有明显相关性, 而与分散介质的化学性质密切相关. 在高剪切速率的剪切增稠阶段, 分散介质黏度越高, 胶体剪切增稠现象越强烈.     

Size and viscoelasticity of spatially confined multilamellar vesicles

We studied viscoelastic properties and scaling behavior of multilamellar vesicles (MLVs) confined between two parallel plates as a function of the shear rate and sample thickness (gap size between parallel plates). The rheological properties are classified into two regimes; the shear-thinning regime at high shear rates and the shear-thickening regime at low shear rates. In the former, the MLV radius results from the mechanical balance between the effective surface tension σ_eff and viscous stress force. The MLV radius is independent of the gap size. σ_eff estimated by van der Linden model is 2.1 ±0.15 ×10^-4 Nm^-1 corresponding to the same value obtained by SANS measurement. Power law exponents for the steady state viscosity and yield stress against pre-shear rate ( , ) well agree with prediction based on the layering of membranes. Therefore, viscoelastic properties in this regime could be modeled by assuming that the dynamics of MLVs are driven by layering of MLV polydomains, which could be accompanied by the viscous dissipation, i.e., the stress relaxation on the MLV, induced by continuous sequence of yields of MLVs. The flow curve is empirically explained by the assumption of a relaxation time for the MLV shape. In the latter, however, scaling laws observed in the shear-thinning regime break down. The MLV radius increases when the gap size is reduced below the threshold value and MLV is no longer formed at very small gap sizes. Different dynamics from the shear-thinning regime seem to dominate the viscoelasticity.     

动脉分岔血管内膜增生过程的数值模拟

内膜增生从发生到阻塞血管是一个复杂的变化过程,在这个过程中,内膜的增生、血管腔体形状的改变和血流动力学之间是相互影响的。为了研究这些变化,本文提出一种单元填充方法数值模拟了三维颈动脉分岔血管在低切应力作用下血管内膜增生的过程。该方法既可以克服节点移动方法所不可避免的内膜增生的不连续性,也可以避免网格重划分的困难。结果发现,如果单纯以切应力阈值作为内膜增生的判据,低切应力的作用将无法导致血管完全阻塞,但内膜增生和血流动力学之间的相互影响是可以通过数值方法进行模拟的。在本数值模拟中,内膜增生的过程分为"增厚"(先)和"扩展"(后)两个阶段,最大狭窄率为34.4%,发生在距血管分岔5mm处动脉窦的外侧壁面。其发生位置和形状与临床观察吻合。