Modified Tricomi Problem for a Nonlinear System of Second Order Equations of Mixed Type

In this paper a nonliuear system of second order equations of mixed type is considered. The existence of H¹ strong solution for the modified Tricomi problem is proved by the energy integral method and the Leray-Schauder's fixed point principle.     

Darboux Transformations Between Δα=sinh α and Δα=sin α and the Application to Pseudo-Spherical Congruences in R2,1

In this Letter, it is proved that by using Darboux transformation, from a known solution of the sinh-Laplace equation (resp. sine-Laplace equation), new solutions of sine-Laplace equation (resp. sinh-Laplace equation) are obtained explicitly. The corresponding geometrical configuration is the space-like pseudo-spherical line congruence with two focal surfaces of negative constant curvature being space-like and time-like, respectively.     

Novel Shape‐Memory Polymer Based on Hydrogen Bonding

Summary: Due to a large difference in storage modulus below and above the glass transition temperature, a novel shape‐memory poly[(methyl methacrylate)‐co‐(N‐vinyl‐2‐pyrrolidone)]/poly(ethylene glycol) semi‐interpenetrating polymer networks structure was synthesized, which is stabilized by hydrogen‐bonding interactions. The recovery ratio of these polymers could reach 99%. In such a system the maximum molecular weight of PEG required for the semi‐IPNs formation reaches 1 000.

Transition from the temporary shape (chem) to the permanent shape (four rods) for a shape‐memory P(MMA‐co‐VP)/PEG1000 semi‐IPNs.     

On a Maximum Principle and Uniqueness Theorems for a System of First Order Equations of Mixed Type

A maximum principle for a system of first order equations of mixed type is established. The uniqueness theorems of solutions t.o the generalized Tricomi type problem and to the Frankl's problem are proved by the method of auxiliary functions.     

Measurement of the τ Polarization Asymmetry in τ→a1 v→π±π0π0v Decay at Z0 Peak

The τ polarization asymmetry in τ→a₁v→π^±π⁰ π⁰v decay at Z⁰ peak is measured using data collected by L3 detector at LEP in 1992 and 1993, A_pol=－0.239±0.126±0.100, where the first error is statistical and the second systematic. And the effective electroweak mixing angle sin²θ_w=0.2197±0.0219 is derived.     

Green Synthesis of 2‐Substituted Imidazolines using Hydrogen Peroxide Catalyzed by Tungstophosphoric Acid and Tetrabutylammonium Bromide in Water

Various 2‐substituted imidazolines were constructed from aromatic aldehydes with ethylenediamine using hydrogen peroxide as an oxidant in water. Tungstophosphoric acid (HPW) was found to be active for this transformation due to its ubiquitous catalytic and oxidative nature, and further combination of tetrabutylammonium bromide (TBAB) made this transformation more efficient and attractive. It was found that the yields of the corresponding 2‐substituted imidazolines were markedly influenced by the position and nature of the substituents on the phenyl ring. A plausible mechanism was also proposed to clarify this catalytic oxidative system.     

钛合金在低温下的高速变形特性和绝热剪切

采用SHPB技术在室温和低温下,对相钛合金TB-2的高速变形特性和绝热剪切进行了宏观和微观研究。结果表明,TB-2是一个对应变率和温度敏感的材料,其热粘塑性本构特性可表为 =(0+E1（1+gln/0）（1-（T-Tn） /Tn）显微观察表明,TB2在低温下比在常温下对绝热剪切更敏感。低温下绝热剪切带的形态和结构也和常温下有所不同。把文献[9]所建议的绝热剪切的热粘塑性失稳准则,应用到不同的环境温度下,本文建议了一个既依赖于应变和应变率,又依赖于环境温度的三变量准则。理论预示和试验结果符合较好。     

Ultrasonically initiated emulsion polymerization of methyl methacrylate

The ultrasonically initiated emulsion polymerization of methyl methacrylate (MMA) was investigated. Experimental results show that sodium dodecyl sulfonate (SDS) surfactant plays a very important role in obtaining a high polymer yield, because in the absence of SDS, monomer conversion is near zero. Thus, the surfactant serves as an initiator and as interfacial modifier in this system (MMA/H₂O), and the monomer conversion increases significantly with increasing SDS concentration. An increase in the reactor temperature also leads to an increase in the monomer conversion. An appropriate increase in the N₂ purging rate also leads to higher conversion. The conversion of MMA decreases with increasing monomer concentration because of the higher viscosity of the system. With the experimental results, optimized reaction conditions were obtained. Accordingly, a high monomer conversion of about 67% and a high molecular weight of several millions can be obtained in a period of about 30 min. Furthermore, transmission electron micrographs show that the latex particles prepared are nanosized, indicating a promising technique for preparing nanoscale latex particles with a small amount of surfactant. In conclusion, a promising technique for ultrasonically initiated emulsion polymerization has been successfully performed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3356–3364, 2001     

超高效液相色谱-串联质谱法同时测定食品级聚苯乙烯和聚乙烯色母粒中33种初级芳香胺

建立了超高效液相色谱-三重四极杆质谱(UPLC-MS/MS)同时测定食品级聚苯乙烯(PS)和聚乙烯(PE)色母粒中33种初级芳香胺(PAAs)的检测方法。PS色母粒用二氯甲烷溶解,超声提取后加入甲醇沉淀,并将提取液过石墨化碳固相萃取柱净化;PE色母粒用二氯甲烷超声溶胀提取;将PS色母粒过柱液和PE色母粒提取液浓缩,浓缩液用甲醇-水(1:9, v/v)定容至2 mL, 0.22 μm膜过滤后上机检测。采用BEH Phenyl色谱柱(100 mm×2.1 mm, 1.7 μm),以0.07%(v/v)甲酸甲醇溶液-水(1:9, v/v)为流动相,梯度洗脱分离,UPLC-MS/MS多反应监测(MRM)模式检测,同位素内标法定量。优化了色谱分离条件、质谱碎裂电压、碰撞能量等,并考察了提取时间、提取溶剂、浓缩方式等对回收率的影响。33种PAAs的方法检出限为6~10 μg/kg,定量限为20~30 μg/kg, 2种不同基质样品在20、100、200 μg/kg等3个添加水平的平均回收率为61.3%~119.8%,相对标准偏差(RSD)为1.4%~14.8%。本方法操作简便、快速、准确、灵敏度高,能满足相关测定要求。