Rapid solidification processing in molten salts chemistry: X-ray analysis of deeply undercooled cryolite-alumina melts

Rapid solidification processing (cooling rate from the interval 10⁵–10⁶ K s⁻¹) was used to prepare deeply undercooled cryolite-alumina melts. Such prepared samples were analyzed by the XRD method. Besides cryolite, XRD patterns belonging to ι-Al₂O₃ were recorded. The influence of annealing on the XRD patterns of deeply undercooled melts was also investigated.     

Thermally degradable maleimides for reworkable adhesives

New thermosetting materials were developed for reworkable adhesive applications by introducing acetal ester groups as thermally degradable linkages into maleimide compounds. The synthesis of compounds containing maleimide functionality and acetal ester linkages was conducted by a one‐step neat reaction from commercially available materials. The polymerization process and thermal degradation of the synthesized materials were studied. It was found that the acetal ester linkage degraded rapidly above 225 °C, and introduction of this linkage into the adhesive formulation led to improved reworkability of adhesively bonded substrates. A mechanism for reworkability was proposed based on the insight provided by experimental and modeling work. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1073–1084, 2009     

Melt fracture of polyisobutylenes

The processability of different grades of polyisobutylene (PIB) was investigated using a capillary rheometer. Direct focus was given to the occurrence of melt fracture phenomena, such as sharkskin and gross melt fracture (GMF). The influence of molecular weight (MW) of PIB, temperature and die entrance angle on melt fracture was examined in detail. Due to their highly elastic nature, high MW PIBs were found to exhibit gross melt fracture instability even at low shear rates, rendering their processing an impossible task. An increase in temperature resulted in postponing both instabilities (sharkskin and gross) to higher shear rates, thus making their processing easier. Finally, decreasing the entrance angle below a critical value resulted in postponing the onset of GMF to higher shear rates.     

A study on the melt viscosity of linear and branched poly(butyleneterephthalate)

Linear and branched PBTP samples were synthesized and characterized in terms of the intrinsic viscosity, the melt-flow-index and, for some, the melt viscosity over a range of shear rates at 250 °C.An exponent of 3.2 in the equation relating to was found for linear samples. Both linear and branched samples exhibited Newtonian behaviour over a wide range of shear rates, but for any given melt-viscosity the branched samples became shear thinning at lower shear rates than the linear ones. Correlation between a branching index,, and melt-visocity ratio (0,b/0,l) was in agreement with a previous theoretical study.     

Elongational properties and molecular structure of polyethylene melts

Summary The viscosity and the recoverable strain in the steady state of elongation have been measured on several polyethylenes of different molecular structures. The elongational viscosity as a function of tensile stress runs through a more or less pronounced maximum in the nonlinear range whereas in the linear range the Trouton viscosity is reached. For low density polyethylenes it could be demonstrated that the maximum of the steady-state elongational viscosity and the elasticity expressed by the steady-state compliances in shear and tension sensitively increase if the molecular weight distribution is broadened by the addition of high molecular weight components. A variation of the weight average molecular weight does only shift the elongational viscosity curve but leaves its shape unchanged. Two of the four high density polyethylenes investigated do not show a maximum of the steady-state elongational viscosity, for the others it is less pronounced than in the case of low density polyethylenes. The influence of branching on the elongational behaviour of polyethylene melts in the steady-state and the transient region is qualitatively discussed.With 11 figures and 4 tables     

Growth and Spectral Characteristics of Yb3＋-doped LiGd（MoO4）2 Crystal

The Yb3+-doped LiGd(MoO4)2 crystal with the size up to Φ20×30 mm3 has been grown by Czochralski technique.The polarized room temperature absorption and emission spectra have been investigated.This crystal exhibits a broad absorption band centered at 975 nm with an FWHM of 43 and 59 nm for π-and σ-polarization,respectively,and the corresponding maximal absorption cross-sections are 3.36 and 2.42×10-20 cm2.The emission broadband has an FWHM of 47 and 54 nm for π-and σ-polarization,respectively,with the corresponding emission cross sections of 3.92 and 3.34 × 10-20 cm2 at 1020 nm.The measured fluorescence lifetime is 287 μs.     

传感器在固体熔化凝固实验中的运用

基于固体的熔化和凝固实验的传统做法,结合现代测量技术对其进行了改进,对晶体海波和非晶体石蜡的熔化和凝固进行了全程监控,取得了很好的实验效果并节省了实验时间,提高了实验的准确性.     

紫外光固化阳离子复合胶粘剂的制备与表征

 以三芳基六氟磷酸硫鎓盐为引发剂,钛酸四异丙酯(TIP)为无机前驱体,3,4-环氧环己基甲基3,4-环氧环己基甲酸酯为单体,制备了含TiO2纳米结构的紫外光固化阳离子复合胶粘剂。通过原子力显微镜对复合胶粘剂形貌进行表征,研究了TIP含量对胶粘剂折射率和光透过率的影响。结果表明：TiO2无机相均匀地分散在胶粘剂的聚合物基体中,平均粒径约为20 nm;随着体系中TIP质量分数的变化,复合胶粘剂折射率为1.510 3~1.540 9可调;胶粘剂的光透过率随TIP质量分数增加略有减低,但当TIP含量高达40%时,光透过率维持在90%左右。     

缩水甘油胺型聚氨酯胶粘剂的性能研究北大核心CSCD

采用商品化SKE-1型环氧树脂对自制聚氨酯预聚体(NCOPU)进行封端,制备了缩水甘油胺型聚氨酯(GAPU),并用傅里叶-红外光谱(FT-IR)对其结构进行表征,用在线FT-IR监控间苯二甲胺固化GAPU过程,用差示量热扫描(DSC)研究其相分离,用扫描电镜(SEM)观察固化物的表面微观形貌,用热重(TG)分析固化物的热力学稳定性。探讨了聚丙二醇(PPG)分子量的大小、不同质量分数的SKE-1对NCOPU封端及不同种类的固化剂对GAPU固化物力学性能的影响。研究表明:在60℃时用间苯二甲胺固化GAPU,2h即可固化完全,固化物热稳定性能良好,其外推起始分解温度为248.3℃,5%的分解温度为282.6℃。GAPU固化物的力学性能随着GAPU的环氧值减小而减小,在室温以上力学性能下降,在-196℃力学性能增加,其环氧值为0.153,在-196℃的拉伸剪切强度最佳,为16.11MPa。     

液态结构与性质关系Ⅱ——Mg-9Al熔体的运动黏度及与熔体微观结构的关系

采用坩埚扭摆振动法测量Mg-9Al熔体的运动黏度,得到890—1190 K温区内高精度的黏度-温度关系曲线ν(T),发现升温过程中黏度随温度升高发生异常变化,当温度升高至1000—1075 K时,黏度由快速增大转变为逐渐减小,即发生转折变化;在随后的降温和第二次升温过程中,黏度随温度变化呈指数规律单调递增(减),符合Arrhenius方程式.在实验研究基础上,采用剩余键结构模型和"平均原子集团"演变行为的计算模型讨论Mg-9Al熔体的黏度与微观结构之间的相关性,结果表明:类关键词： 合金熔体 结构与黏度相关性 剩余键结构 平均原子集团模型