Molecular behaviors in thin film lubrication——Part two: Direct observation of the molecular orientation near the solid surface

Over the past twenty years, thin film lubrication(TFL) theory has been used to characterize the molecular behaviors in lubrication films thinner than 100 nm, effectively bridging the gap between elastohydrodynamic lubrication and boundary lubrication. Unfortunately, to date, the TFL molecular model proposed in 1996 has not been directly proven by experimental detection. Herein, a method based on surface-enhanced Raman spectroscopy was developed to show both the packing and orienting of liquid molecules in the TFL regime. By trapping liquid crystal molecules between a structured silver surface and a glass surface, molecular ordering states dominated by shear effect and surface effect were successfully distinguished. A nanosandwich structure consisting of an adsorbed layer, an ordered-molecule layer, and a fluid layer was demonstrated. Molecule imaging in TFL was achieved. Our results illustrate the molecular behaviors and lubrication mechanism in nanoconfined films and facilitate the lubrication design of nanoelectromechanical and microelectromechanical systems.

RHEOLOGICAL BEHAVIOR OF POLYMER MELTS (Ⅰ)UNIFIED MOLECULAR THEORY OF NON-LINEAR ISCOELASTICITY WITH CONSTRAINTS OF GAUSSIAN CHAIN ENTANGLEMENT FOR POLYMER MELTS

An approach with statistical mechanics and a unified molecular theory of non-linearviscoelasticity with constraints of Gaussian chain entanglement for polymer melts were proposed.Amultimode model structure for a single polymer chain with tail segments and N reversibleentanglement sites on the test polymer chain was developed.The probability distribution function ofthe end-to-end vector for a single polymer chain at entangled state and the viscoelastic free energyof deformation for polymer melts were calculated.Four types of stress-strain relationship and mem-ory function were derived from this theory.The above theoretical relationships were verified by experi-mental data of PS(polystyrene)and LDPE(low density polyethylene)melts.

Advances in studies of the tribological behavior of molecular deposition films

An overview of the advances in studies on tribology of molecular deposition （MD） films is presented here to summarize the studies of nanofrictional properties, adhesion, wear and mechanical behavior, as well as the molecular dynamics simulation of nanotribological properties of the film in the last decade. Some key research topics which need to be investigate further are addressed.

Molecular-Cytological Identification and Chromosome Behavior Analysis of Telotetrasomic in Rice

From the progenies of a telotrisomic of chromosome 9 short arm of an indica rice variety, Zhongxian 3037, a phenotypical variant was selected. The variant plant had rolled leaves, dispersed plant type, as well as a low seed-setting rate. Cytological and molecular cytological investigations revealed two extra chromosomes, which were the shortest in somatic cells of the variant. Fluorescent in situ hybridization （FISH） analysis using a rice centromere specific DNA （RCS2） and a DNA sequence specific for chromosome 9 on premetaphase and pachytene chromosomes showed that these two chromosomes were the short arms of chromosome 9. That is to say, the variant was a telotetrasomic of chromosome 9. Among the 25 pachytene cells, the two telosomic chromosomes paired each other to form a bivalent and didn＇t pair with other normal chromosome 9 as multivalents in 96% cells. However, the bivalent was easy to disassociate in advance.

Molecular Dynamics Studies on Thermal Behavior of a FAU-type Zeolite

Molecular Dynamics (MD) simulations of siliceous FAU-type zeolite were carried out at various temperatures. to investigate its thermal behaviors. From the study. we found that pure silicon fanjasite showed different thermal behaviors below 1500K and above 1500K. its cell volume gradually shrinks with the rising of the temperature below 1500K. the cell volume of the zeolite changes little above 1500K.

Influence of Molecular Weight of Ultra-high Molecular Weight Polyacrylonitrile on Its Rheological Behavior in Dimethylsulfoxide

Ultra-high molecular weight polyacrylonitrile (UHMW PAN ) was prepared by aqueous suspension polymerization, and the effect of molecular weight on its rheological behaviors in dimethylsulfoxide (DMSO) and the spinning stability were investigated. It shows that,compared with common polyacrylonitrile (C-PAN),UHMW- PAN/DMS0 solution has smaller non- Newtonian index, larger structural viscosity index, much longer maximum relaxation time, and no first- Newtonian region appears in the flow curves under the same experimental conditions. The explanations for these phenomena are given in the view of chain- entanglements. The optimal technology of preparing UHMW-PAN fibers and hollow fiber membranes could be obtained based on the theological study.

RHEOLOGICAL BEHAVIOR OF POLYMER MELTS (Ⅱ) MULTIPLE ENTANGLEMENT MODEL FOR PREDICTING THE DEPENDENCE OF VISCOELASTIC BEHAVIOR ON DEFORMATION STATE AND PRIMARY MOLECULAR WEIGHTS AND THEIR DISTRIBUTION

In accordance with the memory function of Gaussian chain constraints in entangled polymermelts a set of material functions related to a certain"test flow"are formulated from the O-W-Ftype constitutive equation by the appropriate selection of the Cauchy-Green and Finger tensors.Thedependences of these material functions on the strain rate and the dependences of the linearviscoelastic functions on the primary molecular weight distribution and the entanglement sites sequencedistribution on polymer chain are derived from a multiple-entanglement model and a couple of mech-anisms of relaxation for the loop and terminal entanglement sites.When the primary polymer chainsare modelled with the Lansing-Kraemer molecular distribution function,a set of new relationshipsamong linear viscoelastic functions(η0,ψ100,ηext0 and τ1)and the molecular weights and their distributionare formulated.These functions and relationships are verified with experimental data.

Methane hydrate formation and dissociation behaviors in montmorillonite

The methane hydrate formation and the methane hydrate dissociation behaviors in montmorillonite are experimentally studied. Through the analyses of the microstructure characteristic, the study obtains the porous characteristic of montmorillonite. It is indicated that methane hydrate in montmorillonite forms the structure I (si) crystal. Meanwhile, molecular dynamics simulation is carried out to study the processes of the methane hydrate formation and the methane hydrate dissociation in montmorillonite. The microstructure and microscopic properties are analyzed. The methane hydrate formation and methane hydrate dissociation mechanisms in the montmorillonite nanopore and on the montmorillonite surface are expounded. Combining the experimental and simulating analyses, the results indicate the methane hydrate formation and methane hydrate dissociation processes have little influence upon the crystal structure of porous media from either micro- or macro-analysis. It is beneficial to the fundamental researches on the exploitation and security control technologies of natural gas hydrate in deep-sea sediments.

Wetting behaviors of methanol,ethanol,and propanol on hydroxylated SiO2 substrate

Water molecules could form a liquid droplet on the water monolayer on a specific solid surface, which has been referred to as ‘‘ordered water monolayer that does not completely wet water'' at room temperature. In contrast to the water molecules, the family of alcohol molecules has the same OH polar head and various lengths of their hydrophobic nonpolar tail; the length of the hydrophobic tail can affect the hydrophobic effect. In this study, using molecular dynamics simulations, we investigated the wetting behaviors of methanol, ethanol, and propanol molecules adsorbed on a SiO_2 surface. The results showed that the methanol, ethanol, and propanol molecules could form an ordered monolayer on the SiO_2 surface and a droplet on top of this monolayer, with different contact angles. The differences in the contact angles were attributed to the differences in the interactions between the alcohol monolayer and droplet.

PHASE SEPARATION IN BIMODAL MOLECULAR WEIGHT HIGH DENSITY POLYETHYLENE WITH DIFFERING BRANCH CONTENTS BY MOLECULAR DYNAMICS AND MESODYN SIMULATION

The phase behavior of bimodal molecular weight high density polyethylene(BHDPE) in solid state was investigated.Hildebrand solubility parameters(δ) were calculated for the models of blends of higher molecular weight branch polyethylene(HBPE) with different branch contents and lower molecular weight linear polyethylene(LLPE),by using molecular dynamics(MD) simulations.These 3 values were then used to calculate the corresponding Flory-Huggins interaction parameter(x) between HBPE and LLPE models.In order to b...

Effect of amorphous lamella on the crack propagation behavior of crystalline Mg/amorphous Mg–Al nanocomposites

The effects of amorphous lamella on the crack propagation behavior in crystalline/amorphous（C/A） Mg/Mg–Al nanocomposites under tensile loading are investigated using the molecular dynamics simulation method. The sample with an initial crack of orientation（1210） [0001] is considered here. For the nano-monocrystal Mg, the crack growth exhibits brittle cleavage. However, for the C/A Mg/Mg–Al nanocomposites, the ‘double hump＇ behavior can be observed in all the stress–strain curves regardless of the amorphous lamella thickness. The results indicate that the amorphous lamella plays a critical role in the crack deformation, and it can effectively resist the crack propagation. The above mentioned crack deformation behaviors are also disclosed and analyzed in the present work. The results here provide a strategy for designing the high-performance hexagonal-close-packed metal and alloy materials.

EFFECT OF TEMPERATURE ON THE RHEOLOGICAL BEHAVIOR OF UHMW-PAN SOLUTION

UHMW-PAN is an important polymer material used to make high strength and high modules fiber as well as hollow fiber membrane with high mechanical strength to resist the backwash process.In this paper, the effect of temperature on non-Newtonian flow index, structural viscosity index,maximum relaxation time and zero shear viscosity of the UHMW-PAN solution has been studied.The relationship between parameters and spinnability has been discussed. On the basis of above works, the suitable temperature of spinning solution by gel-spinning has been found.

铜-铝电磁脉冲焊接界面形成过程的原子扩散行为

电磁脉冲焊接技术以高压脉冲放电驱使异种金属可靠连接而备受关注,但其界面结合机制尚不明确.该文搭建了铜-铝电磁脉冲焊接综合试验平台,捕获了焊接的动力学过程,得到碰撞点速度与碰撞角度的变化规律.在此基础上,构建了基于分子动力学模拟的电磁脉冲焊接典型界面(平直界面与涡旋界面)形成过程的对应模型,探究了焊接中的原子扩散行为,并根据模拟结果计算了典型结合界面的扩散层厚度,同时采用透射电子显微镜分析了结合界面的微观结构.研究结果表明,剧烈碰撞驱使界面材料塑性变形,界面材料塑性形变形成冶金结合和机械咬合是铜-铝电磁脉冲焊接界面的结合机制,且涡旋界面处的原子扩散厚度大于平直界面.该文可为深入理解电磁脉冲焊接机理和调控焊接效果提供科学依据.

Genetic behavior of Triticum aestivum–Dasypyrum villosum translocation chromosomes T6V#4S·6DL and T6V#2S·6AL carrying powdery mildew resistance

T6V#2S·6AL and T6V#4S·6DL translocation chromosomes developed from the cross of wheat and different Dasypyrum villosum accessions have good powdery mildew （PM） resistance, but their pairing and pyramiding behavior remains unclear. Results in this study indicated that the pairing frequency rate of the two differently originated 6VS chromosomes in their F1 hybrid was 18.9% according to genomic in situ hybridization （GISH）; the PM resistance plants in the F2 generation from the cross between T6V#4S·6DL translocation line Pm97033 and its PM susceptible wheat variety Wan7107 was fewer than expected. However, the ratio of the resistant vs. the susceptible plants of 15：1 in the F2 generation derived from the cross between the two translocation lines of T6V#2S·6AL and T6V#4S·6DL fitted well. Plants segregation ratio （homozygous：heterozygous：lacking） revealed by molecular marker for T6V#4S·6DL or T6V#2S·6AL in their F2 populations fitted the expected values of 1：2：1 well, inferring that the pairing of the two alien chromosome arms facilitates the transmission of T6V#4S·6DL from the F1 to the F2 generation. A quadrivalent was also observed in 21% of pollen mother cells （PMCs） of homozygote plants containing the two pairs of translocated chromosomes. The chromosome pairing between 6V#2S and 6V#4S indicates that it will be possible to obtain recombinants and clarify if the PM resistance determinant on one alien chromosome arm is different from that on the other.

茂金属聚乙烯薄膜料结构与性能

对3种茂金属聚乙烯薄膜料的基础性能、分子量及分布、熔融行为、力学性能和光学性能进行了分析测试,考察了不同工艺下制得的茂金属聚乙烯薄膜料的结构与性能的关系。结果表明:三种薄膜料的分子量分布均比较窄,呈单峰分布,且密度在0.921g/cm^(3)左右,其中具有更低分子量和更高支化度的3~#茂金属聚乙烯薄膜料具有更好的加工性能,并且其断裂标称应变较高、鱼眼少、雾度低。

Volatilization behaviors of molybdenum and sulfur in vacuum decomposition of molybdenite concentrate

Thermodynamic calculation, ab initio molecular dynamics(AIMD) and vacuum decomposition experiments were performed to study the volatilization behaviors of Mo and S from molybdenite concentrate by vacuum decomposition. In thermodynamic calculation, starting decomposition temperatures of reactions were calculated, and saturated vapor pressures of Mo, S and Mo S2 were also analyzed. In AIMD, geometries of the Sn(n≤8), Mom(m≤8) and MomSn(m+n≤8) clusters have been optimized using density functional theory(DFT) with generalized gradient approximation(GGA). And these clusters were simulated in DFT with Cambridge Sequential Total Energy Package(CASTEP) code of Material Studio software. Structures and stabilities of these clusters before and after molecular dynamics simulations were discussed, and diffusion coefficients were also calculated. In vacuum decomposition experiments, relationship between heat preservation time and volatilization rate of Mo and S was obtained, while the constant temperature and chamber pressure were 1823 K and 5–35 Pa, respectively. Above all, both the theoretical and experimental results showed that volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate were as follows: Mo could partly evaporate into the condensate in the form of clusters, and S could easily evaporate into the condensate.

RHEOLOGICAL BEHAVIOR OF ERWINIA GUM IN AQUEOUS SOLUTION

Erwinia (E) gum, an extracellular polysaccharide, is composed of fucose, galatose, glucose and glucuronic acid. Its viscosity behavior was investigated by a low-shear-rate multiball viscometer and a rotational viscometer. Its weight-average molecular weight M-w and intrinsic viscosity [eta] in 0.2 mol/L NaCl aqueous solution were measured by light scattering method at 35 degrees C and viscometry at 25 degrees C and found to be 1.06 x 10(6) g/mol and 1050 mL/g, respectively, and its aggregates in aqueous solution were proved by gel permeation chromatography (GPC). These results indicated that E gum in water has exceedingly high viscosity and exhibits Binham fluid behavior, owing to its aggregation. The viscosity of E gum decreased with increasing temperature, and the turning point appeared at 38 degrees C for dilute solution and 80 degrees C for concentrated solution suggesting that the aggregates of E gum in water started to disaggregate under these temperatures. In addition, the aggregates can be disrupted by adding either acid or base. The experimental results indicated that the E gum is a good thickening agent, and its fluid behavior is similar to xanthan.

单晶硅纳米磨削力热行为与亚表面损伤研究

纳米磨削作为实现单晶硅低损伤加工的技术之一被逐渐应用于硅片减薄中,但磨削过程中的力热行为及其对亚表面损伤形成的影响机制仍不清楚;因此,通过分子动力学仿真手段对单晶硅纳米磨削时的力热行为和亚表面损伤之间的联系进行研究。结果表明,单晶硅纳米磨削过程中切向磨削力对材料去除起主要作用,磨粒前下方区域的热量聚集和应力集中现象明显。在力热载荷作用下,非晶化和相变是单晶硅纳米磨削时亚表面损伤的主要形成机制。磨削力的增大会导致单晶硅去除过程中产生较大的亚表面损伤层,而一定的高温由于增强了单晶硅的韧性进而抑制了亚表面损伤层的形成。

TRISO颗粒SiC层辐照行为与力学性能的分子动力学模拟

碳化硅(SiC)材料对TRISO颗粒的安全性能有重要影响,因此有必要对SiC层的辐照行为和力学性能进行研究。本文采用分子动力学模拟对等轴状多晶和长轴状多晶两类SiC层进行辐照行为模拟,计算发现,SiC层的辐照肿胀程度和力学性能的理论值与实验值吻合较好。通过肿胀程度、密度、原子结构类型、点缺陷演化等参量详细考察了SiC层的辐照行为。结果表明,辐照过程中的非晶化存在晶体结构转化为中间态结构,再转化为非晶结构的过程。在辐照早期,点缺陷以C空位、Si间隙原子和C反位原子为主,但在辐照剂量趋于饱和后差异逐渐消失。非晶化和点缺陷演化倾向于从晶界附近开始发展。辐照会导致SiC层力学性能的降低,但在辐照剂量趋于饱和后不再有显著影响。微观分析表明,SiC层力学性能的降低与其在外力作用下的承受能力和塑性变形程度减小、应力应变分布紊乱密切相关。研究结果有助于理解TRISO颗粒SiC层的辐照行为演化和力学性能变化的关系。

医用级可降解聚乳酸材料的合成与应用

聚乳酸(PLA)因具有良好的生物相容性和生物降解性,广泛应用于包装材料、医用缝合线、组织工程支架和药物控制释放体系等医学领域。丙交酯开环聚合是制备聚乳酸的理想之选,其催化体系目前以金属催化剂为主,但金属离子在聚合物中的痕量残留和细胞毒性限制了聚乳酸在生物医学的应用。本文利用改进两步法合成了分子量可控的医用级可降解聚乳酸材料,采用^(1)H NMR、IR、GPC和HPLC确证了其结构,研究了十二醇加入量、聚合时间对合成聚乳酸分子量和性能的影响规律。利用3D打印技术设计成型了PLA骨钉,并研究了其降解行为、溶血率和细胞毒性。结果表明:聚合温度为130℃,反应时间为24 h,加入辛酸亚锡0.05%(质量分数,下同)、十二醇0.03%时,聚乳酸重均分子量在35万以上,纯度可达99.0%以上,分布指数为2.031;骨钉在PBS缓冲液中降解10周(w),残余量为23.4%,降解性能良好;溶血率小于5%,100%试验液的细胞存活率为75%,大于国标参考值70%,未发现潜在细胞毒性。