Thymol在不同分子链长度塑料膜中的释放动力学模拟

目的利用分子动力学模拟并分析不同PVA分子链长度下thymol的释放过程和释放机理。方法研究基体为聚乙烯醇(PVA),添加抗菌剂thymol共建PVA/thymol聚合物模型,进行分子动力学模拟,分别从均方位移、扩散系数、自由体积角度研究分子链长度分别为40,80,160下thymol在PVA聚合物中的释放过程。结果 thymol分子在PVA中的自由体积分数会随着分子链长度的增加而减小,thymol分子在PVA中的扩散系数也会相应减小,这是因为分子链越长,分子间的相互作用力越大,束缚了链的运动性。结论不同分子链长度对于包装材料抗菌剂中的释放具有重要影响。在同等条件下,短链结构中thymol分子在PVA中容易释放,且做缓慢蠕动运动而非跳跃扩散运动。     

基于分子模型的支化聚合物流变性能模拟

通过布朗(Brown)动力学方法模拟了有限伸展非线性弹性(FENE)珠-簧链支化聚合物分子模型在稳态和瞬态剪切流场中的运动,得到分子构象信息,同时对支化聚合物分子力场进行计算分析,模拟支化聚合物流变性质,研究支链长度、弹簧模型参数和剪切速率等对该模型定常与非定常流变性质的影响。结果表明,珠簧链模型能够较好地描述支化聚合物分子结构,随支链长度的增加,支化聚合物黏度升高,随剪切速率的升高,伴随有假塑性剪切变稀现象发生,并且弹簧拉伸常数不同,支化聚合物黏度稳态值也不同。     

FENE珠-簧链聚合物分子模型流变性质Brown动力学模拟

通过布朗(Brown)动力学方法模拟了有限伸展非线性弹性(FENE)珠-簧链分子模型在稳态简单剪切流场和突始剪切流场中的运动,得到分子链的位形变化轨迹,同时对聚合物分子力场进行计算分析,模拟流变性质,研究温度、弹簧参数和剪切速率等对该模型定常与非定常流变性质的影响。结果表明,随温度升高,聚合物黏度下降,随剪切速率的升高伴随有假塑性剪切变稀现象发生,并且弹簧拉伸常数不同,聚合物黏度稳态值也不同。     

气体分子在聚氯代对二甲苯中渗透行为的数值模拟

用数值模拟方法对气体分子在聚氯代对二甲苯(Parylene C)中的溶解、扩散和渗透行为进行了研究。构建了含300个对二甲苯单元的聚氯代对二甲苯分子链模型,经结构优化处理,获得了具有一定有序性,密度、玻璃化转变温度与实验数据接近的聚合物分子链结构模型。计算了该聚合物中的自由体积,结果表明Parylene C聚合物具有很小的自由体积,不易形成分子渗透的有效通道,这是其具有高气体阻隔性能的重要原因。结合分子动力学(MD)和巨正则蒙特卡洛方法(GCMC),计算获得了几种气体分子在Parylene C中的扩散系数、溶解系数和渗透系数,发现相比H2、He、O2、N2等分子,H2O分子更易被Parylene C聚合物吸附溶解。分子动力学模型说明在Parylene C聚合物中,气体扩散和渗透符合跳跃机理。     

受限体系聚合物扩散行为的研究进展

随着纳米技术的发展,纳米尺度聚合物材料的应用变得越来越普及,纳米尺度受限体系中聚合物分子链的扩散行为受到人们的广泛关注。由于受限效应,聚合物分子链的运动行为偏离本体,出现尺寸依赖性。研究受限体系中聚合物的扩散行为,对受限聚合物的结构设计及实际应用有十分重要的意义。文中从聚合物扩散基本理论出发,综述了近30年来聚合物分子链在不同维度受限体系中扩散行为的研究进展,介绍了不同受限状态下聚合物分子链扩散的物理机制、影响因素以及相关的理论模型,并对该领域进行了总结与展望。     

分子复合材料的研究进展

分子复合材料是指刚性高分子链(段)聚合物与柔性链聚合物(树脂)共混，刚性链高分子聚合物作为增强相在接近分子水平上均匀分散在柔性链(段)树脂基体中，达到最佳分子增强效应，形成高强度、高模量的复合材料。制备分子复合材料的关键在于溶混性。文中重点讨论了分子复合材料的制备方法与改进方法，简要介绍了分子复合材料的分类、特点及组成。     

半晶态与非晶态聚合物的微观变形机理

采用粗粒化聚乙烯醇模型,应用大规模分子动力学分别模拟半晶态和非晶态聚合物的单轴拉伸变形,探究两者潜在的变形机理。半晶态聚合物的应变软化过程主要发生在晶区,而应变强化过程主要体现在非晶区。晶区在屈服点发生滑移,初始滑移形成使晶区变化需要的拉伸力变小,而发生应变软化。随着应变的增大,各分子链段协同作用使二者分子链的解缠结达到最大,取向趋向于拉伸方向,从而使拉伸应力增大发生应变强化。重要的是还得到半晶态聚合物中的非晶区,表现出与非晶态聚合物相似的应力-应变行为。     

ATRC法合成蝌蚪型及杠铃型环状聚合物

具有特定臂数的星型聚合物通过分子内链末端偶联反应可以制备出不同形状的环状聚合物。文中以三臂含溴代异丁酸酯为引发剂(Tri-Br),通过先核后臂的方法制备出三臂星型聚苯乙烯(PSt_3)。然后将合成的三臂星型聚苯乙烯(PSt_3)在极稀浓度条件下,利用原子转移自由基偶合(ATRC)技术合成一端为大分子环状,一端为线型聚合物链的蝌蚪型环状聚合物(Tadpole-PSt_3)。最后在高浓度条件下,通过ATRC技术,使三臂聚苯乙烯未参与分子内偶联的线型链段进行分子间偶合制备出杠铃型环状聚合物(Barbell-PSt_3)。使用核磁共振、凝胶渗透色谱对聚合物结构进行了表征。结果表明,成功合成了纯度较高的蝌蚪型环状聚合物,同时进一步合成了中间为聚合物链段两端为大环的杠铃型环状聚合物。     

涤纶树脂的容积变化对热性能的影响

导言聚合物在其任何存在阶段都带有过去的痕迹。当制造成各种纤维、薄膜等形态时,经过温度、应力和溶剂作用,聚合物分子链经常在构型和取向上固定下来。在聚合物以比链段松弛更快的速度冷到玻璃态时,分子链段局限在体积单元中来不及调整到平衡。聚合物有能察觉的和不能察觉的变化潜势取     

橡胶纳米复合材料计算机模拟研究进展与挑战

总结了作者课题组采用分子动力学模拟研究橡胶纳米复合材料目前取得的主要进展,包括不同几何形状的纳米颗粒在橡胶基体中的分散机理、颗粒与橡胶分子链间的界面结合(聚合物玻璃化层是否存在)、颗粒对应力应变增强机理、碳纳米弹簧的引入对橡胶粘弹性的调控以及橡胶纳米复合材料非线性行为(Payne效应)产生的机理。模拟结果表明,存在一个最佳界面相互作用与接枝密度以实现纳米颗粒均匀分散;对于片状颗粒,在类似氢键界面相互作用时,存在聚合物玻璃化层。静态力学增强来自于两个方面:一是颗粒诱导分子链取向与排列,二是分子链吸附临近颗粒形成桥链在大变形下的有限链伸长。同时发现,碳纳米弹簧的加入会明显降低复合材料的滞后损失,并且得出纳米颗粒直接接触聚集与由分子链同时吸附多个颗粒成网对Payne效应非线性行为均有贡献。这些基础问题的澄清,将为制备动静态力学性能兼顾的橡胶纳米复合材料提供重要科学依据与理论指导,进而实现我国轮胎制品的高性能化与绿色化。最后针对橡胶纳米复合材料多层次多尺度结构与性能关系,简要评述了计算机模拟研究存在的挑战。     

Flow dynamics and wall shear-stress variation in a fusiform aneurysm

Pulsatile flow through a tube featuring a sinusoidal bulge is computed in order to determine the flow dynamics and wall shear-stress conditions encountered under conditions representative of blood flow through a human abdominal aortic aneurysm. A high-order spectral-element algorithm is employed to accurately determine velocity and vorticity fields, plus wall shear stresses, which are notoriously difficult to measure experimentally. A greater level of detail in the flow is revealed when compared to recent particle image velocimetry experiments. For both the mean and standard deviation of wall shear stress, minimum levels are found at the widest point of the aneurysm bulge, and maximum levels are recorded in the distal (downstream) region of the bulge. In an aneurysm with length and maximum diameter 2.9 and 1.9 times the artery diameter, respectively, peak instantaneous wall shear stress is 2.4 times greater than the peak wall shear stress recorded in a healthy vessel.     

Shear-flow Instability in Two-component Bose-Einstein Condensates

We theoretically study Kelvin-Helmholtz instability in phase-separated two-component Bose-Einstein condensates with shear flow at T=0. The stability of the shear flow state is investigated with the Gross-Pitaevskii and the Bogoliubov-de Gennes models, compared with a hydrodynamic model. The dynamics of the instability is revealed by numerically solving the Gross-Pitaevskii equation. In the nonlinear development, singly-quantized vortices are released from the interface between the two condensates.     

DYNAMICS OF AGGLOMERATE SIZE DISTRIBUTION IN LINEAR SHEAR FLOW FIELDS

A dispersive mixing model was developed and analyzed for the case when separation of the fragments following rupture of agglomerates primarily determines the dynamics of the mixing process. The model incorporates the effects of van der Waals forces within agglomerates and hydrodynamic interactions between fragments as they are convected apart from each other by the applied shear field. The essential features of the model are illustrated by examining the effect of shear on the breakage of parent agglomerates and their first five generations of fragments for a particular test case. In addition, the dynamics of the agglomerate size distribution produced by linear shear flow fields in gaps of various aspect ratios was analyzed for two types of initial conditions. These results provide insight to the design of the high shear zones in actual mixing equipment.     

磁流变液流变特性的数值模拟分析

建立了微观颗粒的动力学模型,提出了相应的算法,模拟了二维状态下颗粒成链及其剪切流动过程,表明剪切过程中存在旧链断裂和新链形成,得到了流动时磁流变液的剪切应力,并模拟了高剪切速率下微结构的演变过程,发现新链形成速度小于旧链断裂速度,出现了剪应力下降现象.     

Influence of particle elasticity in shear testers

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10% smaller than the microscopic friction coefficient between the grains.     

DYNAMICS OF AGGLOMERATE SIZE DISTRIBUTION IN LINEAR SHEAR FLOW FIELDS

ABSTRACT

A dispersive mixing model was developed and analyzed for the case when separation of the fragments following rupture of agglomerates primarily determines the dynamics of the mixing process. The model incorporates the effects of van der Waals forces within agglomerates and hydrodynamic interactions between fragments as they are convected apart from each other by the applied shear field. The essential features of the model are illustrated by examining the effect of shear on the breakage of parent agglomerates and their first five generations of fragments for a particular test case. In addition, the dynamics of the agglomerate size distribution produced by linear shear flow fields in gaps of various aspect ratios was analyzed for two types of initial conditions. These results provide insight to the design of the high shear zones in actual mixing equipment.     

Non-equilibrium all-atom molecular dynamics simulations of free and tethered DNA molecules in nanochannel shear flows

In order to gain insight into the mechanical and dynamical behaviour of free and tethered short chains of ss/ds DNA molecules in flow, and in parallel to investigate the properties of long chain molecules in flow fields, we have developed a series of quantum and molecular methods to extend the well developed equilibrium software CHARMM to handle non-equilibrium dynamics. These methods have been applied to cases of DNA molecules in shear flows in nanochannels. Biomolecules, both free and wall-tethered, have been simulated in the all-atom style in solvent-filled nanochannels. The new methods were demonstrated by carrying out NEMD simulations of free single-stranded DNA (ssDNA) molecules of 21 bases as well as double-stranded DNA (dsDNA) molecules of 21 base pairs tethered on gold surfaces in an ionic water shear flow. The tethering of the linker molecule (6-mercapto-1-hexanol) to perfect Au(111) surfaces was parametrized based on density functional theory (DFT) calculations. Force field parameters were incorporated into the CHARMM database. Gold surfaces are simulated in a Lennard-Jones style model that was fitted to the Morse potential model of bulk gold. The bonding force of attachment of the DNA molecules to the gold substrate linker molecule was computed to be up to a few nN when the DNA molecules are fully stretched at high shear rates. For the first time, we calculated the relaxation time of DNA molecules in picoseconds (ps) and the hydrodynamic force up to a few nanoNewtons (nN) per base pair in a nanochannel flow. The velocity profiles in the solvent due to the presence of the tethered DNA molecules were found to be nonlinear only at high shear flow rates. Free ssDNA molecules in a shear flow were observed to behave differently from each other depending upon their initial orientation in the flow field. Both free and tethered DNA molecules are clearly observed to be stretching, rotating and relaxing. Methods developed in this initial work can be incorporated into multiscale simulations including quantum mechanical, molecular and the microfluidic continuum regimes. The results may also be useful in extending existing macroscopic empirical models of DNA response dynamics in shear flows.     

Shear controllable synthesis of barium sulfate particles using lobed inner cylinder Taylor-Couette flow reactor

A novel lobed inner cylinder assembled in Taylor-Couette flow reactor (LTC) has been adopted to synthesize barium sulfate particles. The fluid dynamics that affects synthesis of particles using both the LTC and the classical Taylor-Couette flow reactor (CTC) was investigated through CFD modelling and experiments. The results have demonstrated that the Taylor vortices and turbulence induced shear rate distribution in the reactors have a significant influence on the final particle size distribution. The narrower shear rate distribution in the LTC is beneficial to the synthesis of particles with smaller size. The local turbulence intensification in the intra-Taylor vortices in the LTC effectively reduces the low shear strain regions. A strong correlation between the synthesized particle size and the local turbulent dissipation rate is existing. Shear induced by small turbulent eddies can inhibit particle growth. The LTC can be used for effectively shear controllable synthesis of particles.