丙三醇浓溶液玻璃化转变与结构松弛参数DSC分析

利用差示扫描量热仪研究了5种高浓度丙三醇水溶液(60%、70%、80%、90%、100%)的玻璃化转变行为,以考察水分含量和升降温速率对其玻璃化转变行为和结构松弛参数的影响.采用4种线性升降温速率(10、15、20、25K/min)获得玻璃化转变的相关参数.利用GordonTaylor方程对玻璃化转变温度的分析结果表明,水对丙三醇增塑常数的计算结果与升降温速率和玻璃化转变温度的读取方法有很大关系.玻璃化转变过程的比热容变化不仅随水分含量的增加而增加,而且与升降温速率也有一定的依赖关系.结构松弛活化能的计算结果表明,随体系水分含量的增加,体系的结构松弛活化能和动力学脆度都逐渐降低.随水分含量的变化,热力学脆度和动力学脆度表现出相反的变化趋势.     

椭球胶体在圆球胶体体系中扩散行为的实验研究

复杂受限介质中的扩散行为在自然界是普遍存在的,与其相关的研究涉及物理学、材料科学和生物学等多学科领域,受到了这些领域研究者们的广泛关注.然而,相比于众多的圆球受限扩散研究,对形状各向异性的粒子在复杂受限介质中的扩散行为的研究依然比较匮乏.本文提出了一个简单的软物质实验模型—胶体椭球与圆球混合体系,来研究形状各向异性的椭球在圆球的受限环境下的扩散行为.通过描述椭球的运动轨迹和计算粒子的均方位移、范霍夫自关联函数以及非高斯参量,发现随着圆球浓度的增大,椭球的平动和转动都被抑制,出现次扩散行为;并且,平动和转动的位移分布也展现出不同的演化行为,表明这两种运动在高浓度下会发生解耦合.此外,在不同圆球浓度下,椭球都趋向于沿自身长轴方向扩散,因此在沿长轴和短轴方向的平动受到的受限作用的影响也不同,导致二者也发生解耦合行为.综上所述,受限环境会导致各向异性胶体粒子出现反常扩散行为.本文的研究有助于理解复杂环境中各向异性物体的扩散和输运行为.     

活性-非活性二元混合体系玻璃化转变的模耦合理论（英文）

近年来,活性粒子系统的集体行为引起了人们的广泛关注.本文针对活性-非活性(Active-passive, AP)布朗粒子二元混合系统的玻璃化转变行为提出了一个模耦合理论框架,理论从一个有效的Smoluchowski方程出发,它描述了在位置相空间中概率分布函数的演化.之后假设体系存在一个非平衡稳态,就可以得到一个描述中间自散射函数的随时间弛豫的积分微分方程.在此方程中有一个记忆函数,它在模耦合近似下又可以写成中间自散射函数的乘积的线性组合,于是构成了一个封闭的方程组.活性粒子的作用体现在方程中的一个有效扩散系数,它可以通过较短时间的计算机数值模拟求出.通过计算中间自散射函数的长时极限,即Debye-Waller(DW)因子,可以判断体系所处的相态,并进一步得出玻璃化转变的临界体积分数η_C.同之前的模拟结果一致.另外发现对粒子大小相同的AP混合系统,临界体积分数η_C随着活性粒子组分的粒子数占比x_A的增大而增大;而对于粒子大小不同的AP混合系统,发现了一个有趣的非单调行为,即临界体积分数η_C不再随活性粒子占比x_A单调增加.但是,如果活性足够大,这样的非单调行为便会消失.总之,本文的理论框架提供了一个研究活性-非活性混合系统的玻璃化转变行为的有效途径.     

振荡场作用下聚合物/纳米棒混合体系的自组装

采用元胞动力学和布朗动力学联用方法研究了振荡场作用下两嵌段共聚物/均聚物/纳米棒混合体系的自组装相行为.通过计算模拟,探讨了振荡场的振幅和频率对混合体系相形貌形成和演化的影响.研究发现振荡场对体系有序结构的形成和转变有重要作用,随着外加振荡场频率的增大,混合体系形貌从平行于场方向的条纹结构过渡到斜层状结构再转变为垂直于场方向的条纹结构.进一步分析了振荡场作用下体系畴尺寸的演化及纳米棒取向角的变化情况.研究结果为制备和调控聚合物纳米复合材料有序结构提供了新的方法和参考.     

聚甲基丙烯酸甲酯/镓纳米复合物的动力学弛豫行为

通过原位自由基聚合方法制备了聚甲基丙烯酸甲酯(PMMA)以及含镓(Ga)质量分数为11.3%和13.5%的PMMA/Ga纳米复合材料.在玻璃化转变温度及以上温区,利用能量耗散技术研究PMMA/Ga纳米复合物的动力学弛豫行为,发现随着Ga含量的增加,复合物的α弛豫峰移向高温但峰高降低.此外,还定量研究了Ga含量对PMMA/Ga复合物的α' 弛豫的影响,并作出了相应的解释. 关键词： 相对能量耗散 玻璃化转变 力学弛豫     

非晶纳米Ni500团簇等温晶化过程中的结构与动力学研究

采用分子动力学方法和镶嵌原子势, 模拟了500个Ni原子(简称Ni₅₀₀)组成的纳米团簇的等温晶化过程. 通过对纳米Ni团簇的动力学行为和微观结构演变的研究, 发现Ni₅₀₀在高温时是一步晶化的, 在低温时则呈现出多步晶化的特征. 在多步晶化的过程中, 团簇结构会陷入多个亚稳态结构, 经过原子重排, 进入能量更低的亚稳态, 最后完成晶化. 在多步晶化过程中, 原子的位置重排是通过协同跳跃运动实现的, 其协同运动方式不但有常见的线型协同运动, 也有多个原子的集体平移运动等其他形式. 关键词： 分子动力学模拟 纳米Ni团簇 协同运动     

冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响

用分子动力学模拟方法研究了五种不同冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响.采用两种不同的方法来确定玻璃转变温度Tg,并且对结晶温度Tc、径向分布函数g(r)、均方位移函数MSD与扩散系数D、平均配位数进行比较分析.结果表明:冷却速率影响Lennard-Jones体系凝固过程中的结构.当使用足够高的冷却速率冷却时,体系发生玻璃化转变,而且冷却速率越快,玻璃转变温度越高;当冷却速率较小时,体系形成晶体,而且冷却速率越慢,结晶温度越高,结晶程度也越高.同时发现,冷却速率对扩散系数和平均配位数也有很大影响,二者在体系发生玻璃转变时都有一个缓变的过程,表明了过冷液相区的存在.     

基于Logistic函数模型的纳米自组装动力学分析

利用快速混合停流吸收(stopped-flow absorption)技术,研究了半胱氨酸分子介导的金纳米棒线性自组装过程的动力学性质.通过观测金纳米棒的表面等离激元动态吸收光谱,分析其自组装动力学行为及其与组装结构之间的关系.研究表明,传统的二阶反应动力学理论模型在描述金纳米棒自组装动力学行为上存在明显的局限性.由此,我们提出了基于Logistic函数的新的动力学分析模型.与传统的理论模型相比,新的理论模型具有更好的普适性,不仅适用于定量分析不同速率的金纳米棒自组装动力学特征,还提供了一种更加准确地描述组装初期动力学行为的方法.此外,这种新的动力学分析方法还有助于理解和建立金纳米棒组装动力学特征与组装体结构之间的关联.     

共聚聚醚酯酰胺的多重转变及其相态结构

以自行合成的N ,N’ 双 (对甲酯基苯甲酰 )丁二胺 (T4T)为硬段 ,市售平均分子质量为 10 0 0的聚氧丁撑(PTMO10 0 0 )和 1,5 戊二醇 (PDO)的混合体系为软段 ,制备出了PTMO10 0 0 /PDO不同摩尔比的一系列共聚聚醚酯酰胺(T4T PTMO10 0 0 /PDO) .应用H1 NMR分析了单体T4T的化学组成及纯度 ,用差示扫描量热法 (DSC)、动态力学分析(DMA)研究了共聚物的熔融转变及玻璃化转变 ,发现当PDO含量超过 40 %摩尔分数时 ,共聚物显示出两个玻璃化转变和两个熔融转变 ,并且两个玻璃化转变温度均不随组成的改变而变化 ,而熔点则随PDO含量的增加而逐渐提高 .由此认为 ,共聚物中存在两个完全分离的无定形相区和两个具有不同结晶结构的结晶相区 .另外 ,推断有一些PDO链以邻近折叠 (adjacentre entry)的形态位于结晶相的边缘     

冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响

用分子动力学模拟方法研究了五种不同冷却速率对Lennard-Jones体系凝固过程中结构与动力学性质的影响。采用两种不同的方法来确定玻璃转变温度Tg,并且对结晶温度Tc、径向分布函数g(r)、均方位移函数MSD与扩散系数D、平均配位数进行比较分析。结果表明：冷却速率影响Lennard-Jones体系凝固过程中的结构。当使用足够高的冷却速率冷却时,体系发生玻璃化转变,而且冷却速率越快,玻璃转变温度越高;当冷却速率较小时,体系形成晶体,而且冷却速率越慢,结晶温度越高,结晶程度也越高。同时发现,冷却速率对扩散系数和平均配位数也有很大影响,二者在体系发生玻璃转变时都有一个缓变的过程,表明了过冷液相区的存在。     

Phase behavior of dispersions of hard spherical particles

A series of experiments on concentrated dispersions of hard colloidal spheres is discussed. The observed phase behavior is analogous to that of simple atomic systems: colloidal fluid, crystal and glass phases are found. The structure of the crystals, revealed by light diffraction, is a strongly faulted stacking of hexagonally-packed layers of particles. Dynamic light scattering confirms that the concentration of the metastable fluid phase for which long-ranged particle diffusion ceases coincides with the concentration where the glass transition is observed macroscopically. In studies of a binary mixture of colloidal spheres with a size ratio 0.61 eutectics, glass formation and the AB₁₃ type alloy structure have been identified.     

Demixing and nematic behaviour of oblate hard spherocylinders and hard spheres mixtures: Monte Carlo simulation and Parsons–Lee theory

Parsons–Lee approach is formulated for the isotropic–nematic transition in a binary mixture of oblate hard spherocylinders and hard spheres. Results for the phase coexistence and for the equation of state in both phases for fluids with different relative size and composition ranges are presented. The predicted behaviour is in agreement with Monte Carlo simulations in a qualitative fashion. The study serves to provide a rational view of how to control key aspects of the behaviour of these binary nematogenic colloidal systems. This behaviour can be tuned with an appropriate choice of the relative size and molar fractions of the depleting particles. In general, the mixture of discotic and spherical particles is stable against demixing up to very high packing fractions. We explore in detail the narrow geometrical range where demixing is predicted to be possible in the isotropic phase. The influence of molecular crowding effects on the stability of the mixture when spherical molecules are added to a system of discotic colloids is also studied.     

Glass transitions in quasi-two-dimensional suspensions of colloidal ellipsoids

We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as they approach the glass transitions. The clusters of translational and rotational fastest-moving ellipsoids formed mainly within pseudonematic domains and around the domain boundaries, respectively.     

Cooperative rearrangement regions and dynamical heterogeneities in colloidal glasses with attractive versus repulsive interactions

Water-lutidine mixtures permit the interparticle potentials of colloidal particles suspended therein to be tuned, in situ, from repulsive to attractive. We employ these systems to directly elucidate the effects of interparticle potential on glass dynamics in experimental samples composed of the same particles at the same packing fractions. Cooperative rearrangement regions (CRRs) and heterogeneous dynamics are observed in both types of glasses. Compared to repulsive glasses, the attractive glass dynamics are found to be heterogeneous over a wider range of time and length scales, and its CRRs involve more particles. Additionally, the CRRs are observed to be stringlike structures in repulsive glasses and compact structures in attractive glasses. Thus, the experiments demonstrate explicitly that glassy dynamics can depend on the sign of the interparticle interaction.     

Mixtures of anisotropic and spherical colloids: Phase behavior, confinement, percolation phenomena and kinetics

Purely entropic systems such as suspensions of hard rods, platelets and spheres show rich phase behavior. Rods and platelets have successfully been used as models to predict the equilibrium properties of liquid crystals for several decades. Over the past years hard particle models have also been studied in the context of non-equilibrium statistical mechanics, in particular regarding the glass transition, jamming, sedimentation and crystallization. Recently suspensions of hard anisotropic particles also moved into the focus of materials scientists who work on conducting soft matter composites. An insulating polymer resin that is mixed with conductive filler particles becomes conductive when the filler percolates. In this context the mathematical topic of connectivity percolation finds an application in modern nano-technology. In this article, we briefly review recent work on the phase behavior, confinement effects, percolation transition and phase transition kinetics in hard particle models. In the first part, we discuss the effects that particle anisotropy and depletion have on the percolation transition. In the second part, we present results on the kinetics of the liquid-to-crystal transition in suspensions of spheres and of ellipsoids.     

Colloidal glass transition observed in confinement

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.     

Three-dimensional structure of polystyrene colloidal crystal by synchrotron radiation X-ray phase-contrast computed tomography

Colloidal crystal with long-range ordered structure has attracted great attention for their applications in various fields. Although perfect colloidal crystals have been achieved by some fabrications for utilization, little is known about their exact structures and internal defects. In this study, we use synchrotron radiation (SR) phase-contrast computed tomography (CT) to noninvasively access the internal structure of polystyrene (PS) colloidal crystals in three dimensions (3D). The phase-attenuation duality Paganin algorithm phase retrieval was employed to achieve a satisfactory contrast and outline of the spheres. After CT reconstruction, the positions of individual PS particles and structural defects are identified in three dimensions, and the local crystal structure is revealed. Further quantitative analysis of the void system in colloidal crystal illustrates that single voids can be mostly attributed to tetrahedron void of sphere close packing, but the interconnected voids with large volume induce a sphere volume fraction of 59.39 % that reflects a metastable glass behavior of colloidal crystal arrangement. The void orientation result reveals that the 3D close-packing difficulty mainly lies in the stacking of interlayer.     

Digital imaging studies of submicron colloidal spheres confined into a single layer between two smooth glass plates: Two dimensional melting

Digital imaging is used to study the particle positions and trajectories of a layer of highly charged 0.3 μm diameter polystyrene spheres in water suspension, rigidly confined into two-dimensions between two smooth glass plates. As the in-plane sphere density is varied, we find a two-stage melting transition in which we observe separate divergences in the orientational and translational order at different densities, separated by roughly 4%. The topological defect statistics are complex, but qualitatively consistent with the predictions of Halperin and Nelson.     

Rheology of dense suspensions of shape anisotropic particles designed to show pH-sensitive anisotropic pair potentials

Here we investigate the flow properties of suspensions of dicolloidal particles composed of interpenetrating spheres where one sphere is rich in polystyrene and the second is rich in poly 2-vinyl pyridine. The synthesis method is designed to create both anisotropic shape and anisotropic interaction potentials that should lead to head to tail clustering. These particles are referred to as copolymer dicolloids (CDCs). The viscoelastic properties of stable and gelled suspensions of CDC particles are compared with analogs composed of homopolymer dicolloids (HDCs), having the same shape but not displaying the anisotropic attractions. After coating the particles with a nonionic surfactant to minimize van der Waals attractions, the flow properties of glassy and gelled suspensions of CDCs and HDCs are studied as a function of volume fraction, ionic strength and pH. Suspensions of HDC particles display a high kinetic arrest volume fraction (?(g)?>?0.5) over a wide range of pH and ionic strength up to [I]=0.5?M, demonstrating that the particles experience repulsive or weakly attractive pair potentials. Suspensions of CDC particles behave in a similar manner at high or low pH when [I]=0.001?M, but gel at a volume fraction of ?(g)?相似文献   

Multispeckle diffusing wave spectroscopy of colloidal particles suspended in a random packing of glass spheres

We use a multispeckle diffusing wave spectroscopy (MSDWS) method to study the ensemble-averaged dynamics of the fluctuating speckle pattern when illuminating colloidal particles suspended in a static and opaque porous medium with a coherent light source. Experiments were performed with Brownian latex particles in a random packing of glass spheres. The mixing of the light scattered by the moving colloidal particles and the porous matrix gives rise to a plateau value of the intensity autocorrelation function in the long-waiting-time limit. From the plateau in the correlation function, we can determine the fraction of light scattered from moving particles and estimate the photon mean free path in the colloidal solution. The method opens up promising possibilities to probe the static fraction in semisolid materials.