Amorphous solidification in polymer-platelet nanocomposites

Computer simulations are used to understand the molecular basis of the rheology changes in polymer melts when loaded with platelet filler particles, specifically when the polymer and nanofiller interact attractively. With decreasing temperature, there is increasing aggregation between chains and filler and an increase in the polymer matrix structural relaxation time. These lifetimes are predicted to diverge at an extrapolated temperature, which we identify with the emergence of an amorphous solid state. Our findings suggest that filled polymers are phenomenologically similar to solutions of associating polymers and to supercooled liquids near their glass transition.     

Arm retraction potential of branched polymers in the absence of dynamic dilution

We study the stress relaxation of model polymer networks containing low contents of star shaped and linear dangling polymers. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling chains residing in polymer networks, we were able to identify the effects of dynamic dilution clearly. Since in polymer networks the dynamic dilution effect is suppressed, we were able by the first time to experimentally test the validity of the potential for arm retraction proposed by Pearson and Helfand.     

Nanoscopic-confinement effects on local dynamics

The segmental dynamics of 1.5-2.0 nm polymer films confined between parallel solid surfaces is investigated with dielectric spectroscopy in polymer/silicate intercalated nanocomposites. The confinement effect is evident by the observation of a mode, much faster than the bulk-polymer alpha relaxation and exhibiting much weaker temperature dependence. This is discussed in relation to either the interlayer spacing restricting the cooperative volume of the alpha relaxation or to the dominance of the more mobile interphase regions as predicted by simulations; the data qualitatively support the former.     

Structure of a tethered polymer under flow using molecular dynamics and hybrid molecular-continuum simulations

We analyse the structure of a single polymer tethered to a solid surface undergoing a Couette flow. We study the problem using molecular dynamics (MD) and hybrid MD-continuum simulations, wherein the polymer and the surrounding solvent are treated via standard MD, and the solvent flow farther away from the polymer is solved by continuum fluid dynamics (CFD). The polymer represents a freely jointed chain (FJC) and is modelled by Lennard-Jones (LJ) beads interacting through the FENE potential. The solvent (modelled as a LJ fluid) and a weakly attractive wall are treated at the molecular level. At large shear rates the polymer becomes more elongated than predicted by existing theoretical scaling laws. Also, along the normal-to-wall direction the structure observed for the FJC is, surprisingly, very similar to that predicted for a semiflexible chain. Comparison with previous Brownian dynamics simulations (which exclude both solvent and wall potential) indicates that these effects are due to the polymer–solvent and polymer–wall interactions. The hybrid simulations are in perfect agreement with the MD simulations, showing no trace of finite size effects. Importantly, the extra cost required to couple the MD and CFD domains is negligible.     

Simulations of nematic homopolymer melts using particle-based models with interactions expressed through collective variables

We develop a hybrid Monte Carlo approach for modelling nematic liquid crystals of homopolymer melts. The polymer architecture is described with a discrete worm-like chain model. A quadratic density functional accounts for the limited compressibility of the liquid, while an additional quadratic functional of the local orientation tensor of the segments captures the nematic ordering. The approach can efficiently address large systems parametrized according to volumetric and conformational properties, representative of real polymeric materials. The results of the simulations regarding the influence of the molecular weight on the isotropic-nematic transition are compared to predictions from a Landau-de Gennes free energy expansion. The formation of the nematic phase is addressed within Rouse-like dynamics, realized using the current model.     

Stress relaxation in entangled melts of unlinked ring polymers

Stress relaxation in unlinked ring polymer melts poses an important challenge to our theoretical understanding of entangled polymer dynamics. Recent experiments on entangled unlinked ring melts show power-law stress relaxation with no hint of a rubbery plateau, usually the hallmark of entangled polymers. Here we present a theory for stress relaxation in rings analogous to the successful approach for star polymers. We augment our theory with mesoscale Monte Carlo dynamics simulations of equivalent "lattice animal" configurations. We find a stress relaxation function G(t)～t(-α) with α≈1/2 consistent with experiment, emerging ultimately from the disparate relaxation times of more- and less-central portions of ring conformations.     

Structural relaxation and dynamic heterogeneity in a polymer melt at attractive surfaces

Molecular dynamics simulations of polymer melts at flat and structured surfaces reveal that, for the former, slow dynamics and increased dynamic heterogeneity for an adsorbed polymer is due to densification of the polymer in a surface layer, while, for the latter, the energy topography of the surface plays the dominant role in determining dynamics of interfacial polymer. The dramatic increase in structural relaxation time for polymer melts at the attractive structured surface is largely the result of dynamic heterogeneity induced by the surface and does not resemble dynamics of a bulk melt approaching T(g).     

Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

We demonstrate that molecular dynamics simulations are a versatile tool to ascertain the interpretation of spin–lattice relaxation data. For ¹H, our simulation approach allows us to separate and to compare intra- and inter-molecular contributions to spin–lattice relaxation dispersions. Dealing with the important example of polymer melts, we show that the intramolecular parts of ¹H spectral densities and correlation functions are governed by rotational motion, while their inter-molecular counterparts provide access to translational motion, in particular, to mean-squared displacements and self-diffusion coefficients. Exploiting that the full microscopic information is available from molecular dynamics simulations, we determine the range of validity of experimental approaches, which often assume Gaussian dynamics, and we provide guidelines for the determination of free parameters required in experimental analyses. For ²H, we examine the traditional methodology to extract correlation times of complex dynamics from relaxation data. Furthermore, based on knowledge from our computational study, it is shown that measurement of ²H spin–lattice relaxation dispersions allows one to disentangle the intra- and inter-molecular contributions to the corresponding ¹H data in experimental work. Altogether, our simulation results yield a solid basis for future ¹H and ²H spin–lattice relaxation analysis.     

Physical and chemical effects of ultrasound vibration on polymer melt in extrusion

The physical and chemical effects of ultrasound on polypropylene (PP) melts in extrusion were investigated. By applying ultrasound vibration to the entrance of the die, apparent pressure and viscosity of PP can be obviously decreased under the appropriate ultrasound power. Ultrasound has both physical and chemical effects on the polymer melt. In our study with specific polymer and ultrasound system, we determined that the chemical effect makes up 35–40% of the total effect of ultrasound on the apparent viscosity reduction of PP melts at most of the studied intensities. The physical effect plays a more important role in the ultrasound-applied extrusion than the chemical effect. This chemical effect is an irreversible and permanent change in molecule weight and the molecular-weight distribution due to ultrasound. As the ultrasound intensity increases, the molecular weight of PP reduces and its molecular-weight distribution becomes narrower; the orientation of PP molecules along the flow direction reduces (in melt state) and the crystallinity of PP samples (in solid state) decreases by applying the ultrasound vibration. Ultrasound vibration increases the motion of molecular chains and makes them more disorder; it also affects the relaxation process of polymer melts by shortening the relaxation time of chain segments, leading to weakening the elastic effect and decreasing the extruding swell ratios. All the factors discussed above reduce the non-Newtonian flow characteristics of the polymer melt and result in the viscosity drop of the polymer melt in extrusion.     

中空结构Pt纳米粒子热稳定性和形变的分子动力学研究

不同于实心结构纳米粒子,中空结构Pt纳米粒子具有低密度、高孔隙率和大的比表面积等特点,具有特殊的物理化学性质,在催化、光电子和药物输送领域有重要的应用.纳米粒子热稳定性和形变特性对其合成、应用具有重要的影响.利用分子动力学模拟研究中空结构Pt纳米粒子结构稳定性和形变过程,对不同壳层厚度的Pt纳米粒子进行分析,结果表明:在弛豫过程,温度为0.1 K时,壳层厚度为0.5 nm的Pt纳米粒子结构将发生形变,壳层厚度为1 nm、1.5 nm、2.0 nm、2.5 nm和3 nm纳米粒子结构保持几乎不变;升温过程,中空结构Pt纳米粒子塌缩时对应的温度随着壳层厚度增加而升高;塌缩过程所经历的温度区间很窄,空心结构短时间内突变为实心结构,另外,中空结构纳米粒子塌缩后,内部原子重新排列,仍保持有序的fcc结构.     

模耦合理论计算纳米粒子在聚合物熔体中的含时扩散系数与常规扩散常数 (cited: 2)

分析和计算了纳米粒子在聚合物熔体中的含时扩散系数与常规扩散常数. 采用广义朗之万方程描述扩散动力学,并通过模耦合理论计算摩擦记忆内核.为简单起见,只考虑了来自两体碰撞和溶剂密度涨落耦合作用两类微观因素对摩擦记忆内核的贡献. 采用聚合物参考作用点模型以及Percus-Yevick闭合条件计算了聚合物-纳米粒子复合溶液的平衡态结构信息函数;详尽分析了纳米粒子的尺寸与聚合物链的尺寸对扩散动力学的影响. 揭示了结构函数、摩擦记忆内核以及扩散系数等随着纳米粒子半径和聚合物链长的变化关系. 结果表明,对于小尺寸的纳米粒子或者短链的聚合物,短时间的非马尔可夫扩散 动力学特征比较显著,含时扩散系数需要更长的时间弛豫到常规扩散常数. 微观因素对扩散常数的贡献随着纳米粒子尺寸的增加而减小,却随着聚合物链长的增加而增大. 此外,模耦合理论得到的扩散常数与Stokes-Einstein关系的预测值进行比较,发现对于小尺寸的纳米粒子或者长链的聚合物,微观因素对扩散常数的的贡献占主导地位. 相反,当纳米粒子较大或者聚合物链长较短时,流体力学的贡献会发挥重要作用.     

纳米粒子在自吸附半柔性高分子诱导下成有序的环形结构

采用动态蒙特卡洛方法研究了自吸附半柔性高分子及其自吸附半柔性高分子与纳米粒子组成的混合物的构象行为.自吸附半柔性高分子的构象行为的研究,结果显示通过自吸附参数和弯曲能使高分子从一个线团状构象转变成一个紧密螺绕环状构象.紧密螺绕环状构象的折叠过程分为三个阶段:(i)几个分立的螺绕环(ii)一个松散的螺绕环(iii)一个紧密螺绕环.自吸附半柔性高分子与纳米粒子组成的混合物的构象行为的研究,结果表明可以采用具有紧密螺绕环结构的高分子有效地调控纳米粒子的空间排列.另外观察到一个非常有趣的现象,纳米粒子排列成一个环形结构.     

Femtosecond optical investigation of electron–lattice interactions in an ensemble and a single metal nanoparticle

Electron–lattice energy exchange is investigated in an ensemble of silver nanoparticles of mean diameter 9 nm and in a single 30-nm particle using a femtosecond pump–probe technique. The dependences of the measured transient transmission change and of the electron energy loss kinetics on the excitation amplitude are compared to the results of numerical simulations of nonequilibrium electron relaxation and of the two-temperature model. The good agreement between the theoretical and experimental data indicates that, for the studied low particle density samples, hot-electron cooling is dominated by electron–lattice coupling in a nanoparticle both for weak and large electron heating with a minor influence of their surrounding environment (glass or polymer matrix). PACS 78.47.+p; 42.65.-k; 73.20.Mf     

Dispersion of nanoparticles: From organic solvents to polymer solutions

This work is devoted to a systematic study of nanoparticle dispersion by ultrasonication in different solutions: from organic solvents to polymer solutions. The cluster size of nanoparticles at different concentrations in both organic solvents and polymer solutions were directly characterized by Dynamic Light Scattering to study the effect of solid concentration, surfactant and polymer on the dispersion. It reveals that in stabilized suspensions, the smallest attainable size or aggregate size of nanoparticles is independent of solvent type and solid content over the tested range. Furthermore, nanoparticles in simple solvent and in polymer solutions had the similar evolution of cluster size and almost the same final size, which could be very helpful to optimize the dispersion of nanofillers in polymer solutions and nanocomposites. It is also shown that, with appropriate sonication amplitudes, the dispersion procedure developed for very dilute suspensions could be transferred to higher concentration suspensions or even to polymer suspensions.     

Interaction between two solid surfaces across PDMS: influence of chain length and end group

Forces between solid surfaces across polymer melts are poorly understood despite their importance for adhesion and composite materials. Using an atomic force microscope (AFM) this force was measured for poly(dimethyl siloxane) (PDMS) on silicon oxide. The influence of molecular weight (4.0–40 kDa) was studied. Forces are attractive for low and repulsive for high molecular weight. In addition, changing the terminal methyl group for a hydroxyl group leads to an increased probability of bridging.     

On the chemical potential of nanoparticle dispersion

Understanding of the mechanism describing the chemical potential of nanoparticle dispersions, whether from modelling or experimental perspectives, is missing in the literature. As nanofluids are widely used in engineering applications, predicting material properties correctly needs a correct formulation for their behaviour. Often, the chemical potential of mixing is used for such expressions. Although quite appropriate for polymer blends or binary solutions, it is not suitable for nanoparticle dispersions. This work proposes a new mechanism for the chemical potential of dispersions or suspensions from thermodynamic principles, relying on porous flow principles, proposing that it is the fluid that diffuses in between the nanoparticles. The proposed model is applied in the case of mass diffusion and the results compare well with molecular dynamics results and several experimental data, motivating the proposed mechanism for dispersions.     

聚酰亚胺/铜纳米颗粒复合物的分子动力学模拟研究

通过分子动力学模拟对聚酰亚胺/铜纳米颗粒复合物的形态结构、 热力学性质、力学特性进行计算, 分析其随模拟温度和纳米颗粒尺寸的变化规律. 模拟结果表明, 聚酰亚胺/铜纳米颗粒复合物为各向同性的无定形态结构, 铜纳米颗粒与聚酰亚胺基体之间通过较强的范德华作用结合在一起使结构更加稳定, 铜纳米颗粒表面多个原子层呈现无定形状态, 在铜颗粒和聚酰亚胺基体之间形成界面层, 界面区域随颗粒尺寸和温度的增加分别减小和增加. 聚酰亚胺/铜纳米颗粒复合物的等容热容随着颗粒尺寸增大而明显增高, 随温度变化比聚酰亚胺体系更为缓慢, 在较低温度下较小颗粒尺寸复合物的热容比聚酰亚胺体系更低. 聚酰亚胺/铜纳米颗粒复合物的热压力系数随颗粒尺寸增加而显著增大, 比聚酰亚胺体系的热压力系数更小, 且随温度升高而减小的程度要小得多. 聚酰亚胺/铜纳米颗粒复合物的热力学性质表现出明显的尺度效应, 温度稳定性明显高于聚酰亚胺体系. 聚酰亚胺/铜纳米颗粒复合物的力学特性表现出各向同性材料的弹性常数张量, 具有比聚酰亚胺体系更低的杨氏模量和泊松比, 随温度升高分别减小和增大, 与聚酰亚胺体系随温度的变化趋势相反, 且杨氏模量的温度稳定性显著提高, 同时泊松比随纳米颗粒尺寸增大而减小, 具有明显的尺度效应. 加入铜纳米颗粒形成复合物可获得与聚酰亚胺体系显著不同的力学新特性. 关键词： 分子动力学模拟 聚合物纳米复合物 聚酰亚胺 纳米颗粒     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.