Structural Interpretation of Eyring Activation Parameters for Tensile Yielding Behavior of Isotactic Polypropylene Solids

The temperature and strain rate dependence of the stress-strain curves of isotactic polypropylene were analyzed on the basis of Eyring kinetic theory. The molecular and structural basis underlying the parameters, such as activation volume and activation energy, used by the Eyring analysis were studied using the lamellar cluster theory, where a stacked lamella acts as a basic structural unit under yielding and necking deformation. It was suggested that, at lower temperatures, the activation volume corresponds to chain slippage within crystalline lamellae in the lamellar clusters whereas at higher temperature the increase in activation volume results in intralamellar slip corresponding to the α₂ relaxation.     

Structure and mechanical properties of nylon 6.12 prepared by temperature slope crystallization. II. Rolling deformation and microhardness of the oriented negative spherulite

Oriented negative spherulites of nylon 6.12 were crystallized by the temperature slope method. Rolling deformation of the negative texture was performed in three directions. The deformation mechanism of the negative spherulites was investigated by x-ray diffraction and microhardness measurements. Slip deformation between crystalline lamellae (interlamellar slip) was observed in the first stage of deformation. The results are compared with the rolling deformation of other polymer textures such as the positive spherulites of nylon 6.12 and β-phase isotactic polypropylene. In the rolling deformation of nylon 6.12, hydrogen-bonded (010) planes play an important role. For large deformations (λ > 1.5), preferential slip between (010) planes appears, resulting in a lamellar inclination of 60° and a decrease of the lamellar thickness. Microhardness measurements after rolling deformation of the oriented negative texture show good agreement with the structural analysis. The rapid decrease in the microhardness with increasing deformation can be explained by the lamellar thickness decrease brought about by the preferential slip between the (010) planes.     

Structure and Properties of Polypropylene/Polyolefin Elastomer/Organic Montmorillonite Nanocomposites

The crystallinity, mechanical properties, and thermal stability of polypropylene (PP)/organic montmorillonite (OMMT) and PP/polyolefin elastomer (POE)/OMMT composites, with polypropylene-g-maleic anhydride/styrene (PPMS) as a compatibilizer for both, were compared. The results showed that the strong interaction between the clay platelets and compatibilizer, which were generated by the maleic anhydride (MAH), improved the compatibility of the polymer matrices with the OMMT. A unique lamellar, flocculated structure of OMMT was formed after introduction of the POE. The highly dispersed clay layers could act as nucleating agents, resulting in smaller spherulites and higher crystallization temperatures. Compared with pure PP, the PP/OMMT nanocomposite showed enhanced mechanical properties and thermal stability; however, the PP/POE/OMMT had the best impact toughness.     

New and Simple Staining Method for Visualizing UHMWPE Lamellar Structure in TEM

Ultra-high-molecular-weight polyethylene (UHMWPE) is used in arthroplasty as a key component of total joint replacements. In order to increase its wear resistance, the polymer is often cross-linked in a two-step procedure comprising irradiation and thermal treatment. Both modification steps impact UHMWPE lamellar structure. In this study we introduce a new staining method that makes it possible to visualize the UHMWPE lamellae by transmission electron microscopy (TEM). The method consists of one-step staining with oleum (H ₂ SO ₄ solution of SO ₃ ) for 4 days and ultramicrotomy; this is significantly simpler than previously described techniques and yields micrographs of the same quality. The lamellar thicknesses from TEM correlated well with those from small- and wide-angle X-ray scattering and differential scanning calorimetry. It has also been demonstrated that the exact values of lamellar thicknesses obtained from TEM micrographs strongly depend on the place selected for image analysis.     

Cluster synchronization in networks of distinct groups of maps

In this paper, we study cluster synchronization in general bi-directed networks of nonidentical clusters, where all nodes in the same cluster share an identical map. Based on the transverse stability analysis, we present sufficient conditions for local cluster synchronization of networks. The conditions are composed of two factors: the common inter-cluster coupling, which ensures the existence of an invariant cluster synchronization manifold, and communication between each pair of nodes in the same cluster, which is necessary for chaos synchronization. Consequently, we propose a quantity to measure the cluster synchronizability for a network with respect to the given clusters via a function of the eigenvalues of the Laplacian corresponding to the generalized eigenspace transverse to the cluster synchronization manifold. Then, we discuss the clustering synchronous dynamics and cluster synchronizability for four artificial network models: (i) p-nearest-neighborhood graph; (ii) random clustering graph; (iii) bipartite random graph; (iv) degree-preferred growing clustering network. From these network models, we are to reveal how the intra-cluster and inter-cluster links affect the cluster synchronizability. By numerical examples, we find that for the first model, the cluster synchronizability regularly enhances with the increase of p, yet for the other three models, when the ratio of intra-cluster links and the inter-cluster links reaches certain quantity, the clustering synchronizability reaches maximal.     

团簇连接对Cu10Ag90快速凝固中晶体结构的影响

快速凝固得到的晶体通常都会形成多种孪晶结构,但其成因一直是一个谜。本文采用分子动力学模拟Cu10Ag90合金在0、20和40 GPa压强下的快速凝固过程,分别形成了五重孪晶、层片状孪晶和复杂晶体,通过最大标准团簇方法(LaSCA)分析了短程有序结构的演变和晶体团簇之间的连接形式。结果发现,结晶转变温度Tc随着压强的增大而升高,平均原子势能也随之增加。非晶共有近邻子团簇(CNS)会在温度降至Tc时转变为晶体CNS,同时晶体团簇数量急剧增加,结构有序度提升。此外,通过分析晶体团簇之间的连接方式发现,五重孪晶中心团簇截边十面体(tDh)只能与具有相同S422的HCP相连形成五重孪晶轴;FCC与HCP有相同的S421,但只能以彼此的顶层或者底层原子作为彼此的中心原子连接形成层片状结构;BCC与FCC、HCP虽然没有共同的CNS,但是可以通过D-S6结构及其变形体连接,其连接方式的多样性是形成复杂晶体的主要原因。这些结果为研究晶体结构不同排列方式提供了新的理解。     

Influence of structure on the dynamic mechanical properties of crystalline polymers

A dynamk mechanical theory is presented whose structural elements correspond to such physically measurable material parameters as the compliance of the phases, the percent crystallinity, and the orientation of the crystalline and noncrystalline regions. The theory is tested by using both structural and dynamk mechanical data from samples of isotactic polyproylene films of varying crystallinity and orientation. For the specific case of isotactic polypropylene the general theory is shown to reduce to a simple two-parameter model. It is further shown that the two-parameter model not only identifies the unique properties of each of the phases separately, but also predicts the dynamic mechanical properties of the polymer over a temperature range of—130°C to 120°C, for samples ranging from unoriented to highly oriented, and varying in crystallinity from 40 to 70%. The model can also predict the dynamic mechanical properties over the same temperature range for strips of these samples cut at varying angles to the machine direction.     

Structure,energetic and phase transition of small nickel-palladium heterogeneous clusters

Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni _n Pd_(13?n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.     

Structural and electronic properties of Bin (n = 2-14) clusters from density-functional calculations

The structural and electronic properties of Bi_n (n = 2-14) clusters have been systematically studied using gradient-corrected density-functional theory. For each cluster size, a number of structural isomers were constructed and optimized to search for the lowest-energy structure. The competition of several structural patterns such as cages, superclusters, and layered structures leads to the alternating appearance of these configurations as global minima. Although the tendency of Bi to form puckered-layer structures is already well-known, the electronic states of Bi_n clusters are still far from that of the bulk. As well, a remarkable even-odd atom number oscillation is observed in the structural and electronic properties of the clusters, implying that the stability of Bi_n clusters is mainly dominated by the electron shell effect rather than by geometrical packing. The theoretically calculated values for electron affinities agree well with available experimental data.     

Morphological changes in isotactic polypropylene as a function of cooling rate

A new method for monitoring rather than controling thermal history has been applied recently to analyze the crystallization kinetics at high cooling rate of a highly pure isotactic polypropylene, proving that the cooling history relevant to the structure of quenched samples is in the neighborhood of 90°C. A continuous variation of morphology and crystal structure was obtained with cooling rate. At cooling rates above 20°C/s, mesomorphic and α-monoclinic phases coexist with a morphology characterized by negatively birefringent spherulites surrounded by a weakly birefringent medium; below this value of cooling rate only α-monoclinic phase is formed, composed of mixed spherulites.     

Magnetism and magnetic isomers in free chromium clusters

We have used the Stern-Gerlach deflection technique to study magnetism in chromium clusters of 20-133 atoms at temperatures between 60 and 100 K. We observe that these clusters have large magnetic moments and respond superparamagnetically to applied magnetic fields. Using superparamagnetic theory, we have determined the moment per atom for each cluster size and find that it often far exceeds the moment per atom present anywhere in the bulk antiferromagnetic lattice. Remarkably, our cluster beam contains two magnetically distinguishable forms of each cluster size with > or =34 atoms. We attribute this observation to structural isomers.     

Quantitative structural characterization of the mechanical properties of isotactic polypropylene

This investigation introduces quantitative morphological criteria for defining the structural state of a polymer sample and develops a general structural state deformation model for characterizing mechanical behavior. This general model discards the present fragmented view of the autonomy of observed mechanical processes (yielding, fracture, recovery, etc.) and brings the fabrication process, the mechanical tests, and even the construction of the sample (fiber or film) into focus as simply different aspects of a single process of deformation. The fabrication, fracture, yielding, and recovery behavior of two series of uniaxially drawn isotactic polypropylene films and two series of drawn isotactic polypropylene fibers, totaling thirty different structural states, have been deter-mined. The structural state of each of these samples is known quantitatively. The measurements were made over a range of strain rates from 1 to 1,000,000%/min and temperatures from 23° to -196°C. This study has resulted in new quantitative relationships between mechanical properties and morphological structure and has developed a deeper insight into the underlying deformation mechanisms that are occurring.     

Formation of nanoparticles of Ag in poly(vinyl methyl ether) hydrogel network by electron beam irradiation

Ag⁺ ions, in poly(vinyl methyl ether) (PVME) hydrogel network have been reduced by electron beam irradiation to produce Ag clusters as nanoparticles in fully reduced and highly pure state. XRD (X-ray diffraction) technique confirmed the zero valent state of silver. UV-visible absorption spectral characteristics of Ag cluster confirm that the PVME network is a very efficient stabilizer and prevents further aggregation of Ag clusters. From the results of TEM, it was found that Ag particles were homogenously dispersed in the PVME matrix.     

Lamellar model of the left-handed metamaterials composed of metallic split-ring resonators and wires

Left-handed materials composed of split ring resonators (SRRs) and wires are investigated in the viewpoint of lamellar composite with epsilon-negative (ENG) and mu-negative (MNG) materials staked alternatively. Several configurations of the SRR-wire metamaterial are numerically simulated to confirm its left-handed response as an analogy to the ENG-MNG lamellar model.     

Lamellar changes during heat treatment in isotactic polystyrene crystals

In the present investigation, the melting and thickening processes in lamellar crystals of isotactic polystyrene have been studied by transmission electron microscopy. It is shown that under properly chosen experimental conditions for the polymer, one can continuously follow the physical changes involved during the thickening as well as melting of lamellar crystals on heat treatment. The study of crystals grown at different temperatures reveals that melting of a single lamella starts at various areas. A commonly observed feature is the preferential melting of elastically bent parts of a lamella. It is indicated that the occurrence of melting in the various parts is due to a structural variation along the surface of lamellae resulting in a hindrance of the lamellar thickening process. At particular temperatures, melting of lamellar crystals is followed by recrystallization. The occurrence of a solid-stage thickening process is the major process so far observed during slow heat treatments. Considerable change in surface structure of the crystals grown at different temperatures is clearly reflected during the heat treatment. The rates of heating have marked influence on the resulting morphology of the crystalline superstructures.     

Nucleation Based Explanation of Crystalloluminescence

In crystalloluminescence light is produced during crystallization. While there was work on crystalloluminescence during the 18th, 19th and very early 20th centuries, crystalloluminescence is a phenomena without scientific explanation. A new nucleation theory, based on a cluster population balance accounting for all possible cluster–cluster collisions, is presented that uses the energetics of clusters of various geometries determined from ab initio quantum mechanical calculations. This theory predicts cluster population dynamics during nucleation, as well as the spectrum of light emitted during nucleation due to the energy released by clusters activated by collision. Further experimental work is suggested to determine the nature of the light spectrum resulting from crystalloluminescence to validate this mechanism of light production.     

以团簇加连接原子模型解析Cr-C共晶成分

Cr-C体系材料是重要硬质防护涂层的代表,具有共晶特征.我们的前期工作指出,共晶合金满足双团簇近程序结构模型,由两种稳定液体亚单元构成,各自满足理想非晶团簇成分式,这里的第一近邻团簇来自相关共晶相.显然共晶成分解析的关键在于获得团簇,而相结构中往往存在多种团簇,进入到非晶/共晶团簇成分式的主团簇定义是关键环节.本文通过应用Friedel振荡理论及原子密堆,以团簇分布的球周期性及孤立度为判据,以Cr-C共晶相为例,进一步细化了共晶相中的主团簇选择流程,再搭配以2,4或6个连接原子,获得了描述共晶成分Cr_(86)C_(14)和Cr_(67.4)C_(32.6)的双团簇成分式:[Cr-Cr_(14)+C-Cr_9]Cr C_3和[C-Cr_9+C-Cr_8]C_6,其中四种团簇分别来自共晶相Cr,Cr_(23)C_6,Cr_7C_3和Cr_3C_2.该工作进一步证实了团簇加连接原子模型在共晶点解析中的普适性,并从理论上支持了相关的材料设计.     

The development of spherulite morphology in polymers

Morphology of polyethylene spherulites has been investigated by the low-dose technique using transmission electron microscopy (TEM) in a scanning mode for dark field and microdiffraction. Specimens were prepared by solvent casting and subsequent recrystallization at different temperatures. The dark-field studies provide spatial information on the lamellar morphology. Two major types of spherulite morphology have been observed: At high temperatures (low crystallization rate) the dark fields show a preferred orientation of the lamellae along the 020 axis. At low temperatures the preferred orientation is 110. The microdiffraction patterns confirm the dark-field results and also show that for intermediate temperatures, regular alteration of regions with 020 and 110 growth planes is responsible for the ringed spherulite appearance. The twisted lamellae model cannot be excluded, but it is shown that it is not responsible for the regular changes in contrast. A model of dendritic growth of spherulite lamellae is used in which the plane of crystal growth is temperature dependent and, for intermediate temperatures, results in regular fluctuation in the mode of crystal growth, branching, and ringed spherulites appearance.     

Multimolecule glasses and crystals of isotactic polystyrene

When spreading dilute solutions of isotactic polystyrene (iPS) on water, it was observed that different concentrations produce different morphologies in the resulting amorphous films. From 10^?1 to 10^?2 wt% solutions, ribbonlike films and circular platelets of multimolecule aggregates were obtained. After isothermal crystallization of these amorphous films, lamellae, incipient spherulites, and single-molecule crystals were observed. The formation of lamellar crystals lying flat on the substrate is an indication of the absence of epitaxy by the substrate, which would require the molecular chains to be in the surface plane and thus yield lamellae normal to the substrate. The dense packing in the center of sheaflike spherulites is caused by screw-dislocation growth, and it produces structures similar to shish kebabs. It was also observed that the lamellae grown from the center develop secondary growth spirals, causing the branching and splaying of the structure typical for spherulitic growth.     

A density-functional theory study of microphase formation in binary Gaussian mixtures

We use density-functional theory to study the formation of inhomogeneous phases in a binary mixture of particles interacting by repulsive, athermal Gaussian potentials with suitably chosen strengths and ranges. Both the potential parameters and the free-energy functional are the same as those adopted in a previous investigation by other authors (Archer A J, Likos C N and Evans R 2004 J. Phys.: Condens. Matter 16 L297), but here a fully numerical minimization of the functional is performed, without any assumption about the functional form of the density profile. We find lamellar, rod and cluster phases. In the lamellar phase, the two species arrange into intercalating stripes; in the rod and cluster phases, the minority species is localized at the site of a periodic lattice, either triangular (for rods) or body-centred cubic (for clusters), while the other species is distributed non-uniformly in the remaining region, so that it forms a percolating network. The order of the transition from the homogeneous to the inhomogeneous phase and the phase diagram of the mixture are also discussed.