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.     

强迫对流影响二元合金非等温凝固枝晶生长的相场法模拟

在二元合金相场模型的研究基础上,建立了耦合溶质场、温度场和流场的相场模型,采用Simple算法求解质量和动量守恒方程,用交替隐式有限差分法求解温度控制方程,模拟了流场作用下二元合金等温和非等温凝固过程中枝晶的生长过程,研究了流场对枝晶生长形貌、溶质场和温度场分布情况的影响,将流场作用下二元合金等温和非等温凝固枝晶生长过程进行比较,分析了由于凝固潜热的释放对流场作用下凝固枝晶生长的影响． 关键词： 相场法 对流 非等温凝固 枝晶生长     

Crystallization Kinetics of Nucleated Polypropylene with Organic Phosphates

The crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP with two organic phosphates, sodium salt (NA7) and triglyceride ester (NA8) of 2,2'-methylene-bis(4,6-di-tert-butylphenyl) phosphoric acid, were investigated by means of a differential scanning calorimeter under isothermal and nonisothermal conditions. During isothermal crystallization, a modified Avrami equation was used to describe the crystallization kinetics. Moreover, kinetics parameters, such as the Avrami exponent, n, the crystallization rate constant, k, and the half-time of crystallization, τ_1/2, are compared. The results showed that a dramatic decrease of the half-time of crystallization, as well as a significant increase of the overall crystallization rate, were observed in the presence of the organic phosphates. During nonisothermal crystallization, the primary crystallization was analyzed using the Ozawa model, leading to similar Avrami exponents for iPP and iPP/NA7, which means simultaneous nucleation with three-dimensional spherulitic growth. However, for iPP/NA8, the Avrami exponent in nonisothermal crystallization is evidently different from that in isothermal crystallization, which would indicate a different mechanism of crystal growth. Adding the nucleating agent to iPP makes the overall crystallization activation energy increase.     

Study on the Non-isothermal Melt Crystallization Kinetics of PTT/PBT Blends

Macro-kinetic models, namely the modified Avrami, Ozawa, Mo, and Kissinger models, were applied to investigate the non-isothermal melt crystallization process of PTT/PBT blends by DSC measurements. It was found that the modified Avrami model can describe the non-isothermal melt crystallization processes of PTT/PBT blends fairly well. When the cooling rates range from 5 to 20°C/min, the Ozawa model could be used to satisfactorily describe the early stage of crystallization. However, the Ozawa model didn't fit the polymer blends in the late stage of crystallization, because it ignored the influence of secondary crystallization. Under the conditions of the non-isothermal melt crystallization, it was found that the cooling rates and the blend composition affect the crystallization for blends according to Kissinger crystallization kinetics parameters. The crystallization kinetics constant K_a increases with increasing cooling rate, indicating the crystallization rates of PTT, PBT, and PTT/PBT blends were improved. The crystallization kinetic activation energy parameters are good agreement with the results from isothermal crystallization processes of the polymer blends. The crystallization activation energy of PTT/PBT blends is higher than the activation energy of PTT and PBT.     

Isothermal Crystallization Kinetics and Melting Behavior of a Luminescent Conjugated Polymer,Poly(9,9-Dihexylfluorene-Alt-2,5-Didodecyloxybenzene)

A study of the isothermal crystallization behaviors of poly(9,9-dihexylfluorene-alt-2,5-didodecyloxybenzene) (PF6OC12) was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. The Avrami exponent n ranges from 3.43 to 3.71 for PF6OC12 at crystallization temperatures between 100.0°C and 90.0°C, indicating a three-dimensional spherical crystal growth with homogeneous nucleation in the primary crystallization stage for the isothermal melt crystallization process. In the DSC scan, after the isothermal crystallization, multiple melting behavior was found. The multiple endotherms could be attributed to melting of recrystallized materials produced originally during different crystallization processes. According to the Arrhenius equation, the activation energy was determined to be 211.29 kJmol^?1 for the isothermal melt crystallization of PF6OC12.     

Effect of Organoclay Platelets on Crystallization of Polytrimethylene Terephthalate/Polyethylene-octene-g-maleic Anhydride Blends: Isothermal Crystallization

The blends of poly(trimethylene terephthalate) (PTT) with maleic anhydride-grafted poly(ethylene-octene) (POE-g-MA) and organoclay (OMMT) were prepared by melt-blending. The effects of organoclay platelets on the isothermal crystallization behaviors of PTT/POE-g-MA blend were examined using differential scanning calorimetry. The crystallization kinetics of the primary stage under isothermal conditions could be described by the Avrami equation, with values of the Avrami exponent between 2.01 and 2.81 for all samples. The crystallization rate parameter, K, decreased with increase of melt-crystallization temperature for all samples. The activation energies for isothermal crystallization were determined by the Arrhenius equation.     

二元合金等温凝固过程的相场模型

基于Ginzberg-Landau理论，发展了一个与WBM模型和KKS模型一致的新相场模型.并利用该相场模型与溶质场耦合计算，以Al-65%Cu合金为例模拟了不同过冷度条件下，二元合金凝固过程的等轴枝晶生长过程.研究过冷度对二元合金等温凝固过程的等轴枝晶生长以及溶质场分布的影响.结果表明：随着过冷度的增大，枝晶的二次枝晶更加发达，浓度Peclet数和枝晶尖端的生长速率增大，而枝晶尖端的曲率半径减小，枝晶前沿的溶质富集现象也更严重；另外，计算结果与Ivantsov理论符合较好. 关键词： 相场法 枝晶生长 溶质场 Ivantsov理论     

Convective instabilities in films of binary mixtures

We present a model for the evolution of films of isothermal binary liquid mixtures with a free evolving surface. The model is based on model-H supplemented by appropriate boundary conditions at the free surface and the solid substrate. The equations account for the coupled transport of the concentration of a component (convective Cahn-Hilliard equation) and the momentum (Korteweg-Navier-Stokes equation). The inclusion of convective motion makes surface deflections possible, i.e., the model allows to study couplings between the decomposition of the mixture and the evolving surface corrugations. We present selected steady layered film states for representative polymer mixtures, and show that convective motion favors their destabilization and qualitatively changes the linear instability modes in experimentally accessible ranges of parameters.     

Density functional theory for nonuniform polymer melts: Based on the universality of the free energy density functional

A density functional theory is proposed for nonuniform freely jointed tangential hard sphere polymer melts in which the bonding interaction is treated on the basis of the properties of the Dirac δ-function, thus avoiding the use of the single chain simulation in the theory. The excess free energy is treated by making use of the universality of the free energy density functional and the Verlet-modified (VM) bridge function. To proceed numerically, one of the input parameters, the second-order direct correlation function of a uniform polymer melt is obtained by solving numerically the Polymer-RISM integral equation with the Percus-Yevick (PY) closure. The predictions of the present theory for the site density distribution, the partition coefficient and the adsorption isotherm, near a hard wall or between two hard walls are compared with computer simulation results and with those of previous theories. Comparison indicates that the present approach is more accurate than the previous integral equation theory and the most accurate Monte Carlo density functional theories. The predicted oscillations of the medium-induced force between two hard walls immersed in polymer melts are consistent with the experimental results available in the literature. Received 18 April 2000     

Gamma irradiation of poly(phenylene sulfide): Effects on crystallization behavior

The irradiation of polymers can bring about significant changes in the polymer structure and ultimately in their performance. Poly-(phenylene sulfide) (PPS) is an engineering polymer which is mostly used in applications where it is subjected to high-energy radiation. This paper reports the results of gamma irradiation of PPS. The technique of differential scanning calorimetry was used to monitor the changes in the melting and crystallization parameters of irradiated PPS. The heat of fusion was found to decrease upon irradiation, indicating a lower degree of crystallinity; however, a marginal increase was observed in the melting point. The nonisothermal crystallization of irradiated PPS indicated reduced crystallizability. This contention is also supported by the isothermal crystallization studies in which the crystallization rate for irradiated PPS was found to be decreased. The retardation in the crystallization of irradiated PPS has been attributed to the possible changes in the structure of the polymer.     

The Role of Poly(phenylene sulfide) Microfibrils in Isothermal Crystallization of Isotactic Polypropylene under Shear

An optical polarizing microscope with a hot shear stage was used for an in‐situ investigation of the influences of poly(phenylene sulfide) (PPS) microfibrils on isothermal crystallization of isotactic polypropylene (iPP) under shear. As the nucleation sites on the PPS microfibril's surface are not able to induce a transcrystalline layer, there are only spherulites generated in a PPS/iPP in‐situ microfirbillar blend in quiescent condition. Applying shear during isothermal crystallization, the crystalline morphology greatly changes. There are fibrillar nuclei induced after steady shear with a shear rate of 5 and 10 s^–1, and these nuclei formed fibrillar crystals after crystallization completion. Two opposite effects coexist in PPS/iPP in‐situ microfibrillar blends during shear‐induced isothermal crystallization; one is the obstructive effect of PPS microfibrils on the iPP molecular chains orientation; the other is the positive effect provided by stress between fiber and matrix, generated by shear, which reduces the potential barrier of crystallization. The results of wide angle x‐ray diffraction (WAXD) show that there are β‐iPP crystals generated in neat iPP and PPS/iPP blends, but that PPS microfibrils have an inhibiting influence on the formation of β‐iPP.     

Influence of isothermal approximation on the phase-field simulation of directional growth in undercooled melt

By using the phase-field approach,we have simulated the directional growth of alloys in undercooled moten states under the isothermal and nonisothermal conditions.The influences of the isothermal approximation on simulation results are discussed.We found that for undercooling greater than 25K,the isothermal approximation overestimates the interface growth velocity and reduces a critical velocity for an absolute stable planar interface,thus in this simulation,the uinterface morphology shows the plane-cell-plane transition with increasing initial undercooling of the mele,and the planar interface obtained under a large undercooling is absolutely stable.Whereas in the nonisothermal simulation,only plane-cell transition occures in the same range of the initial undercoolings of the melt,and the planar interface tends to be destabilized and evolve into cells.     

Sharp interface tracking using the phase-field equation

A general interface tracking method based on the phase-field equation is presented. The zero phase-field contour is used to implicitly track the sharp interface on a fixed grid. The phase-field propagation equation is derived from an interface advection equation by expressing the interface normal and curvature in terms of a hyperbolic tangent phase-field profile across the interface. In addition to normal interface motion driven by a given interface speed or by interface curvature, interface advection by an arbitrary external velocity field is also considered. In the absence of curvature-driven interface motion, a previously developed counter term is used in the phase-field equation to cancel out such motion. Various modifications of the phase-field equation, including nonlinear preconditioning, are also investigated. The accuracy of the present method is demonstrated in several numerical examples for a variety of interface motions and shapes that include singularities, such as sharp corners and topology changes. Good convergence with respect to the grid spacing is obtained. Mass conservation is achieved without the use of separate re-initialization schemes or Lagrangian marker particles. Similarities with and differences to other interface tracking approaches are emphasized.     

Scale-bridging phase-field simulations of microstructure responses on nucleation in metals and colloids

In the present studies we investigate the connection between atomistic simulation methods, i.e. molecular dynamics (MD) and phase-field crystal (PFC), to the mesoscopic phase-field methods (PFM). While the first describes the evolution of a system on the basis of motion equations of particles the second uses a Cahn–Hilliard type equation to described an atomic density field and the third grounds on the evolution of continuous local order parameter field. The first aim is to point out the ability of the mesoscopic phase-field method to make predictions of growth velocity at the nanoscopic length scale. Therefore the isothermal growth of a spherical crystalline cluster embedded in a melt is considered. We also show simulation techniques that enable to computationally bridge from the atomistic up to the mesoscopic scale. We use a PFM to simulate symmetric thermal dendrites started at an early stage of solidification related to nucleation. These techniques allow to simulate three dimensional dendrites from the state of nuclei (≈50?Å) converted from MD up to a size of some μm where ternary side-arms start to grow.     

Evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with different external magnetic field conditions

The evolution of modulated structure in Fe–Cr–Co alloy during isothermal ageing with and without external magnetic field was investigated by using transmission electron microscopy (TEM) and phase-field simulation. The isotropic modulated structure in Fe–Cr–Co alloy formed during isothermal ageing without external magnetic field could be converted to be anisotropy during further isothermal ageing under an 8 kOe external magnetic (thermo-magnetic treatment), and the formation of anisotropy was slower than the direct formation from spinodal decomposition under external magnetic field and is time dependent. The anisotropy characteristic of modulated structure in Fe–Cr–Co alloy subjected to thermo-magnetic treatment for 1 h remained during further isothermal ageing without external magnetic field for up to 20 h. Novel modulated structure could be obtained through specific thermo-magnetic treatment processes, which was confirmed by phase-field simulation.     

二元合金多晶粒的枝晶生长的等温相场模型

基于Ginzburg-Landau理论和单晶粒的枝晶生长模型,发展了一个单相场控制的多个晶粒的枝晶生长模型. 采用相场和溶质场耦合的方法,以Al-2%Cu合金为例模拟了二元合金等温凝固过程中多个晶粒的生长过程. 结果表明,这个模型的计算结果展现了多个晶粒枝晶的竞争生长,能较真实的再现凝固过程中的枝晶的生长过程. 关键词： 相场法 多晶粒 等温凝固 二元合金     

二元合金多晶粒的枝晶生长的等温相场模型

基于Ginzburg-Landau理论和单晶粒的枝晶生长模型，发展了一个单相场控制的多个晶粒的枝晶生长模型. 采用相场和溶质场耦合的方法，以Al-2%Cu合金为例模拟了二元合金等温凝固过程中多个晶粒的生长过程. 结果表明，这个模型的计算结果展现了多个晶粒枝晶的竞争生长，能较真实的再现凝固过程中的枝晶的生长过程.