压力对非晶铜形成影响的分子动力学模拟

用分子动力学模拟法研究了Cu熔体在冷却过程中形成非晶的能力. 结果表明，压力使Cu熔体在冷却过程中形成晶体的倾向增加. 高压下冷却形成的Cu非晶中含有较多的晶态原子团，有序度较高.     

FeCoCrCuNi高熵合金裂纹及孔洞结构的力学与微观构象演化的分子动力学模拟研究

本文采用分子动力学模拟研究了FeCoCrCuNi高熵合金裂纹和孔洞结构在不同轴向拉伸速率下的力学与微观结构演化机理.结果表明:应变速率越高FeCoCrCuNi裂纹结构对应更高的过冲应变和过冲应力,其主要原因是高拉伸速率会导致高强度的BCC结构及孪晶结构的生成,而BCC结构及孪晶结构的产生进而会抑制应力的下降,通过应力-应变曲线,可知FeCoCrCuNi裂纹模型在轴向应力作用下表现为塑性形变.对于不同尺寸的孔洞FeCoCrCuNi裂纹模型的应力结构分析,可以得出:孔洞尺寸越大, FeCoCrCuNi裂纹结构对应的过冲应变和过冲应力越小,其主要原因是大尺寸的孔洞造成孔洞之间产生裂纹的,进而会影响这个材料的屈服应变和屈服强度.     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

FeCoCrCuNi高熵合金裂纹及孔洞结构力学与微观构象演化机理的分子动力学模拟研究

本文采用分子动力学方法研究了FeCoCrCuNi高熵合金裂纹及孔洞模型结构在不同轴向拉伸应变速率下的力学与微观结构演化机理. 结果表明：应变速率越高FeCoCrCuNi裂纹结构对应更高的过冲应变和过冲应力,其主要原因是高拉伸速率会导致高强度的BCC结构及孪晶结构的生成,而BCC结构及孪晶结构的产生进而会抑制应力的下降,通过应力-应变曲线,可知FeCoCrCuNi裂纹模型在轴向应力作用下表现为塑性形变. 对于不同尺寸的孔洞FeCoCrCuNi裂纹模型的应力模拟与结构分析,可以得出：孔洞尺寸越大, FeCoCrCuNi裂纹结构对应的过冲应变和过冲应力越小,其主要原因是大尺寸的孔洞造成孔洞之间产生裂纹的,进而会影响这个材料的屈服应变和屈服强度.     

均聚物结晶与熔融化的稳态和亚稳态转变统计理论

基于多重微晶网络结构模型和分子分凝机制建立了高分子晶体的微晶核 和微晶粒 高分子链组模型 ,推导出了平衡态下高分子预结晶动力学方程 ,计算出了平衡态下不同尺寸微晶核 和微晶粒 高分子链组的几率分布函数 .建立了非稳态下不同尺寸的微晶核 高分子链组的成核演化方程和微晶粒 高分子链组的增长演化方程 ,求解一般状态下的两个演化方程后 ,得到了不同时间和不同尺寸的微晶核 和微晶粒 高分子链组的一般密度分布函数 .最后根据成核自由能和增长自由能对晶核和晶粒的尺寸大小的依赖性 ,提出了微晶核 高分子链组和微晶粒 高分子链组存在稳定性的热力学条件和动力学条件 ,成功地表征为三个特征区 (稳态、亚稳态和非稳态 )     

PCL/SiO2杂化纳米相微结构与晶态成核生长特性

应用扫描电子显微镜、广角X射线衍射和差示扫描量热手段研究了有机高分子/无机组分间以物理次价力(氢键)键合的高分子量PCL/SiO2杂化材料纳米相微结构和PCL高分子链在该微结构环境中的结晶成核生长特性及其影响因素.研究结果表明:杂化体系中高分子/无机组分间的微相分离尺度在纳米数量级,高分子微区的平均相畴尺寸在70nm左右,无机相形态呈现不规则的颗粒状.两相均匀分布程度与体系中组分间的氢键键合强度有关.PCL杂化后结晶度减小,对应的微晶尺寸明显改变,平衡熔点随无机组成含量的增加而下降.高分子链在晶核表面折叠形成结晶结构所需的能量增加.这一结果归因于无机非晶SiO2和键合强度的影响.     

二维X射线衍射光谱原位检测单轴拉伸过程中等规聚丙烯晶体的结构演变

聚合物制品在使用过程中，人们最关心的是它的使用失效条件，失效的重要体现就是材料的屈服。目前为止，人们普遍利用位错理论来解释聚合物材料的屈服现象，该理论关注的是晶体的取向和破坏现象，而忽略了晶体的形变和受力情况。事实上，晶体的取向和破坏只是屈服的结果，晶体承受应力的能力才是屈服的直接原因。因此，从晶体的受力和非均匀形变入手研究了聚合物制品的屈服行为，期望为理解聚合物材料的失效行为提供新思路。这里选取被人们广泛使用的等规聚丙烯(iPP)材料作为研究对象，将iPP熔体在不同温度下等温结晶制备出具有不同片晶厚度iPP样品，利用二维广角X射线衍射光谱原位监测了拉伸过程中iPP样品的晶体破坏和晶体取向过程。首次利用“覆盖法”对二维X射线衍射图进行了处理，原位观察了(110)晶面在拉伸过程中的2θ角的变化，区分出了两个方向上(平行于拉伸方向和垂直于拉伸方向)晶体形变的非均一性。结果表明：对于不同片晶厚度的iPP晶体，在单轴拉伸过程中，晶体在不同方向上的受力和形变均是不同的，即晶体的非均一形变是一种普遍现象；晶体的破坏和取向总是同时发生，都是从屈服点位置处开始，这和片晶厚度无关；而晶体破坏时对应的临界...     

环状聚合物及其对应的线性链熔体在启动剪切场下流变特性的分子动力学模拟研究

采用非平衡分子动力学方法研究了环状聚合物及其对应的线性链熔体在启动剪切场下的结构与流变特性.模拟结果显示:低剪切速率下(■1×10~(-4)τ~(-1))环状链分子体系相比于同分子量的线性链体系并没有出现明显的过冲现象.该结果表明,在启动剪切场下环状分子与其对应的线性链相比较表现出了更弱的分子形变,同时模拟结果也与最近实验观察的结果一致.为了进一步探究这种现象背后的分子机理,在分子层面统计了不同流场强度下,链段的长度和取向角分布随着应变的演化,统计结果证明了环状聚合物分子链段弱的形变是导致其弱的剪切变稀和峰值应变的主要因素.本文还给出了过冲点和稳态在不同剪切速率下环状分子与其对应的线性链的流变特性(过冲应变、最大应力、最大黏性和稳态黏性)、结构和取向参数与维森伯格数(Wi_R)所满足的标度关系.     

高压下尼龙1010-单壁碳纳米管复合材料的结晶行为

 采用XKY-6×1200MN型六面顶压机,在不同温度、压力条件下处理30 min后制备了尼龙1010(PA1010)-单壁碳纳米管（SWCNT）复合材料的高压结晶样品,通过X射线衍射（XRD）、差热分析仪（DSC）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）,研究了高压处理样品的结晶行为、结构变化及形貌特征。结果表明：在1.0~2.5 GPa压力下,属于高压熔体结晶;在3.0和4.5 GPa压力下属于高压退火处理;高压结晶或高压退火均有助于聚合物片层晶体的增厚,并且高压熔体结晶的增厚效果优于高压退火处理。XRD结果表明,PA1010的三斜晶型在高压处理后保持不变,高压熔体结晶或高压退火都可以使(100)晶面和(010)晶面间距减小,即高压处理致使聚合物分子链紧密堆积。DSC结果表明：在高压熔体结晶过程中,升高压力和温度可以得到片层厚度较大的PA1010晶体;在2.0 GPa、350 ℃下获得的高压结晶样品的熔点和结晶度最高,分别达到208.5 ℃和64.6%。SEM和TEM结果表明：与常压结晶样品相比,高压结晶样品内部出现c轴厚度超过150 μm的大尺寸晶体;SWCNT与PA1010基体之间形成相互穿插的网络结构,刚性的SWCNT作为高压成核剂促进PA1010晶体生长和增厚。     

Ni熔体凝固过程中临界晶核和亚临界晶核的分子动力学模拟

本文用分子动力学模拟研究了Ni熔体以不同冷速凝固后微观结构的演变规律, 并通过理论计算确定出了Ni熔体凝固后获得理想非晶的临界条件. 模拟结果发现冷速小于10¹¹ K/s时, Ni 熔体凝固后形成晶态组织; 冷速在10¹¹ K/s到10^14.5 K/s之间时, Ni熔体凝固后形成由晶态结构与非晶态结构组成的混合组织. 冷速小于10¹⁰ K/s, Ni 熔体凝固后形成的晶态组织具有fcc结构; 冷速在10¹⁰ K/s到10^14.5 K/s之间时, Ni熔体凝固后组织中的晶态由fcc和hcp结构层状镶嵌排列构成. 通过分析模拟结果和计算结果, 确定出了Ni熔体凝固后形成理想非晶的临界冷速为10^14.5 K/s. 并发现Ni熔体中临界晶核(冷速等于10^14.5 K/s)和亚临界晶核(冷速大于10^14.5 K/s) 均由fcc和hcp组成层状偏聚结构, 这表明Ni熔体中生长的晶体、临界晶核和晶胚的结构是相同的. 关键词： 分子动力学模拟 晶体团簇 临界冷速 结构     

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.     

Morphology and Properties of Poly(vinyl alcohol)/MMT Nanocomposite Prepared by Solid-state Shear Milling (S3M)

Poly(vinyl alcohol) (PVA)/montmorillonite (MMT) nanocomposites were prepared by combining solid-state shear milling (S³M) technology with melt intercalation. Compared with the composite obtained by simple melt intercalation, more MMT layers were exfoliated and apparently oriented along the injection molding direction in the nanocomposite prepared by combining S³M technology and melt intercalation, which greatly increased the orientation degree of MMT, resulting in the greater interactions between PVA and MMT layers. Simultaneously, this also promoted the orientation of PVA molecules and produced effective nucleation of the crystallization of PVA. Consequently, the thermal stability and mechanical properties of PVA were obviously improved. For instance, when the MMT content was 3 wt%, the tensile strength and modulus of the nanocomposite with MMT prepared by S³M were 98.9 MPa and 3.1 GPa, respectively, increasing by 52% and 63.2% compared with PVA.     

Relaxation dynamics of poly(vinylidene fluoride) studied by dynamical mechanical measurements and dielectric spectroscopy

The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 °C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamic-mechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 °C. Dynamic-mechanical measurements were conducted between -60 °C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (α (c)-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the α (c)-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.     

Nanomechanical modeling of interfaces of polyvinyl alcohol (PVA)/clay nanocomposite

Abstract

We study interfacial debonding of several representative structures of polyvinyl alcohol (PVA)/pyrophillite-clay systems – both gallery-interface (polymer/clay interface in the interlayer region containing polymer between clay layers stacked parallel to each other) and matrix-interphase (polymer/clay interphase-region when individual clay layers are well separated and dispersed in the polymer matrix) – using molecular dynamics simulations, while explicitly accounting for shearing/sliding (i.e. Mode-II) deformation mode. Ten nanocomposite geometries (five 2-D periodic structures for tension and five 1-D periodic structures for shearing) were constructed to quantify the structure-property relations by varying the number density of polymer chains, length of polymer chains and model dimensions related to the interface deformation. The results were subsequently mapped into a cohesive traction–separation law, including evaluation of peak traction and work of separation that are used to characterise the interface load transfer for larger length scale micromechanical models. Results suggest that under a crack nucleation opening mode (i.e. Mode-I), the matrix-interphase exhibits noticeably greater strength and a greater work of separation compared to the gallery-interface; however, they were similar under the shearing/sliding mode of deformation. When compared to shearing/sliding, the tensile peak opening mode stresses were considerably greater but the displacement at the peak stress, the displacement at the final failure and the work of separation were considerably lower. Results also suggest that PVA/clay nanocomposites with higher degree of exfoliation compared with nanocomposites with higher clay-intercalation can potentially display higher strength under tension-dominated loading for a given clay volume fraction.     

半晶态聚合物的分子动力学模拟

采用粗粒化聚乙烯醇模型,应用分子动力学方法模拟熔融态聚合物经过缓慢冷却、局部结晶形成半晶态聚合物的过程.静态结构因子的演变显示出在结晶初期小角散射强度的增大先于布拉格峰的出现,这与小角/大角X射线散射实验现象相一致.模拟得到的半晶态聚合物呈现为折叠链构成的晶区与非晶区交杂在一起的结构形态,与缨状微束结构模型相一致.研究发现在不同的冷却阶段具有不同的有序结构形成机制.从结晶温度到玻璃化温度的凝固过程中,存在分子链的伸展和伸直分子链之间平行排列两种形式的结构转变;而在玻璃化温度之后,材料的活性只允许调整伸直分子链之间的相对排列位置.     

Stretch-Induced Shish-Kebabs in Rubbery Poly(L-Lactide)

The oriented crystallization in stretched rubbery poly(L-lactide) has been studied with the aid of in-situ rheo-Fourier transform infrared spectroscopy (FTIR) measurements and morphological observations. The oriented segments that survived after stretching are first transformed into shish structure composed of helical sequences via intra-chain conformational ordering and propagation, followed by the transverse growth of kebabs from the coiled chains in the surrounding matrix. Moreover, the formation of shish structure and kebabs shows different dependences on the stretching temperature as a result of different controlling molecular processes.     

Shear‐Enhanced Nucleation of Isotactic Polypropylene in Limited Space

The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s^?1 compared with the quiescent state, at 134°C. The ratio of the shear‐enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 134°C to 590 times at 140°C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product σ _s ² σ _e decreased to 3.2×10^?7 for the sheared melt, from 6.0×10^?7 (J m^?2)³ for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three‐dimensional folded chain nuclei to two‐dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.     

Influence of deformations of a polymer matrix on the characteristics of dual fluorescence of 3-hydroxyflavone

The possibility of considerably changing the conditions for the proton transfer reaction in 3-hydroxyflavone molecules in polyvinyl alcohol (PVA) polymer matrices by stretching deformations is demonstrated. Samples of this kind are traditionally used to obtain ensembles of fluorophore molecules oriented along a chosen axis and for polarization measurements. The fluorescence spectrum of 3-hydroxyflavone in PVA has two characteristic bands in the violet and green spectral regions, which indicates excited-state proton transfer. Stretching leads to a strong reduction in the violet band, whose contribution in undeformed samples is comparable to the contribution of green fluorescence. Even twofold stretching of PVA films strongly decreases the violet band intensity, which is more pronounced in the case of sixfold stretching. In the latter case, the fluorescence spectrum behavior is very close to the pattern observed in nonpolar and aprotic solvents, in which the violet fluorescence intensity is very low. The data obtained indicate that mechanical stretching allows one to eliminate the main intermolecular factors that slow down the proton transfer between the active groups in PVA.     

Crystallization of Carbon Nanotube and Nanofiber Polypropylene Composites

A variety of semicrystalline isotactic polypropylene composites containing carbon nanotubes and nanofibers were produced by melt and solution techniques. The effect of the nanofillers on the crystallization process was investigated by transmission electron microscopy, scanning electron microscopy, and differential scanning calorimetry. Under the processing conditions applied in this study, the surfaces of the carbon nanomaterials act as nucleation sites in bulk samples and highly oriented composite films. This conclusion is confirmed by the calculation of Avrami exponents and, in particular, by direct microscopy evidence.