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。     

Effect of Unplasticized Poly Vinyl Chloride (UPVC) Molecular Weight and Graft-Acrylonitrile-Butadiene-Styrene (g-ABS) Content on Compatibility and Izod Impact Strength of UPVC/g-ABS Blends

In this study three grades of rigid poly vinyl chloride (PVC) having different molar masses were melt blended with graft-acrylonitrile-butadiene-styrene (g-ABS) in different compositions. The effect of PVC molecular weight and g-ABS composition on the compatibility and Izod impact strength of the blends were investigated. Differential scanning calorimetry (DSC) results showed a single glass transition temperature (T_g) for all the blends, representative of miscibility between the PVC phase and the styrene-acrylonitrile copolymer (SAN) phase of g-ABS which, in turn, led to compatibility of the PVC/g-ABS blends. It was observed that in all the PVC grades the blends Izod impact strength increased with increasing g-ABS content. Also, at a given composition of g-ABS, by increasing the molecular weight of the PVC phase the impact strength of the blends increased. The morphology of the fracture surfaces from the impact tests were analyzed using scanning electron microscopy (SEM) micrographs and the results showed that with increasing g-ABS content in the blend, cloudy regions increased and eventually begin to overlap each other, and the deformed material on the fracture surfaces increased. This was attributed to the blend compatibility causing greater energy dissipation in the fracture process.     

The effect of phase change materials on the frontal polymerization of a triacrylate

The production of smoke and fumes is a major obstacle to the practical use of thermal frontal polymerization. The front temperature and the amount of smoking can be reduced by adding inert fillers, such as clay and silica, to the reactive mixture. Here we investigate the possibility of incorporating inert materials that melt (so-called phase change materials) to the mixture. By performing both experiments and mathematical modeling, we demonstrate that, in addition to the standard parameters of frontal polymerization, the front temperature and velocity depend on the melting point and heat of fusion of the phase change material. We use the method of matched asymptotic expansions to develop an explicit expression for the velocity of the reaction front. The expression demonstrates that the behavior of the front is determined by the difference between the reaction temperature and the melting temperature, with the front being slower and cooler if melting occurs farther ahead of the reaction front. The theoretical trends are hard to confirm directly because different characteristics of the phase change material cannot be varied separately.     

Preparation and Evaluation of the Morphology,Flammability, and Mechanical Properties of the Acrylonitrile‐Butadiene‐Styrene/Poly (vinyl chloride) Blends

Blends of two grades of acrylonitrile‐butadiene‐styrene (ABS) with three different compounds of poly (vinyl chloride) (PVC) were prepared via melt processing and their morphology, flammability, and physical and mechanical properties were investigated. SEM results showed that the ABS/PVC blend is a compatible system. Also, it can be inferred from fracture surface images that ABS/PVC blends are tough, even at low temperatures. It was found that properties of these blends significantly depend on blend composition and PVC compound type; however, the ABS types have only a small effect on blend properties. On blending of ABS with a soft PVC compound, impact strength, and melt flow index (MFI) increased, but tensile and flexural strength decreased. In contrast, blending of ABS with a rigid PVC compound improved fire retardancy and some mechanical properties and decreased MFI and impact strength.     

分子动力学模拟Cu(010)基体对负载Co-Cu双金属团簇熔化过程的影响

纳米团簇负载到基体上的结构演化和热稳定性是其走向技术应用的关键. 本文用分子动力学结合嵌入原子方法模拟了具有二十面体初始结构的Co₂₈₁Cu₂₈₀ 混合双金属团簇在Cu(010)基体上的熔化过程, 考察了基体的Cu原子可以自由移动(自由基体)和固定(固定基体)两种条件对负载团簇熔化的影响. 发现基体条件对团簇的熔化有明显的影响. 在自由基体上团簇原子的温度-能量曲线存在明显的团簇熔化时的能量突变点, 熔点为1320 K, 低于固定基体上团簇的熔点1630 K. 在升温过程中团簇的二十面体结构会在基体表面发生外延生长. 外延团簇随着温度增加发生表面预熔, 预熔原子会逐渐向基体表面扩散形成薄层, 直至完全熔化. 自由基体上团簇原子的嵌入行为会使原子的分布状态产生不同于固定基体上的演变.     

The effect of molecular weight on the structure and thermal properties of polyethylene

The morphology of polyethylene single crystals prepared isothermally in solution was found to be independent of molecular weight. The enthalpy of fusion, lamellar fold period, and optical appearance were invariant for samples grown from fractions ranging from 20,000 to 2,000,000 in molecular weight. The mass fraction of lamellae which thicken during heating decreased linearly with increasing log molecular weight. The melting temperature of the crystals was also nearly independent of molecular weight.

The superheating of polyethylene crystals was observed to be a function of molecular weight and morphology. At a comparatively high molecular weight the heating rate of the calorimeter exceeded the crystal melting rate, which shifted the observed melting temperature to an anomalously high value. The incorporation of defects within the crystals by irradiation-induced cross-links or chain entanglements increased the melting rate of the high molecular weight samples and thereby minimized the effects of superheating.

The apparent heat of fusion of melt crystallized polyethylene decreased linearly with increasing log molecular weight. In contrast to this behavior the crystallinity of single crystals from dilute solution was independent of molecular weight.

In previous papers we have shown that reorganization of polymer single crystals is suppressed by cross-linking [1—3]. With the appropriate selection of heating rate and irradiation dose, the melting temperatures of solution grown crystals of various morphologies were determined in the absence of lamellar thickening. The observed melting temperatures of polyethylene single crystals with different X-ray fold periods were found to fit the following expression:

T_m = T_m0[1—2σ_e/H_f?] with an equilibrium melting temperature (T_m0) of 145.8 ± 1.0°C and a surface free energy (σ_e) of 89 ± 5 ergs cm^?2 for a polyethylene crystal of infinite dimensions. In addition, at a constant heating rate it was observed that the fraction of crystals which thickened prior to melting decreased with increasing fold period.

Since cross-linking polyethylene increases the molecular weight of the material, it is instructive to investigate the reorganization characteristics of single crystals prepared from polyethylene fractions. Single crystals were prepared in xylene from molecular weight fractions of polyethylene and the effect of molecular weight upon the structure and thermal properties of the crystals was determined.     

The Effect of Melt Vibration on Polystyrene Melt Flowing Behavior During Extrusion

Vibration extrusion (VE) is achieved by superimposing a mechanical vibration on the flowing melt during extrusion. The effect of melt vibration on the melt flow behavior of polystyrene (PS) was studied. The melt flow behavior during conventional extrusion (CE) was studied for comparison. With the application of the melt vibration technology, the melt flow behavior of PS was greatly improved. The melt viscosity during the VE strongly depends on the vibration frequency and vibration amplitude. Extruded at constant vibration amplitude, the melt viscosity decreases sharply with increasing vibration frequency and also does so for increasing vibration amplitude when extruded at a constant vibration frequency. The improved melt flow property is explained in terms of shear-thinning criteria. The effect of melt vibration on the melt flow behavior is also related to the melt temperature and extrusion pressure; the greatest decease in viscosity is obtained at low temperature and low extrusion pressure.     

ABSENCE OF ISOTHERMAL LAMELLAR THICKENING IN A LINEAR POLYETHYLENE BLEND

Melting and lamellar morphology of a polyethylene blend were studied. Two linear polyethylene (LPE) samples were used. A commercial LPE and a low molecular weight LPE fraction (M _n ≈ 2015) were solution blended. The pure LPE and a blend (30% commercial LPE content) were held in the melt at 126° C for up to 48 h, above the equilibrium melting point of the fraction, but below the crystallization temperature of the commercial LPE. The melting behavior of both materials as a function of storage time was studied using differential scanning calorimetry (DSC), in addition to transmission electron microscopy (TEM) of chlorosulfonated samples. Results showed that, although the LPE lamellae grew at the same temperature, those in the pure LPE were thicker than in the blend. Correspondingly, isothermally grown lamellae in pure LPE melt at higher temperatures.     

Thermal behavior of gamma-irradiated low-density polyethylene/paraffin wax blend

The thermal properties of low-density polyethylene (LDPE)/paraffin wax blends were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and melt flow index (MFI). Blends of LDPE/wax in ratios of 100/0, 98/2, 96/4, 94/6, 92/8, 90/10 and 85/15 (w/w) were prepared by melt-mixing at the temperature of 150°C. It was found that increasing the wax content more than 15% leads to phase separation. DSC results showed that for all blends both the melting temperature (T_m) and the melting enthalpy (ΔH_m) decrease linearly with an increase in wax content. TGA analysis showed that the thermal stability of all blends decreases linearly with increasing wax content. No clear correlation was observed between the melting point and thermal stability. Horowitz and Metzger method was used to determine the thermal activation energy (E_a). MFI increased exponentially by increasing the wax content. The effect of gamma irradiation on the thermal behavior of the blends was also investigated at different gamma irradiation doses. Significant correlations were found between the thermal parameters (T_m, ΔH_m, T₅%, E_a and MFI) and the amount of wax content and gamma irradiation.     

Rheological,thermal, and mechanical properties of poly(ethylene naphthalate)/poly(ethylene terephthalate) blends

Structural, Theological, thermal, and mechanical properties of blends of poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET) obtained by melt blending were investigated using capillary rheometry, differential scanning calorimetry (DSC), scanning electron microscopic (SEM) observation, tensile testing. X-ray diffraction, and ¹H nuclear magnetic resonance (NMR) measurements. The melt Theological behavior of the PEN/PET blends was very similar to that of the two parent polymers. The melt viscosity of the blends was between that of PEN and that of PET. Thermal properties and NMR measurement of the blends revealed that PEN is partially miscible with PET in the as molded blends, indicating that an interchange reaction occurs to some extent on melt processing. The blend of 50/50 PEN/PET was more difficult to crystallize compared with blends of other PEN/PET ratios. The blends, once melted during DSC measurements, almost never showed cold crystallization and subsequent melting and definitely exhibited a single glass transition temperature between those of PEN and PET during a reheating run. Improvement of the miscibility between PEN and PET with melting is mostly due to an increase in transesterification. The tensile modulus of the PEN/PET blend strands had a low value, reflecting amorphous structures of the blends, while tensile strength at the yield point increased linearly with increasing PEN content.     

Synthesis and Properties of Thermotropic Poly(oxybenzoate-co-oxynaphthoate) Copolyester Modified by a Third AB Type Monomer

Abstract

The synthesis and modification of high performance thermotropic liquid crystal polyesters (TLCP) are still being investigated due to their excellent properties. In this study modification of poly(oxybenzoate-co-oxynaphthoate) (P-HBA/HNA), a random copolymer of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA), by 5?mol% of one of three AB type monomers (AB type monomers are those who have hydroxy and carboxylic end groups), vanillic acid (VA), 4-hydroxyphenylacetic acid (HPA) and 4-hydroxycinnamic acid (HCA), was conducted. The influence of the AB type third monomer on the liquid crystal properties, melting behavior, thermal stability, microstructure and fiber morphology of the resulting modified terpolyesters was comparatively studied relative to the parent copolymer. When 5?mol% of the HBA units were replaced, the resulting terpolyesters had much lower melting temperatures and could be easily melt processed, while the modified terpolyesters exhibited higher melting and glass transition temperatures when 5?mol% of the HNA units were replaced. Weaker melting peaks and larger melting ranges were observed for the modified terpolyesters, attributed to the existence of nonperiodic layer crystallites caused by the modifiers. The obtained terpolyesters showed good thermal stability and could be melt processed in a broad nematic phase temperature region. The molecular chain packing of the modified terpolyesters was almost destroyed by the third AB type reactants. Fibers prepared from the resulting terpolyesters exhibited well developed fibrillar structure and higher orientation factors, indicating that modification of the P-HBA/HNA by using the AB type monomers to improve its properties would be useful.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Au-Pd共晶纳米粒子熔化行为的分子动力学研究

采用分子动力学方法结合嵌入原子势, 对Au-Pd共晶纳米粒子的热稳定性进行了模拟研究. 计算结果表明: Au-Pd纳米粒子的熔点明显高于Au单质纳米粒子而低于Pd纳米粒子. 通过计算Lindemann指数发现Au-Pd共晶纳米粒子中的Au原子首先熔化, 然后带动Pd原子的熔化; 熔化所经历的温度区间明显要宽于单质纳米粒子.     

Diamagnetic susceptibility of solid and liquid paraffins

Experimental values for the diamagnetic susceptibility of a series of mono-disperse n-paraffins ranging between 6 and 50 carbon atoms in the solid (melt and solution crystallized) and in the liquid state are reported. From the dependence of the molecular susceptibility, ScHM, on the molecular weight, information about the intermolecular interactions between adjacent chain molecules and on the arrangement of the methyl end-groups is obtained. The ScHM values of melt- and solution-crystallized paraffins are, within experimental error, indistinguishable from each other. For C₁₂H₂₆ and C₄₄H₉₀ the specific susceptibility ScHM /M rises at the melting point within a few °C and reaches a plateau which characterizes the liquid state II. For the paraffin C₂₄H₅₀ an intermediate plateau between the melting point and the final true liquid state is observed and is called liquid state I. After cooling below the melting point TM, the ScHM value of the equilibrium state can be obtained after a short time only from liquid state I, while from liquid state II days and weeks are needed for this. By combining this information with the observed ScHM values, it is concluded that in liquid state I a higher or smectic-like order exists similar to mesophases, well-known in liquid crystals, while in the real liquid state II this order is lost.     

Thermal behavior of vinylidene chloride—methyl acrylate copolymers

The thermal behavior of random copolymers of vinylidene chloride-methyl acrylate (VDC-MA) was monitored with differential scanning calorimetry. The effects of composition, annealing, and heating rate on the copolymer melting process were studied. Increasing levels of comonomer decreased the crystalline melting point and percent crystallinity of the copolymers. A combination of information obtained from heating rate and small-angle x-ray scattering studies led to a model of the melting behavior of PVDC and its copolymers. Apparently a simultaneous melting/recrystallization phenomenon occurs in these materials during heating.     

The Effect of Peroxide Cross-Linking on the Properties of Low-Density Polyethylene

Low-density polyethylene (LDPE) was chemically cross-linked with various amounts of dicumyl peroxide (DCP). The cross-link density, determined by Flory–Rehner theory, showed an increase with increasing DCP. The gel content, densities of cross-linked LDPE, thermal stability, crystallization, melting behavior, and tensile properties were studied. The results showed a new finding about the change of weight loss after cross-linking; with increase in temperature, the weight loss showed an increase below the temperature of about 450°C and then showed a decrease at temperatures from about 450°C to 500°C after being cross-linked. The crystallinity, melting point, crystallization temperature, and elongation at break decreased with the increase in DCP. However, the maximum tensile stress increased with the increase in DCP, and the cross-linked samples showed a rubber-like behavior with no flow.     

Effects of variable properties on MHD heat and mass transfer flow near a stagnation point towards a stretching sheet in a porous medium with thermal radiation

The effect of variable viscosity and thermal conductivity on steady magnetohydrodynamic(MHD) heat and mass transfer flow of viscous and incompressible fluid near a stagnation point towards a permeable stretching sheet embedded in a porous medium are presented,taking into account thermal radiation and internal heat genberation/absorbtion.The stretching velocity and the ambient fluid velocity are assumed to vary linearly with the distance from the stagnation point.The Rosseland approximation is used to describe the radiative heat flux in the energy equation.The governing fundamental equations are first transformed into a system of ordinary differential equations using a scaling group of transformations and are solved numerically by using the fourth-order Rung-Kutta method with the shooting technique.A comparison with previously published work has been carried out and the results are found to be in good agreement.The results are analyzed for the effect of different physical parameters,such as the variable viscosity and thermal conductivity,the ratio of free stream velocity to stretching velocity,the magnetic field,the porosity,the radiation and suction/injection on the flow,and the heat and mass transfer characteristics.The results indicate that the inclusion of variable viscosity and thermal conductivity into the fluids of light and medium molecular weight is able to change the boundary-layer behavior for all values of the velocity ratio parameter λ except for λ = 1.In addition,the imposition of fluid suction increases both the rate of heat and mass transfer,whereas fluid injection shows the opposite effect.     

Effect of Benzoic Acid on the Crystallization Behavior of Poly(3-Hydroxybutyrate)

The effect of benzoic acid (BA) on the crystallization behavior of poly(3-hydroxybut yrate; PHB) was studied. A differential scanning calorimeter was used to monitor the crystallization kinetics and thermal behavior. During the crystallization process from the melt, the presence of BA led to a decrease of the crystallization temperature of PHB compared with that for pure PHB. From the depression of the melting and glass transition temperatures of PHB, it can be concluded that BA is miscible, and has a strong plasticization effect on PHB. Isothermal crystallization at 80°C results showed that the addition of BA caused a decrease in the overall crystallization rate of PHB. Polarized optical micrographs of PHB/BA showed that the nucleation density of PHB spherulites decreased with increasing weight percentage of BA.     

Investigation on the Self-nucleation Behavior of Controlled-rheology Polypropylene

Self-nucleation (SN) behavior is one of the most fascinating behaviors of semicrystalline polymers; it is of great scientific and practical importance. In this study, the crystallization and melting behavior of a series of controlled-rheology polypropylene (CRPP) with different molecular weights were studied by differential scanning calorimetry (DSC) scanning and successive self-nucleation and annealing (SSA) fractionation, and their self-nucleation behavior was studied according to the conventional SN measurement, in order to provide a new understanding on the relationship between the molecular weight and the SN behavior of polymers. The results of DSC scanning and SSA fractionation showed that the decrease of molecular weight led to higher molecular chains mobility, making the crystallization temperature higher and the crystalliation peak narrower; in addition, it significantly restrained the formation of thick lamellae. The results of SN measurements showed that both the upper and lower temperatures of the samples decreased as the molecular weight decreased, which was suggested to be closely related to the reduction of the formation of thick lamellae. The related mechanism is discussed, and it was found that the key parameter determining the occurrence of SN and annealing of CRPP was the ability to form thick lamellae with high melting point, which might be related to the ability for ordered structures to persist at high temperature.