剪切流场下聚丙烯结晶行为的研究进展

由剪切引起的分子链非平衡取向状态,及其冷却固化过程中的热松弛,对聚丙烯(PP)的结晶行为有很大影响。系统研究剪切流场下PP的结晶行为将对PP的加工和应用有重要意义。从国内外研究者的工作出发,对剪切流场下PP的结晶动力学改变、晶型转变和晶体形貌变化做了系统综述。     

剪切应力对聚乳酸结晶性能的影响

通过改变升降温循环次数、热历史消除时间及温度,研究了剪切作用力对聚乳酸树脂原料及流延片结晶行为的影响规律。结果表明,聚乳酸的结晶行为受其分子的缠结状态或相对有序程度的影响,在剪切作用力的影响下,聚乳酸分子解缠结或者相对有序程度提高,因此更容易出现冷结晶过程。     

剪切应力场对聚丙烯等温结晶的影响

在等温条件下,通过考察不同剪切应力条件下聚丙烯(PP)的储能模量,并将其储能模量与PP的结晶行为相联系,研究了剪切应力条件下PP的等温结晶行为,研究了各种剪切应力条件对PP结晶的影响。结果表明:剪切应力施加的时间延迟长和应力施加速率的变快均能缩短PP结晶诱导时间、加快结晶速率。此外,应力场作用下消除热历史后的降温速率越快,会延迟结晶出现的时间。     

Crystallization and Precipitation: The Optimal Supersaturation

In this article the optimal supersaturation for cooling and evaporation crystallization are presented. This information is based on a simplified model which describes the metastable zone width of activated nucleation. The model assumes that the nucleation step is very fast and the occurrence of the shower of nuclei which is used to define the metastable zone width is caused by the growth of activated nuclei. In future new sensors for the measurement of supersaturation will help to operate batch crystallizers at the optimal supersaturation during the entire batch time.     

Influence of Interfacial Shear Stress on First-Matrix Cracking Stress in Ceramic-Matrix Composites

The first-matrix cracking stress and fiber-matrix interfacial shear stress were measured in zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide filaments to study the role of intentional changes in interfacial shear stress on first-matrix cracking stress. The first-matrix cracking stress was measured by mechanical tests performed in either tension or flexure, and the filament-matrix interfacial shear stress was measured by a fiber pushout test. The first-matrix cracking stress was independent of the measured interfacial shear stress and did not conform to the predictions of a number of energy-based micromechanics models. In contrast, the first-matrix cracking stress showed a good correlation with the first-matrix cracking strain, which is hypothesized to be a more realistic criterion for first-matrix cracking in this class of filament-reinforced ceramic-matrix composites.     

填料对PP结晶行为的影响

用差示扫描量热法(DSC)和偏光显微镜研究了聚丙烯(PP)/无机填料复合体系的非等温结晶过程以及填料含量、尺寸、形状、偶联处理对PP结晶行为的影响。结果表明:玻璃微珠的含量增加引起结晶度降低,但不会影响球晶尺寸;小尺寸的玻璃微珠能使球晶尺寸更均匀;对玻璃微珠进行偶联处理能提高材料的结晶度;同时发现,纳米碳酸钙引发了聚丙烯由α晶型向β晶型的转变。     

Measurements on Cohesive Powder with two Biaxial Shear Testers

At the Technical University of Braunschweig (Germany) and the Delft University of Technology (The Netherlands) two True Biaxial Shear Testers were used to perform comparative measurements. The Braunschweig Biaxial Tester (BBT) and the Flexible Wall Biaxial Tester (FWBT) are part of a new generation of advanced powder testers whose goal is to fundamentally investigate powder flow based on the same principles. The general aim of these investigations is to compare the results obtained with both testers and gain new experience in handling powders.     

Influence of molecular composition on the development of microstructure from sheared polyethylene melts: Molecular and lamellar templating

A combination of in situ and ex situ X-ray scattering techniques and transmission electron microscopy has been used to study the crystallization behaviour of polyethylene, following the imposition of melt shear. In the case of a branched material, the imposition of shear flow up to a rate of 30 s⁻¹ was found to induce no anisotropy. Although shearing the linear material only ever induced a very small degree of anisotropy in the melt, for shear rates >0.15 s⁻¹, subsequent crystallization resulted in increasing anisotropy. Blends of the above two polyethylenes were produced, in which the linear material constituted the minority fraction (∼10%). Isothermal crystallization at temperatures where extensive crystallization of the branched material does not occur demonstrated that the behaviour of the linear component of the sheared blend mirrored that of the linear polyethylene alone. However, in addition, it was found that when crystallized in the presence of an oriented morphology, the branched polymer also formed anisotropic structures. We have termed the process templating, in which the crystallization behaviour of the bulk of the system (∼90% branched material) is completely altered (spherulitic to oriented lamellar) by mapping it onto a pre-existing minority structure (∼10% linear polymer).     

Crystallization of polyethylene under shear flow as studied by time resolved depolarized light scattering. Effects of shear rate and shear strain

We have studied structure formation during crystallization of polyethylene (PE) under shear flow using time resolved depolarized light scattering (DPLS) in order to elucidate the formation mechanism of the so-called shish-kebab structure. Two-dimensional (2D) DPLS pattern clearly showed streak-like scattering normal to the flow direction in the early stage during the crystallization after pulse shear, suggesting the formation of the shish-like structure in μm scale. In order to analyze the 2D DPLS pattern we defined measures for the acceleration of the crystallization rate and the degree of anisotropy and found that there are critical shear rates for both of the acceleration and the anisotropy at a given shear strain: the former is much smaller than the latter. We also determined the critical shear rate for the anisotropy as a function of the shear strain. Extrapolating to inverse of the infinite shear strain=0, we found the critical shear rate for the anisotropy at the infinite shear strain to be 1.5 s⁻¹. The results were discussed in relation to a competition between the relaxation rate of polymer chains and the orientation-induced crystallization rate.     

Effects of High Intensity Ultrasound Frequency and High-Speed Agitation on Fat Crystallization

The objective of this research was to examine the effect of ultrasound frequency and high-speed agitation on lipid crystallization. Interesterified soybean oil was crystallized at 44 °C without and with the application of high intensity ultrasound (HIU—20 and 40 kHz) or with high-speed agitation (6000 and 24,000 rpm). Two tip amplitudes (24 and 108 µm) and three pulse durations were evaluated (5, 10, and 15 s) for the acoustic frequencies tested. Sonication at 20 kHz of frequency significantly reduced crystal size, increased (p < 0.05) elasticity (435.9 ± 173.3–80,218 ± 15,384 Pa) and SFC (0.2 ± 0.0–4.5 ± 0.4%). No significant difference was observed in the crystallization behavior of these samples when sonicated at different amplitudes for 5 and 10 s. The crystallization behavior was significantly delayed (p < 0.05) in samples sonicated using 108 µm amplitude for 15 s. Larger crystals were formed in samples sonicated at 40 kHz compared to those obtained with 20 kHz and lower SFC (3.7 ± 0.0%) and elasticity (3943 ± 1459 Pa) values were obtained. High-speed agitation at 24,000 rpm increased SFC (5.5 ± 0.1%) and crystallized area and decreased the elasticity (42,602 ± 11,775 Pa) compared to the samples sonicated at 20 kHz.     

力场下聚合物的结晶行为研究进展

介绍了聚合物在力场作用下结晶行为的研究进展;概述了不同力场(剪切、拉伸、振动)对聚合物结晶形态和结晶动力学的影响;重点论述了剪切力场下,剪切速率、剪切方式、分子量和分子量分布等对聚合物成核和生长的影响,简述了拉伸、振动对结晶的影响。认为力场的加入对聚合物的成核和晶体生长都有促进作用。     

Analysis of Shear Stress Growth Experiments for Linear Constitutive Equations

Shear stress growth curves for viscoelastic fluids at low shear rates are analyzed using two linear rheological constitutive equations, an integral constitutive equation and a mixed type constitutive equation. It is shown that some published solutions do not satisfy all of the pertinent boundary conditions. For the low shear rate region, available experimental shear stress curves show a monotonic increase with decreasing slope in the shear stress. Shear stress curves calculated using a mixed type constitutive equation are found to exhibit this type of behavior while curves calculated using an integral constitutive equation do not. For the mixed type constitutive equation, the calculated developing velocity distribution is used to examine its effect on the developing shear stress distribution. For low values of E (the elasticity number), there is a moderate effect, but, for sufficiently large values of E, the developing velocity distribution has a negligible effect. It is also shown that results consistent with experimental data obtained at low shear rates can be attained using a single relaxation time. Additionally, incompressible Newtonian fluids are considered, and it is found that there can be single maxima in some shear stress curves with no maxima occurring in the velocity curves. Multiple maxima were not obtained in the Newtonian shear stress results unlike some published results.     

轻质碳酸镁对PP结晶行为的影响

用熔融共混法制备了聚丙烯(PP)/轻质碳酸镁复合材料。利用差示扫描量热法(DSC)、X射线衍射(XRD)、冲击试验机和原子力显微镜(AFM)等对复合材料进行了测试与表征。结果表明:少量轻质碳酸镁能够诱导PP中β晶的产生,从而增强其韧性;随着轻质碳酸镁用量的增加,PP的结晶度降低,从而使其冲击强度降低;经硬脂酸改性的轻质碳酸镁较未改性的可进一步提高复合材料的结晶度和结晶速率,其在复合体系中的分散性较未改性的也有很大提高。     

Effect of Lactose Monolaurate and High Intensity Ultrasound on Crystallization Behavior of Anhydrous Milk Fat

Crystallization behavior of anhydrous milk fat (AMF) was studied with the addition of 0.025 and 0.05 % lactose monolaurate (LML). The crystallization behavior was studied at low (ΔT = 3 °C) and high supercooling (ΔT = 6 °C). Polarized light microscopy and laser turbidimetry indicated a delay in crystallization on addition of 0.025 % and 0.05 % LML or Tween 20 to AMF. High intensity ultrasound (HIU) was applied to AMF samples with 0.05 % LML and lower supercooling (T _c = 31 °C; ΔT = 3 °C). HIU application in AMF and AMF + 0.05 % LML induced crystallization (p < 0.05) changing the induction time (τ) at 31 °C from 34.20 ± 1.67 min (AMF) and 47.07 ± 1.27 min (AMF + 0.05 % LML) to 23.23 ± 3.26 min (AMF) and 25.00 ± 0.87 min (AMF + 0.05 % LML). Melting enthalpies (ΔH) of AMF were significantly higher (p < 0.05) than the ones observed for AMF + 0.05 % LML when crystallized without HIU, while enthalpy values increased significantly in AMF + 0.05 % LML samples when crystallized with HIU reaching similar values to the ones obtained for AMF without LML. The viscosity of AMF significantly decreased (p < 0.05) on addition of 0.05 % LML and significantly increased on HIU application.     

Effect of High Intensity Ultrasound on Crystallization Behavior of Anhydrous Milk Fat

The need to eliminate trans-fatty acids from foods’ formulation resulted in the exploration of new lipid sources and alternative processing conditions that will improve the physicochemical characteristics and nutritional qualities of lipid-based foods. In general, the physicochemical characteristics of lipid networks depend on the microstructure and crystallization behavior of the system. The objective of this work was to use high intensity ultrasound (HIU) as an additional processing condition to alter the crystallization behavior of a lipid model system (anhydrous milk fat). Results show that HIU application not only decreases the induction time of crystallization (faster crystallization) at a constant crystallization temperature, but also generates smaller crystals. In addition, higher viscosities are obtained when samples are crystallized after HIU application. The degree of supercooling, ultrasound application settings and a combination of both parameters influence the degree of ultrasound effect on the crystallization behavior.     

Influence of Emulsifiers on the Formation and Crystallization of Solid Lipid Nanoparticles

The production of solid lipid nanoparticles by means of hot homogenization uses two different techniques: emulsification (liquid state) and crystallization (solid state). Both these processes depend on many parameters, such as composition, energy input, or geometry of the emulsification device. In former investigations known from the literature, the influence of the emulsifier concentration and the potential of controlling the polymorphic forms of the solidified particles were not investigated in detail. Here, these two factors were analyzed on the example of palm oil particles stabilized with emulsifiers suitable for foods. The influence of the emulsifier concentration on the resulting droplet and particle sizes is very high, up to a certain level. However, beside this influence, the polymorphic form of the solidified particles can be controlled by the right choice of emulsifier type and concentration.     

基于匀变速剪切对含蜡原油触变性的研究

含蜡原油的胶凝问题是管道运输过程中一个重要的研究问题。通过对经历不同降温速率温度降至凝点附近的原油做匀加速剪切,剪切至某值后再做匀减速剪切,形成一个环形剪切。测量剪切应力随剪切速率变化的曲线关系,结果表明:在这样循环匀变速剪切的作用下,样品的剪切应力各点连线也呈现一个环形,即称作含蜡原油的滞回环。在胶凝原油发生屈服之前,剪切应力随剪切速率增大急剧增大,持续的剪切会有效阻止蜡晶结构的形成;降温速率越低,原油形成的蜡晶结构强度越大。     

成核剂对尼龙66熔融与结晶的影响

通过差示扫描量热分析法研究了14种成核剂对尼龙66熔融与结晶的影响。结果表明：大部分成核剂的加入对尼龙66熔点影响不大，而熔程△Tm、结晶度和熔融熵降低；结晶峰值温度Tc升高，过冷度△Tc和半高宽D降低；低温峰所占的面积比例增加。其中滑石粉、石墨、氧化镁、聚醚砜和蒙脱土成核效果较好。     

Crystallization of waxes in sunflowerseed oil: Effects of an inhibitor

The mechanism of action of a commercial inhibitor on the crystallization of waxes present in sunflowerseed oil was analyzed. The results showed the inhibitor favored nucleation, leading to a decrease in the amount of waxes available for the growth of the crystals already formed. The inhibitor decreased the crystal size, increased the number of crystals and possibly caused slower crystallization of waxes.