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首页> 外文期刊>The Journal of Supercritical Fluids >High-pressure crystallization and melting of polyethylene in n-pentane
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High-pressure crystallization and melting of polyethylene in n-pentane

机译：聚乙烯在正戊烷中的高压结晶和熔融

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摘要

The fluid-solid phase separation and crystallization in relatively dilute solutions of polyethylene in n-pentane at high pressures were studied using time- and angle-resolved laser light scattering techniques. Majority of the experiments were carried out by cooling (crystallization) or heating (melting) while holding the pressure constant at selected pressures in the range from 10 to 54 MPa. Crystallizations were also carried out via two other pathways: (1) cooling without pressure adjustment and (2) first crossing the L-L phase boundary via pressure reduction at constant temperature followed by cooling. Crystallization and melting transitions were assessed from the variations of the transmitted light intensity or the scattered light intensity (averaged over all angles) with temperature. Kinetics of phase separation during both the crystallization and the melting were followed by the time evolution of the angular distribution of the scattered light intensities. The kinetics were typical of nucleation and growth processes. From the time evolution of the light scattering patterns the mean particle radii and their evolution were also followed. The polymer particles that form upon crystallization were collected and characterized using a field emission scanning electron microscope (FESEM) and differential scanning calorimetry (DSC). The results show that the PE crystals that form from these high-pressure conditions display a plate-like morphology, which tend to aggregate into ellipsoid-shape structures. DSC studies show that the PE crystals show higher crystallinity compared to the original polyethylene sample and display multiple melting peaks during the first heating scan, which however collapse to a single melting transition peak after the initial heating scan. The crystals that formed from the experiments in which L-L phase boundary was crossed first were found to display two distinct particle groups. These were attributed to crystals that form from polymer-rich and polymer-lean phases that form when L-L phase boundary is crossed. (c) 2006 Elsevier B.V. All rights reserved.
机译：使用时间分辨和角度分辨激光散射技术研究了聚乙烯在正戊烷中的相对稀溶液中在高压下的流固相分离和结晶。大多数实验是通过冷却（结晶）或加热（熔融）进行的，同时将压力恒定在10到54 MPa范围内的选定压力下。结晶还通过其他两个途径进行：（1）无需调节压力即可冷却;（2）首先通过在恒定温度下进行降压而越过L-L相界，然后进行冷却。根据透射光强度或散射光强度（在所有角度上的平均值）随温度的变化评估结晶和熔化转变。在结晶和熔化过程中的相分离动力学是随散射光强度角分布的时间演变而变化的。动力学是成核和生长过程的典型特征。从光散射图案的时间演变，还遵循平均粒子半径及其演变。收集在结晶时形成的聚合物颗粒，并使用场发射扫描电子显微镜（FESEM）和差示扫描量热法（DSC）对其进行表征。结果表明，在这些高压条件下形成的PE晶体显示出板状形态，并倾向于聚集为椭圆形结构。 DSC研究表明，与原始聚乙烯样品相比，PE晶体显示出更高的结晶度，并且在第一次加热扫描过程中显示出多个熔融峰，但是在初始加热扫描后却塌陷至单个熔融转变峰。发现由首先越过L-L相界的实验形成的晶体显示出两个不同的粒子组。这些归因于由富聚合物相和贫聚合物相形成的晶体，这些相是在L-L相界交叉时形成的。 （c）2006 Elsevier B.V.保留所有权利。

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