Identifying the Transient Cooperative Processes behind the Dynamic Behavior of Sheared Polymer Melts

聚合物熔体在强烈的剪切场作用下表现出丰富的动力学行为。例如，无定形聚合物表现出剪切变稀行为，而可结晶聚合物在面向流动的结构中形成相关的链段。然而，这种关联在可结晶聚合物熔体的动力学行为中的作用还没有得到很好的探索。在此背景下，我们最近通过去湿技术展示了这种相关段在剪切聚合物熔体粘弹性行为上的表现。我们比较了无规聚苯乙烯(APS，非晶态)和等规聚苯乙烯(iPS，可结晶)薄膜在熔点(Tm)以上的动力学。对于APS薄膜，由去湿实验得出的表观粘度(Hf)比零剪切体积粘度(HBulk)要小。令人惊讶的是，对于iPS薄膜，Hf总是大于HBulk，甚至在高于熔化温度约50K时也是如此，Hf/HBulk遵循Arrhenius行为。相应的活化能为160±10kJ/mol，表明链段之间存在协同运动，这些链段通过快速脱湿过程排列并凝聚在一起。由于温度高于Tm，观察到的协作性只能是暂时的。当前建议的目的是建立在这一前提下，通过进行系统的去湿、振荡流变学(宏观粘度的测量)和时间分辨的原位X射线散射实验(局部结构和动力学)来识别可结晶聚合物中瞬时合作过程的来源，并研究它们对聚合物熔体粘弹性性质的影响。为此，具有不同化学结构和相对分子质量的聚合物将被剪切，并使用互补技术进行原位探测，以提供局部动力学(异质性)及其宏观后果之间的联系。在斯特拉斯堡的外部合作者谢梅诺夫博士的帮助下，确保了来自理论战线的支持。此外，我们预计我们的研究将为调节聚合物熔体的性质提供新的策略和途径。

Polymer melts exhibit a rich dynamic behavior when exposed to intense shear fields. For example, amorphous polymers display a shear thinning behavior, while crystallizable polymers form correlated segments within the flow-oriented structures. However, the role of such correlations on the dynamic behavior of crystallizable polymer melts is highly unexplored. In this context, we have recently shown the manifestations of such correlated segments on the visco-elastic behavior of sheared polymer melts through the dewetting technique. We have compared the dynamics of atactic polystyrene (aPS, amorphous) and isotactic polystyrene (iPS, crystallizable) films above the melting point (Tm). For aPS films, the apparent viscosity (hf) deduced from the dewetting experiments was expectedly smaller than the zero shear bulk viscosity (hbulk). Surprisingly, for iPS films hf was always greater than hbulk, even at about 50K above the melting temperature, with hf/hbulk following an Arrhenius behavior. The corresponding activation energy of 160±10 kJ/mol suggested a cooperative motion of segments, which were aligned and agglomerated by the fast dewetting process. Since the temperatures are above Tm, the observed cooperativity can only be transient. It is the aim of the current proposal to build on this premise by performing systematic dewetting, oscillatory rheometry (measure of the macroscopic viscosity) and time resolved in-situ X-ray scattering experiments (local structure and dynamics) for identifying the origin of transient cooperative processes in crystallizable polymers and examining their influence on the viscoelastic properties of polymer melts. To this end, polymers with different chemical structures and molecular weights will be sheared and probed in-situ using the complimentary techniques to provide a connection between the local dynamics (heterogeneities) and its macroscopic consequences. Support from the theoretical front is assured with the help of our external collaborator Dr. Semenov, Strasbourg. In addition, we expect that our studies will indicate novel strategies and pathways for tuning the properties of polymer melts.