熔体静电纺丝的电场力作用机制及射流形成机理研究

Melt electrospinning is of wide applications and scientific significance in various fields, such as biomedicine, sensors, catalysis, and environmental protection. There exist two challenges for the melt electrospinning that it is very difficult to prepare nanoscale polymer fibers, and the production is too low. Following the previous experimental investigations on the melt electrospinning in our group, molecular dynamics simulations and experiments will be carried out to understand the molecular dynamics of polymers in the processes of Taylor cone and jet formation, and jet extension in strong electrostatic fields. The interactions between the polymer chains at the tip of the Taylor cone will be explored by taking into account the induced polarization of polymers and their screening effects on the external electrostatic fields. The proposal will get microscopic insights into the origin of electrostatic forces and the mechanism of jet formation in the melt electrospinning. Furthermore, the profiles of the electric susceptibility, the induced charge density, the orientation parameter and the entanglement degree over the space will be calculated to identify the key factors that prevent the initial jet and the equilibrium distance between two adjacent Taylor cones from being thinner and shorter, respectively. To our best knowledge, there is no work reported so far on the molecular dynamics of polymers in the processes of the melt electrospinning. This proposal will give a novel insight into the origin of the electrostatic forces acting on the molten polymers in the melt electrospinning, which is distinct from the well known leaky dielectric theory. It will also help to seek new approaches to completely break through the bottlenecks for the melt electrospinning to get finer polymer fibers and to realize their mass production.

熔体静电纺丝在生物医药、新能源、传感器、催化、环保等领域具有广泛的应用价值和重要的科学意义。面对熔体静电纺丝目前存在的纤维太粗和产量太低这两个瓶颈，本项目将以本课题组的前期实验探索为基础，针对泰勒锥的形成、射流的形成、射流被拉伸这三个过程，利用分子动力学模拟和实验验证，从分子水平深入理解它们的分子动力学。结合诱导极化效应和极化电荷对外加电场的屏蔽效应，通过分析分子链在泰勒锥尖端的相互作用及受力，重点揭示外加强电场对熔体的分子作用机制和熔体形成射流的微观机理。在此基础上，通过考查分子链的电极化率、极化电荷密度、附加电场强度、取向和缠结度等的分布，进一步明确减小初始射流直径和缩短相邻泰勒锥平衡间距的关键制约因素。国内外尚未见有关熔体静电纺丝的分子动力学的报导，本项目将为熔体静电纺丝揭示一种新的有别于现有漏电介质理论的电场力作用机制，从根本上为突破熔体静电纺丝的两个瓶颈提供新的思路。

在国家自然科学基金青年基金的资助下，本项目构建了具有温度普适性的聚丙烯粗粒度模型及其可极化Drude振子模型。用分子动力学模拟，研究了聚丙烯熔滴在强电场中的形变，发现诱导极化效应并不足以引起聚合物熔体发生形变。借助原位摄像、原位拉曼光谱和分子动力学模拟，以聚氯乙烯凝胶和聚丙烯熔滴为例，研究了电场引起聚合物体系变形的作用机制，证实了熔体静电纺丝过程中，引起聚合物熔体发生变形和形成泰勒锥的电场作用力，产生于从纺丝喷头负极金属表面逸出并进入熔体的电子。进一步研究了聚丙烯熔体与电荷间的相互作用，发现负电荷比正电荷，更容易吸附于聚合物熔体。电子吸附于聚合物分子链，并由于聚丙烯的介电效应，在熔体表面聚集，在电场作用下，拖拽着表层聚合物形成泰勒锥和射流。设计了近场直写熔体静电纺丝可控成型实验及装置，研究了在一维、二维、三维有序沉积中，射流直线下落运动与平面移动的速度匹配机制，探究了喷头移动速度、纺丝电压和接收距离等参数对射流运动及沉积状态的影响，并分析了这三个因素之间的相互关联机制；证实了射流下落运动及被拉伸的动力，来源于电场对其中的静电荷的持续作用，而非泰勒锥在破裂时的喷射动能。可以认为在从自由面进行熔体静电纺丝的过程中，制约泰勒锥数目增加及射流变细的关键因素，是此前未被关注的从负极逃逸进入熔体的电子数量。该研究所获得的结果和结论，除了有助于优化熔体静电纺丝的工艺条件和创新纺丝设备，还在电响应变焦透镜、致动器等智能器件领域及生物组织工程支架领域，具有潜在应用。