熔体拉伸聚乙烯硬弹性膜制备及后拉伸过程原位红外研究

The precursor film with row-nucleated lamellar structure can be fabricated under melt stretching process. These films show typical hard elastic material characteristic, such as high elasticity modulus and elastic recovery. The melt is stretched in a narrow space between the T-slot die and chill roll and the crystallization process is synchronously affected by the stretching force field and temperature field. In general, the melt-extrusion stretching process is a non-isothermal elongational flow induced crystallization process. In this project, we design the in-situ experimental platform using the time-resolved FTIR, which can be combined with some equipment to study polyethylene melt-stretching induced crystallization process. The rheological parameters of the melt extrusion from a slit die and the FTIR data will be collected simultaneously to confirm the theory relationship between the stretching induced nucleation and molecular structure parameter, process condition. A minitype temperature control tensile device is used to simulate the crystallization process when polymer melt is extruded from the T-slot die. This tensile device is combined with the FTIR equipment to clarify the competition between orientation and relaxation effect on lamellar growth, establishing the lamellar growth model. The two-dimensional correlation infrared technology is used to establish the molecular conformation model during the tensile and recovery process of polyethylene hard elastic materials. The viscoelastic response of crystal and amorphous region under external force is verified by rheo-optical Fourier-transform infrared spectroscopy. The in-situ FTIR study will help to perfect the hard elastic mechanism research. Finally, this project will be essential to expanding the understanding of elongational flow induced crystallization, improving the hard elastic mechanism, extending the application of in situ infrared spectroscopy.

熔体拉伸方法可以制备具有垂直于挤出方向排状成核的片晶材料，此类材料在拉伸过程呈现高模量、高弹性回复的硬弹性特征。熔体拉伸过程发生在口模到流延辊间的狭窄区间，结晶受复杂应力场和多级温度场的共同影响，是典型的非等温拉伸流动诱导结晶问题。本项目以聚乙烯为研究对象，以时间分辨红外光谱为核心，搭建多设备联用实验平台。通过同步采集熔体从狭缝口模挤出的流变学参数和红外光谱，建立分子结构参数、加工工艺与拉伸诱导成核的理论关系；利用温控拉伸装置模拟熔体出口模后的结晶过程，同步采集红外光谱数据，阐明取向与松弛的竞争对片晶生长的影响，建立片晶生长模型；结合二维相关红外技术建立聚乙烯硬弹性材料拉伸和回弹过程的分子构象模型，利用流变-红外光谱明确晶区和非晶区在外力作用下粘弹性响应，借此完善硬弹性机理研究。这项研究将有助于加深拉伸流动诱导结晶的科学认识，完善硬弹性机理研究，拓展原位红外光谱技术的应用。

熔体拉伸方法可以制备具有垂直于挤出方向排状成核的片晶材料，此类材料在拉伸过程呈现高模量、高弹性回复的硬弹性特征。充分理解硬弹性制品材料-加工-结构-性能之间的多维关系将有助于深入理解硬弹性材料在形变过程的结构演化规律，为弹性纤维、微孔膜等工业制品的制造提供理论基础。.本项目采用原位和离线信号采集相结合的方法，利用SAXS、WAXS和自行搭建的FTIR-力学实验平台探究了熔体拉伸流场内具有不同分子量和分子量分布的聚乙烯和聚丙烯得到的硬弹性膜的结构与性能的差异，研究分子量分布影响的链缠结对拉伸粘度和分子网链松弛两个参数的影响极其对初始片晶形态的影响。结果显示非晶区缠结临界分子量越大，在拉伸硬化阶段出现的拉伸硬化越弱，而非晶区由于具有越大的伸展空间，所形成的微孔尺寸越小，分布越窄，微孔的贯穿性越差。适当提高非晶区的分子链缠结将有助于提高微孔膜的力学刚度，有助于后期的拉伸成孔。.另外，我们还研究了聚烯烃材料在拉伸过程的结构演化，包括两部分其一是室温下硬弹性形变周期内的结构演化规律，其二是拉伸成孔过程的结构演化。对硬弹性形变周期而言，拉伸屈服去首先出现晶区和非晶区的界面银纹，在拉伸硬化起点开始在银纹内出现空穴，并在拉伸硬化及二次屈服阶段出现银纹内空穴的尺寸增长，但片晶本身没有出现明显的结构破坏。硬弹性材料的回复是由表面张力和片晶弹性变形的能弹性和非晶区分子链蜷曲的熵弹性共同作用的结果。从拉伸成孔的过程看，片晶堆分离后非晶区取向成纤的二次结晶和局部破碎的细小晶粒的熔融再结晶共同促使了架桥晶的形成，而热定型的过程使结晶充分，非晶区的分子链出现横向收缩使微孔扩大，孔径增加，贯穿性提高。再此两方面的协同作用下形成了结构稳定的微孔。.本项目通过设计原位FTIR-拉伸信号采集的实验装置，围绕硬弹性材料的熔体拉伸诱导结晶和形变过程的结构演化展开，基本建立了熔体拉伸诱导结晶和硬弹性材料拉伸过程片晶分离和成孔及微孔稳定化的模型