Flow Induced Nonequilibrium Crystallization Phase Transition Of Polymers

Crystalline polymers have been widely applied to many areas of the modern society, including national economy, defense construction, science and technology, etc.Studying flow-induced crystallization (FIC) has profound guiding significance for polymer industry. Due to the long length, polymer chains can be easily oriented or stretched by flow, which brings about the change of crystallization behavior,such as an acceleration of crystallization kinetics, the formation of new crystal forms and the variation of crystal morphology. These phenomena are closely connected to the processing and using properties of polymers. It is well accepted that crystallization under flow is a typical nonequilibrium thermodynamic phase transition occurring in daily industrial processing of polymers. However, past experiments and theories always adopted a coarse-grained and near-equilibrium approach to describe this issue, which actually do not touch the essential nonequilibrium nature of FIC. First, crystallization under the far-from equilibrium conditions like high-speed and large-strain flow are rarely involved in common FIC experiments. Second, no single nonequilibrium thermodynamic phase diagram of FIC in polymers has been conducted yet. Thus the fundamental research in academia runs into the bottleneck to give guidance on the industrial processing.This thesis introduces the FIC studies by focusing on the requirement of polymer industry. To in situ follow the structural evolution of polymers under the real industrial processing conditions, a small-angle X-ray scattering system with a vertical layout is developed. With the most used polyethylene (PE), isotactic polypropylene (iPP) and poly(1-butene) (PB-1) as model samples, the kinetic processes from initial amorphous melt to final ordered crystal are investigated by a combination of synchrotron radiation ultrafast X-ray scattering and extensional rheology measurements. Meanwhile, flow-induced nonequilibrium structural and morphological diagrams are constructed in temperature-flow strength space. The results and conclusions are summarized as follows:(1) A small-angle X-ray scattering system with a vertical layout is accomplished in University of Science and Technology of China. It can perform in situ detection on the structural evolution of polymers under the complex industrial environment by combining with the large-sized industrial equipments. With a maximum sample-to-detector distance of about 7 m, the largest measurable length scale is about 420 nm,meeting the testing requirements from lamellae to micropore. The capability of this system on in situ study has been confirmed by a drying experiment of a free latex droplet.(2) The protocol of two-step extension is proposed to investigate the early stage of FIC in supercooled iPP melt. By analyzing the dependences of crystallization kinetics,crystal orientation and mechanical stress response of sample on the interval time between two extensional operations, experimental results reveal a three-stage process of structural evolution involving chain relaxation, crystal nucleation and growth. It corresponds to a dynamic transition from chain- to crystal-network.(3) With the crosslinked PE as model sample, flow-induced structural formation is connected to the extensional stress, based on which the nonequilibrium phase diagrams of crystallization and melting are constructed in temperature-stress space. Four phases of melt, non-crystalline 8 phase, hexagonal and orthorhombic crystal are included. The interplay of thermodynamic stabilities and kinetic competitions of the four phases creates rich kinetic pathways for FIC and diverse final structures. The diagrams have been demonstrated to be universal and applicable to the non-crosslinked PE, which provide a detailed roadmap for precisely processing of PE with designed structures and properties.(4) The structural and morphological transitions of PB-1 melt under flow at different temperatures and strain rates are investigated. By comparing the thermodynamic stabilities and kinetic competitions between different crystal forms,flow is demonstrated to kinetically favor the formation of structure with high entropy and low order. In addition, increasing temperature would lead to a transition of crystal morphology from network to shish. Based on experimental results, we construct the nonequilibrium structural and morphological phase diagrams in temperature-strain rate space, respectively, which may guide the industrial processing of PB-1 material.The main innovations are summarized as follows:(1) A small-angle X-ray scattering system with a vertical layout is designed and constructed. It can serve as a large-scaled testing platform for polymer materials by in situ monitoring the structural evolution under the industrial processing conditions.(2) Two-step extension is employed to investigate the structural evolution involving chain relaxation, crystal nucleation and growth in the early stage of FIC. This experimental protocol has the advantage that exploring the crystallization behavior after the second extension can reveal the undetectable structural information induced by the first extension. Thus it provides a new experimental method for the study of FIC.(3) The nonequilibrium phase diagrams of flow-induced crystallization and melting of PE are constructed in temperature-stress space. This treatment actually adopts a nonequilibrium approach and breaks through the traditional understanding on FIC. On the other hand, the crystallization and melting diagrams are essentially the processing diagrams of PE, which provides a detailed roadmap for precisely processing of PE with designed structures and properties.(4) The ultrafast X-ray scattering technique is used to in situ follow the structural transition of polymers under high-speed deformation. It can capture the transient intermediate phase with very short life-time, like the non-crystalline 8 phase of PE and the Form III crystal of PB-1, which is necessary and critical for the understanding on the nonequilibrium physics of FIC.