Flow-Induced Polymer Crystallization

As an extremely external driven and kinetics controlled process, flow-induced crystallization (FIC) of polymer is inevitably involved in industrial processing of semicrystalline polymers, the most widely used polymeric materials due to low cost and structural diversity. The macroscopic flow forces polymer chains to be oriented or stretched, and further controls the structural hierarchy from sub-nanometer to micrometer length scales. Based on large amount of experiments in the last60years, it is generally accepted that flow can enhance crystallization rate by orders of magnitude, induce morphological transition from spherulite to shish-kebab or row-nuclei structure, and lead to new crystal forms. Nevertheless, though experimental observations are widely confirmed by different groups, the understanding of mechanisms of FIC are far from satisfactory. Discrepancies on the structures of shish/row-nuclei or precursors and the roles of molecular and flow field parameters still exist, which are debated intensively in recent years.This thesis focus on the dynamic and ordering kinetic of polymer induced by external flow field. The main objective of this thesis is to understand and manipulate the "processing-structure-poroterities" relationship of plolymic material. To resolve the fundamental challenges of polymer crystallization under processing-revelant conditions, a novel apparatus combing polymer extrusion processing with X-ray scattering and rheo-ultrafast X-ray scattering technique are developed, which allow studies on pre-ordering and crystallization under flow field simultaneously. Meanwhile, the work on model systems provides basic understanding of the role of molecular and flow parameters on structure and thus properties of polymer. The main results and conclusions are summarized as follows:1) The formation mechansim of shish-kebab is investiged in polyethylene (PE) bimodal blends by combing a unique homemade extensional rheometer and synchrotron radiation small-angle X-ray scattering (SR-SAXS). It is found that the critical strain for shish formation decreases with increasing long chain concentration, which contradicts the role of CST but agrees well with stretched network model (SNM). Quantitative analyses indicate that the formation of shish is determined by the degree of network deformation rather than solely by strain or long chain concentration at a specific temperature. In addition, three types of shish with different stability are observed sequentially by increasing strain. Based on our results, the entire molecular picture of shish formation is revealed. When stretched to a critical deformation degree, the aligned segments couple with each other to form fibrillar-like type I shish, which further transform into type II shish embedded with sporadic lamellae and type III shish embedded with well-defined periodic lamellae sequentially by increasing flow intensity.2) The role of long chains in FIC is studied in a model bi-disperse poly(ethylene oxide)(PEO) blends with a combination of extension rheological and in situ SR-SAXS measurements. To our knowledge, this is the first set of extension induced crystallization experiments in which the strain rate reaches the threshold for chain stretch for long chains of well-defined length and concentration. Rheological data of step extension on PEO melt are divided into two regions, corresponding to distinctly different features of crystallization kinetics and crystal morphologies. A new mechanism based on entanglement network perspective is proposed, in which the second entanglement network constructed by long chains has three effects:(i) helping flow to change the free energy of polymer melt more effectively;(ii) ensuring the specific work can impose on the system;(iii) favoring the formation of precursors.3) Extension flow induced crystallization of isotatic polypropylene (iPP) is studied with a combination of extension rheological and SAXS/WAXS measurements. Weak and strong accelerations of crystallization accompanied by transition from point nuclei to shish coincide well with the rheological behavior in before and beyond fracture strain zones. The microrheological model explains the acceleration of nucleation in the "before fracture strain zone" well, while a "ghost nucleation" mechanism is proposed to interpret the strong acceleration of nucleation in the "beyond fracture strain zone". In ghost nucleation model, the acceleration of nucleation on several orders and formation of shish is due to a self-acceleration effect by structural flow during extension rather than entropy loss of simple polymer melt.4) In-situ investigation on FIC under strong flow by combining extensional rheometer and ultrafast X-ray scattering reveals a constant critical strain or time for nucleation in iPP melt in a wide temperature range from130to170℃. Our discovery contradicts the strain-temperature equivalence predicted by classical entropy reduction model but unveils the non-equilibrium nature of FIC. To account for the temperature independence of flow-induced nucleation, a tentative kinetic pathway of nucleation describing stretch-induced hierarchical structural transitions is proposed through which the capability of flow as driving force is justified.5) A novel apparatus combining polymer extrusion processing and X-ray scattering is designed and constructed. It allows direct, real time monitoring of structure and temperature development in polymer material during extrusion. The apparatus involves a vertical industrial extruder equipped with a four-roll stretching device to mimic the processing environments of uni-axially oriented films or sheets, a simultaneous SAXS/WAXS system and an infrared thermometer as detection unit. By moving the sample along the center line, structure and temperature development as a function of position can be obtained. The performance of the apparatus was verified by a test experiment, which allows us to establish the relationship between processing parameters and evolution of structure with different length scales.The main innovations involved in this thesis:1) Unveil the dynamic process of shish formation from initial chain configuration to final stable nuclei in PE melts.2) Propose a phenomenological model based on entanglement network perspective, which offers a new viewpoint for FIC study.3) Propose a "ghost nucleation" in non-linear region which explains well the acceleration of nucleation in orders of magnitude and the formation of shish.4) Develop ultrafast X-ray scattering technique in flow-induced crystallization; Unveil the nonequilibrium nature of flow-induced nucleation and propose an non-classical kinetic pathway to nucleation containing stretch-induced conformational ordering and isotropic-nematic transition.5) Design and construct a novel apparatus combing polymer extrusion processing and X-ray scattering, which extends the application of X-ray scattering methods to extrusion processing for the first time.