In-situ Investigation Of The Structural Evolution Mechanisms Of Polyethylene And Its Copolymers During Film Blowing Process By Synchrotron Radiation Technology

Polyethylene and its copolymer films are the most widely used polymer films,and are used in packaging,agriculture,energy and other fields.Extrusion film blowing is the main molding method of polyethylene and its copolymer films.A deep understanding of the structural evolution mechanism of polyethylene and its copolymer blown film processing is of great significance for developing the basic theory of polymer film processing and guiding the development of film products.The molecular weight and its distribution,the type and content of comonomers and branched structures,etc.,endow polyethylene and its copolymers with abundant chemical structural parameters,enabling them to meet different service requirements.During the process of film blowing,under the action of multiple process steps and parameters,polyethylene and its copolymers experience complex external fields such as non-uniform flow field and temperature field.It also involves the evolution of the multi-scale,multi-morphic condensed matter structure inside the film from sub-nanometer-scale unit cells to micro-scale spherulites and fibers.Therefore,it is undoubtedly a great challenge to study the coupling effects of molecular structural parameters and multi-scale structural evolution under non-uniform external fields.Based on synchrotron radiation X-ray scattering technology,this work develops on-line research technology and methods for film blowing,and conducts systematic research around different molecular structure parameter systems and external field conditions.This work promotes the understanding of the basic physical problems such as flow-induced crystallization,crystal deformation and phase transition involved in the blown film forming and service process of polyethylene and its copolymer films,and accumulates basic theories for the development of high-performance polymer film products and engineering ideas.The specific research work and results are as follows:(1)Using the self-developed online film blowing device,combined with synchrotron radiation wide and small angle X-ray scattering technology,by optimizing two polyethylene(PE1 and PE2)with significantly different chain relaxation kinetics,the synergistic and competitive effects of temperature and flow field on polyethylene crystallization during film blowing were studied.According to the evolution process of structural parameters and the characteristics of crystal morphology,three types of structural evolution during the film blowing are proposed.ⅰ)For PEI with fast chain relaxation,the temperature field dominates the crystallization process during film blowing.ⅱ)PE2 with slower chain relaxation,at high take-up ratio,the film blowing process is dominated by flow field-induced crystallization,and fibrous nuclei are formed at the early stage of crystallization.ⅲ)For PE2 at low take-up ratio,temperature and flow field synergistically controls the crystallization process,and two structures with different orientation and distribution are generated in the system.Physical models of the three processes will help to understand the effects of flow-induced crystallization and temperature-induced crystallization on structural evolution during film blowing.(2)Using synchrotron radiation X-ray scattering,solid-state NMR and other characterization methods,the influence of molecular chain structure on film blowing process and film structure/performance was studied by selecting three polyethylene with different branched chain structures as model systems.The three polyethylene are l-PE with long chain branches,h-PE with butyl branches and o-PE with hexyl branches.The film blowing process of l-PE is affected by the synergistic effect of flow field and temperature,which makes l-PE have excellent bubble stability.The microstructure and macroscopic properties of the l-PE film depend on the take-up ratio.The film blowing process of h-PE and o-PE are dominated by the temperature field,and the formation of spherulite structure in the film makes it have excellent mechanical properties.In this work,the process-structure-property relationship in film blowing of polyethylene and ethylene-α-olefin copolymers was constructed,providing theoretical guidance for film production and performance regulation.(3)Using the self-developed stretching device,through the online analysis of the stretching process of ethylene-vinyl acetate copolymer in a wide temperature range,the effects of comonomer and chain movement ability on the tensile deformation and phase transition behavior of the film were studied.According to the structural evolution process and the relaxation characteristics of the molecular chain,the stretching temperature range is divided into three regions.In the high temperature region,the crystal deformation and failure modes include shear slip and crystal melting,and stressinduced melt recrystallization occurs under large strain.In the mid-temperature region,large-scale amorphization of the crystal occurs in the nonlinear deformation region,and the content of pseudo-hexagonal structure with low order degree in the destruction and reconstruction stage begins to increase.In the low temperature region,the film undergoes uneven deformation,and a non-periodic pseudo-hexagonal structure is mainly formed in the later stage of stretching.By analyzing the phase structures in different strain and temperature spaces,the non-equilibrium structure map of ethylenevinyl acetate copolymer films during tensile service was constructed.(4)A set of three-layer co-extrusion film blowing experimental equipment was built,and the film blowing of high-performance liquid crystal polymer film was realized.By optimizing the overall design scheme and adjusting the wind ring structure,the device can meet the requirements of being used in conjunction with a synchrotron radiation X-ray station.By adding a snap ring structure at the connection between the die and the extruder,the device can achieve up and down blowing.Using this device,combined with the research on the crystallization behavior of liquid crystal polymers,the stable film blowing and performance regulation of liquid crystal polymers are realized,which provides theoretical guidance for breaking through the industrial difficulties in the preparation of liquid crystal polymer films.