In-Situ Study Of Flow Induced Crystallization And Phase Transition Based On Synchrotron Radiation Scattering And Nuclear Magnetic Resonance

Polymer materials are crucial to modern industry and essential for both industrial production and daily life.They are among the four major materials,along with metal,inorganic non-metal,and composite materials.During polymer formation and use,they undergo non-linear rheological phenomena such as flow-induced crystallization,crystal phase transformation,and chain dynamics due to flow fields.Additionally,their complex multi-scale condensed structure,ranging from sub-nanometer atoms to micrometer-sized spherulites,makes their characterization challenging.Unfortunately,the current process of developing polymer material products is inefficient and expensive,relying on extensive trial and error experiments.Therefore,understanding the non-equilibrium structural and dynamic evolution laws and mechanisms of polymer materials under external fields,such as flow fields,is critical for efficient and accurate research and development of high-performance polymer materials and products.To capture the rapid structural and dynamic evolution of polymer materials under non-equilibrium conditions,high-time-resolution in-situ characterization techniques are necessary.In-situ synchrotron radiation scattering technology,based on highbrightness and high-collimation synchrotron radiation sources,is ideal and widely used for studying non-equilibrium states of polymer materials.However,while scattering methods provide excellent structural resolution for polymer crystal structures,they lack the same resolution for amorphous regions with low scattering contrast and without long-range ordered structures.This is where nuclear magnetic resonance(NMR)technology comes in.Due to its excellent chemical structure and dynamic discrimination capabilities,NMR technology complements the shortcomings of scattering methods when studying amorphous regions under non-equilibrium conditions in polymer materials.To enable NMR technology to characterize the structure and dynamics of non-equilibrium states,in-situ NMR characterization technology and methodology must be developed.This thesis developed an in-situ tensile device combined with the low-field nuclear magnetic resonance(LF-NMR)spectrometer and utilized NMR in-situ tensile research methodology.To complement the national "dual carbon" strategic plan,biobased poly[R-3-hydroxybutyrate-co-4-hydroxybutyrate](P(3HB-co-4HB))and epoxidized natural rubber(ENR)were selected as research systems.The study examined the flowinduced crystallization and phase transition mechanisms of P(3HB-co-4HB)and ENR using synchrotron X-ray scattering and NMR in-situ research.The approach of NMR in-situ tensile research methodology was applied to semi-crystalline polymer systems to investigate the chain orientation and relaxation dynamics during uniaxial deformation.The study focused on two types of semi-crystalline polymers,namely αc relaxation and crystal-fixed.The specific research work and results are as follows:(1)This work addressed the lack of in situ equipment and methods for characterizing non-equilibrium crystalline-amorphous structures and dynamics by designing and developing an in situ stretching device,named Rheo-Spin NMR.This device combines low-field nuclear magnetic resonance(LF-NMR)and enables the simultaneous acquisition of stress-strain curves with temporal resolution and corresponding NMR signals.The strain rate can be adjusted within the range of 10-5 s-1-10-2 s-1,and the maximum stretching ratio can reach up to 3.8.Using natural rubber as a standard test sample,in-situ tensile experiments were successfully carried out.Time resolved T2 relaxation spectra were collected to evaluate molecular segment dynamics during uniaxial deformation,coupling macroscopic mechanical rheological information and microscopic molecular dynamics information together for the first time.(2)The structure and dynamics of P(3HB co 4HB)crystal region and amorphous network have been revealed,and the mechanism and necessary conditions for α-βcrystal phase transition have been proposed.The chain dynamics and crystal network structure of P(3HB co 4HB)have been systematically studied using various solid-state NMR techniques.The composition of the crystalline region was determined and revealed that the 4HB unit was excluded from the crystalline region.The amorphous structure was found that there was no microphase separation.There is the absence of chain dynamics in crystal region.The molecular chain network density,including the physical cross-linking point network and molecular entanglement network,was linked to the macroscopic mechanical properties.The modulus of P(3HB-co-4HB)was found to decrease significantly as the network density decreased.The stress-induced crystallization mechanism of P(3HB-co-4HB)during stress loading and unloading was systematically studied using in-situ synchrotron radiation wide-angle X-ray scattering(SR-WAXS).During deformation of P(3HB-co-4HB),the β-form was mainly formed from fully extended tie chains between lamellae,with a small fraction derived from the melting of the α-form.Upon retraction,the β-form transformed back into the highly oriented α-form.Notably,the stress-induced crystallization rate of the β-form was independent of temperature,indicating a constant nucleation barrier.The formation of the β-phase required fully extended lamellar tie chains and an applied stress exceeding the critical stress for β-form formation.(3)The effect of molecular chain polarity on the strain induced nucleation mechanism of ENR was revealed.The nuclear magnetic resonance(NMR)were conducted on the dynamics and molecular chain network structure of various ENR.The results revealed that the inclusion of epoxidized polar functional groups resulted in an increase in the polarity of molecular chains,with no significant change in network density,but an increase in network distribution.In situ NMR studies of the molecular chain dynamics evolution process during the strain-induced crystallization(SIC)of various ENR revealed that crystallization led to a reduction in molecular chain dynamics,with this decrease being less pronounced in ENR with lower crystallization ability.The effect of molecular chain polarity on SIC of natural rubber was studied in situ using synchrotron wide-angle X-ray scattering.The crystals of ENRs of SIC are mainly composed of isoprene segments,with some epoxidized isoprene potentially included in the crystal region.We calculated normalized critical nucleation barrier(G*),end surface free energy(γe),the onset of SIC(λonset),and amorphous orientation degree(P2)data.The findings demonstrate that as the epoxidation degree rises,the Gibbs free energy(ΔG)required for crystallization also increases.In ENR,the process of SIC accompanied by microphase separation between isoprene segments and epoxidized isoprene segments.Increasing the degree of epoxidation results in higher free energy consumption during microphase separation,causing ΔG of the system to increase.Epoxidized isoprene enrichment on the crystal nucleus surface after phase separation causes a reduction in end surface free energy of the crystal.(4)The mechanism of the influence of orientation on NMR dynamics of semi crystalline polymers was revealed.Based on the Rheo-Spin NMR apparatus,the structure and dynamic evolution of low-density polyethylene(LDPE),linear lowdensity polyethylene(LLDPE),and polycaprolactone(PCL)during uniaxial stretching were investigated.We explored the impact of orientation differences on NMR signals resulting from variations in orientation degrees by comparing PE samples with different orientation degrees and magnetic field angles,revealing the anisotropy of the chemical shift.We quantitatively studied the effects of temperature,molecular chain structure,and material type on in situ uniaxial stretching experiments for PE,and identified the sources of dynamic differences in different phase components based on these factors.