Micro/nano Hierarchal Structure Control Of High-performance Polyolefin Based Shape Memory And Microcellular Foamed Materials

The energy-saving,emission-reduction,and environmental protection have become critical issues in the development of automobile and appliance industry.Consequently,the light-weight and recyclable materials are widely used to improve fuel efficiency,save electricity,reduce emissions and recycle and reuse wastes.Polyolefin-based thermoplastic elastomers?TPEO?and their compounded thermoplastic polyolefins?C-TPO?are new generation polyolefins with some advantages,such as light-weight,high ratio between performance and cost,outstanding comprehensive properties,recyclability and reusability.TPEO,C-TPO and their foams are widely used as interior and exterior parts of cars,cable and wire,washer drum,waterproof,heat insulation materials,logistics packaging,sole and bicycle tyres.In this dissertation,high energy radiation is used to tailor the long chain branching and heterogeneous crosslinked topologies of isotactic poly?1-butene??iPB-1?,olefin block copolymes?OBC?and soft TPO.As a result,their corresponding crystal polymorphism and transformation,microphase separation structure and orientated multiphase morphology are controlled.The supercritical carbon dioxide?Sc-CO₂? assisted batch foaming is used to regulate the cell structure and morphology of crosslinked OBC and its hard TPO based microcellular foamed materials.Moreover,advanced characterization and testing methods(such as high temperature GPC combined with triple ditectors,high temperature ¹³C-NMR,ARES,XRD,DSC,SEM,2D-SAXS and DMTA)are applied to establish the quantitative correlations between hierarchical structure and properties.1)The slow transformation of form ? to form I in the high-performance green tube and pipe material of iPB-1 limited its larger commercial diffusion.In this part,the correlations among long chain branched?LCB?structure,coil contraction conformation,and crystal polymorphism and their transformation are established.Among the coil contraction conformation of LCB chains,length of crystallizable 1-butene sequence and crystallization condition,the coil contraction conformation plays a critical role in crystal polymorphism and their transformation.As crystallized at a high crystallization rate,the low-to-moderately branched iPB-1 crystallizes from melt into the mixtures of form ? and form ?,because of the lower coil contraction conformation and larger crystallizable 1-butene sequence.The highly coil compact conformation and shorter crystallizable 1-butene sequence in the highly branched iPB-1 result in crystallization from melt into pure form ?.The weakly birefringent spherulites of form ? contains unconventional highly twined lamellae,and the form ? used to be crystallized only from iPB-1 solution.When crystallized at a low crystallization rate,the former crystallizes form melt into pure form ?,and the latter crystallizes from melt into the mixtures of form ? and form ?.The higher inter-crystalline links?entangled loops,tie molecule and form ??,acting as self-nucleation sites for form I,means the higher transformation rate of form ? into form I.The young`s modulus and yield strength of LCB-PB decrease with an increase in the content of form ?.Form ? partially transformed into form I under stretching,and form ? in the highly branched iPB-1 do not melt-recrystallized into form ? upon heating.2)The shape memory polymers?SMPs?,based on TPEO blends,exhibit low-melt strength and lower high-temperature resistance,narrow range of shape transition temperatures.In this part,high-strength and high-temperature resistance triple shape memory elastomers?SMR?are prepared via radiation crosslinking.A moderate molecular weight?M_w?polyolefin elastomer?POE?is melt-blended with yarn?YP?and high-melt-strength?HP?grade polypropylene?PP?,respectively.The blends are subsequently irradiated using electron beam and gamma radiation.After irradiation,the uncrosslinked crystallizable ethylene and crosslinked amorphous sequences of POE chains and LCB-PP are interlinked via chemical bounds.The discoid-like PP phase with an average size of 1.6×11?m,dispersed in the POE matrix,is aligned parallel to the plane of the films.The PP lamellar crystals is partially orientated,and the PP lamellar crystals with the?040?_?and?110?_?planes are arranged parallel and perpendicular to the surface of the highly crosslinked film,respectively.The integration of heterogeneous macromolecular structure and aligned multiphase morphology contributes to high-tensile strength and excellent triple shape memory effects.At the solid state,the tensile strength??_b?of 31–38 MPa,elongation at break??_b?of 900–1100%,and strength at 100% strain??_y?of 8.5–12 MPa are observed for the SMRs.Two temporary shapes with the fixity ratios?R_f?of 82–97%are programmed by the reversible aggregation and solidification of the segregated PP lamellar crystals and POE bundle-like crystals.Under thermal triggering at the switching temperatures of 90 and 180?,the entropy-driven elasticity of the separated POE and PP switching segments provides a good shape recovery?R_r?of 88–94%to the memorized complex permanent shape.3)The low-melt-strength of olefin block copolymers?OBCs?leads to poor dual shape memory effect?D-SME?and foaming ability.In this part,the influence of absorbed doses on the crosslinking degree,crystallization behavior and tensile property of the OBCs with different chain structures are studied.Moreover,the Sc-CO₂ assisted batch foaming property of the crosslinked OBCs at different foaming temperatures is investigated.A high-molecular-weight OBC with a high 1-octene content can be crosslinked at a lower absorbed dose,yet the moderate-molecular-weight OBCs crosslinked at a higher absorbed dose still exhibits high elasticity.As the dose increases,the ?_y and ?_b decrease.The ?_b nonmonotonically correlates with dose,it increases with the low-to-moderate doses,then subsequently decreases at the high doses.Irradiation results in mainly crosslinking in the amorphous regions of OBCs,which results in no significant changes in the crystallization and melting behaviors.The crosslinked OBCs exhibit outstanding D-SME with R_f and R_r high than 93%?The Sc-CO₂ batch foaming property is comprehensively influenced by the lamellar microphase structure,selective crosslinked chain structure and diffusivity of Sc-CO₂.At the foaming temperatures of 122 and 126?,the cells are highly orientated parallel to the long surface of the foamed OBCs.As the dose increases,the average cell size decreases from 220?m to 34?m.The distinct cell size distribution results from different swelling properties of the soft and hard segments of OBCs in Sc-CO₂.Under pressure drop,the diffusivity of Sc-CO₂ in OBC sheets gradually decreases from skin layer to core layer,which contributes to a gradient increased distribution of cell size from skin layer to core layer of the foams.The average cell size is the same at different foaming temperatures.4)The nanocellular foamed polypropylene materials manufactured using Sc-CO₂ assisted mold-opening foam injection molding show lower toughness and lower low-temperature-impact resistance.This process also produces skin-layer cell morphology.In this part,the high-performance thermoplastic polyolefins?H/Y-TPOs?are prepared via melt mixing of the moderate-molecular-weight OBC with YP and HP.The Sc-CO₂ assisted batch foaming properties of the TPOs are investigated,and the quantitative correlation of cell structure and morphology with tensile property is established.The multiphase morphology of Y-TPO is sea-island,and the average domain size of the OBC dispersed phase is 1.05?m.Introducing of an OBC into the YP not only reduce the complex viscosity of YP,but also improve the melt elasticity and crystallization temperature.Consequently,the foaming window temperature of YP increased from 5? to 22? for Y-TPO.At the foaming temperatures of 135–150?,the average cell size of 3.66–6.62?m are observed for the microcellular foamed Y-TPO materials.Besides,the e_b is higher than 450%,and apparent s_y at 50% strain and s_b are about 17.5 and 23 MPa,respectively.The Y-TPO exhibits potential application in manufacturing high strength and high toughness nano/microcellular foaming materials using Sc-CO₂ assisted foam injection molding.The compression molded H-TPO exhibits hierarchical heterogeneous skin?co-continuous?-core?sea-island?layer morphology,the average domain size of the dispersed OBC phase in the core layer is 1.16?m.After foaming,the high-melt-strength and heterogeneous cell morphology contribute to lower cell size and higher tensile property.As the foaming temperature increases from 135? to 153?,the average cell size increases from 2.84?m to 8.75?m.Accordingly,the e_b decreases from 635% to 350%,and apparent s_y at 50% strain decreases from 20.3 MPa to 16.05 MPa,and the apparent s_b decreases from 27.8 MPa to 20.5 MPa.In conclusion,as the average cell size is lower than the critical dimensions of 3–4?m,the microcellular foamed TPO materials exhibit outstanding high toughness and high mechanical strength,because of the coordinated toughening effect of the cell and elastomer.However,they show brittleness when the average cell size is higher than a dimension of 30?m,and they can not be used as materials.