| Polymer materials are developing in the direction of high performance due to advances in technology.Among them,ultra-high molecular weight polyethylene(UHMWPE)is favored by the public for its excellent comprehensive performance.It is widely used in textile,pipe,biomedical,national defense,chemical industry,sports and other fields.However,there are relatively few studies on the development and preparation of high-performance UHMWPE materials,and the understanding of the structure,structural evolution mechanism and the relationship between structure and properties in the current research is not still clear.In this paper,different nascent UHMWPE powders were used to study the morphology,porous structure,thermal properties,initial crystal structure and structural changes under variable temperature with different primary molecular weight prepared by different polymerization methods.The influence on morphology,structure,degree of compactness and flatness of UHMWPE hot pressed sheets under different hot pressing conditions(temperature,pressure and time)was explored.The effects of polymerization conditions,molecular weight,temperatures and draw ratio on the mechanical properties,thermal properties,crystallization properties,structure and the change of chain entanglement of UHMWPE hot-drawn films with ultra-high draw ratios were investigated by different testing and characterization methods.The thermal stability of nascent UHMWPE powders was great,and its melting enthalpy,crystallization enthalpy and crystallinity decreased with the increase of molecular weight.The crystallization temperature of the UHMWPE melt decreased slightly with the extension of isothermal time.Most of the parallel non-penetrating extended chains of nascent UHMWPE powders were well reserved when they melted at the temperature above the equilibrium melting point of UHMWPE.The nascent UHMWPE powder was composed of secondary particles with a diameter of several tens of micrometers.There was a certain gap between the particles.The particles were connected by fibrous filaments.It can also be found that the secondary particles had a finer internal structure.Hierarchical porous structures of nascent UHMWPE powders with different sizes were basically formed.When the draw ratio is 10,the tensile strength and Young's modulus of UHMWPE hot-drawn films increased with the increase of molecular weight.The diameter of nascent UHMWPE powders prepared under different polymerization conditions,the degree of chain entanglement and the degree of crystallization were different,which have a great influence on the UHMWPE films prepared by compression molding.The mechanical properties of UHMWPE hot-drawn films were quite different.The higher the molecular weight,the lower the melting temperature,crystallization temperature,melting enthalpy,crystallization enthalpy and crystallinity,probably due to more chain entanglement had formed with the increase of molecular weight and the alignment regularity of molecular chains became lower.At the two different temperatures,the tensile strength of the selected UHMWPE hot-drawn films increased with the increase of draw ratios.At lower draw ratio(5× and 10×),the tensile strength of UHMWPE hot-drawn films was not much different.After drawing to higher ratio(50× and 100×),the difference of tensile strength of samples became more significant.As the hot-drawing ratio and time increased,the lateral dimension of the kebab crystal decreased rapidly,and the shish-kebab structure gradually transforms into microfiber(imperfect shish)structure.Finally,all shish-kebab structures were transformed into microfiber structures.As the draw ratio increased at higher temperature,a small amount of shish-kebab crystals gradually transform into microfibrils,while a portion of the existing microfibers were converted into extended chain crystals(perfect shish).Finally,the hot drawn films contained only microfibers and extended chain crystals. |
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