Study Of The Deformation Mechanism Of Ultra High Molecular Weight Polyethylene And The Establishment Of Morphological Diagram

Ultra high molecular weight polyethylene fibers(UHMWPE)and membranes are produced by the extrusion and post-stretching processes of gel silk(film).During the post stretching prcess,the different processing parameters(temperature,strain and strain rate,etc)are of great significance to the final morphologies and properties.Therefore,to improve the performance properties and truly provide theoretical guide in the actual manufacture processing step,the stretching-induced crystal morphological transformation mechanism during post-stretching process should be understood deeply.Actually,the post-stretching step of semicrystalline polymer materials is a nonequilibrium process driven by external stress field(strain or strain rate)and temperature,during which either stress field or temperature field may play the dominant roles.Though many efforts have been devoted to explore the structural evolution of semicrystalline polymer materials during stretching process,most studies are limited in narrowed temperature range(e.g.cold stretching or hot stretching at near melting point).Thus the coupling effects of external stress field and temperature field on driving structural evolution is far from well understood.In this work,the structural evolutions of extracted UHMWPE precursor fibers during stretching in wide temperature range and at different starin rates are studied with the combination of in-situ synchrotron radiation scattering techniques.In order to deepen understanding of the effect of external parameters(tempearture,strain rate,etc)on deformation-induced structural evolution,a semi-quantitative equation to reveal the intrinsic relationship of different structural parameters have been established.Meanwhile,the morphological evolutions of PE gel films during uniaxial and biaxial stretching are studied.And the morphological diagram of PE gel films in two-dimensional(2D)temperature-strain space and 2D machine direction(MD)-tansverse direction(TD)strain space have been established.The main results and conclusions are listed as follows.(1)Temperature effect on deformation behaviors of extracted ultrahigh molecular weight polyethylene(UHMWPE)precursor fibers is studied with in situ synchrotron radiation wide angle X-ray scattering technique(WAXS)during tensile deformation at temperatures from 25 ℃ to 130 ℃.The structural and mechanical evolution behaviors during tensile deformation can be divided into four temperature regions with boundaries located at temperatures of a,relaxation(TαⅠ)and αⅡ relaxation(TαⅡ)and the onset of melting(Tonset),respectively,which reveal that the deformation behaviors of polymer crystals are determined by the interplay between intrinsic structural dynamic or chains mobility and external stress field.Irrespective to temperature yield and strain-softening proceed via partially melting while crystal slip via cutting crystal planes occurs in strain-hardening zone.Finally we construct morphological diagrams containing crystallinity,crystal size and orientation in 2D temperature-strain space,which may serve as a roadmap for UHMWPE fibers processing.(2)Strain rate effect on the structural evolutions of extracted UHMWPE precursor fibers is studied by in situ synchrotron radiation WAXS technique at 25 ℃。The shape of stress-strain curve shows transformation from similarly brittle fracture behavior to typically ductile fracture manner as strain rate increases.The discrepant deformation behaviors at different strain rates are determined by the coupling effects of nucleation density of strain-induced crystal-amorphous transformation(n)and nucleus size(L).(3)Deformation-induced structural evolution of polyethylene(PE)gel films were investigated by in situ synchrotron radiation small and wide angle X-ray scattering(SAXS and WAXS)techniques during stretching at temperatures from 25 ℃ to 110 ℃.The structural evolutions along tensile strains showed distinct behaviors in the four temperature regions divided by TaⅠ and TaⅡ given by dynamic mechanical analysis(DMA)and Tonset measured by DSC,respectively,which was determined by the coupling effects of external stretching field and temperature.Finally,the morphological diagrams of PE gel films on distinct scales including long period of lamellae,crystallinity and crystal size in 2D temperature-strain space were established,which can give a direct sight of membrane morphologies and act as a map for processing parameters selection.(4)Morphological evolutions of PE gel films after different MD strains during TD stretching is studied with in situ synchrotron radiation SAXS and WAXS techniques at 90 ℃.Three different deformation mechanisms during TD stretching are proposed,which are caused by the differently initial structures after MD stretching.Moreover,the’morphological diagram of PE gel films in 2D MD-TD space is established to enhance the understanding of the coupling relationship of MD and TD strain on the final morphologies and tracing the distribution of these discrepant morphologies.