The Study On Crystalline Structural Evolution Of Ultra-High Molecular Weight Polyethylene Fibers During Hot-stretching Process

The key point for UHMWPE fibers obtaining higher performance is to make more molecules involve in the formation of extended chain crystals,and constitute the uniform continuous crystallization matrix in fibers.Therefore,the study for the crystalline structural evolution of UHMWPE fibers in hot-stretching process plays an important role on the improvement of the fiber properties.In this thesis,the effects of high spinning concentration on the ultimate structure and performance of UHMWPE fibers,the influence of stretch ratio and temperature of the hotstretching process on the structure of UHMWPE fibers and the structural evolution of fibers with different spinning concentration during hot-stretching process were studied by using small-angle X-ray scattering(SAXS),wide-angle X-ray diffraction(WAXD),scanning electron microscopy(SEM),and differential scanning calorimetry(DSC).The main results are as following:1.The effects of stretch ratio and temperature of the hot-stretching process on the structure of UHMWPE fibers were studied by using the equipment simulating the hot-drawing process of industrial production line.The results suggested that the formation of kebab crystals mainly occurred at low stretch ratio,while the formation of shish crystals took place at high stretch ratio.The increase of stretch ratio was good to the formation of shish crystals.The appropriate increase of the stretching temperature was conducive to the formation of shish and kebab crystals,but had no influence on the shish length.2.Through studying the evolution of shish-kebab crystals of fibers from low concentration solution during hot-stretching process,we found that the stretching stress firstly forced on the amorphous layer between the stacked lamellae,and led to the elongation of amorphous layers.Meanwhile,the highly oriented lamellae began to slip,and some shish-kebab crystals formed during this process.When the elongation of amorphous layers between lamellae reached to the maximum,the stretch stress was transferred to the lamellae,and then the stress-induced fragmentation recrystallization or melting recrystallization process of many lamellae occurred with new shish-kebab crystals forming.At late stage of hot stretching,the new shish-kebab crystals continually transformed to shish crystals via the stress-induced local fragmentation recrystallization or melting recrystallization process,and finally the structure composed of shish crystals formed in fibers.3.Through studying the structural evolution of shish-kebab crystals of fibers from high concentration solution during hot-stretching process,we found that the initial increase of long period of lamellae at beginning of stretching was mainly caused by the elongation of amorphous layer between the lamellae.When the amorphous layers were stretched to the limitation,the stretching stress conducted on the lamellae.Subsequently,the lamellae via the local fragmentation recrystallization or stress-induced melting recrystallization continually transformed to shish crystals.But finally,few shish-kebab crystals were still not transformed to shish crystals.On the whole,the transformation of shish-kebab crystals of UHMWPE fibers with high spinning concentration was not smooth as that of fibers with low spinning concentration.4.Through researching the formation of shish-kebab crystals of fibers from low concentration solution during hot-stretching process,we found that the formation of shish-kebab crystals was from the stress-induced melting recrystallization process of randomly oriented lamellae mainly at earlier stage of hot-stretching process,while the shish-kebab crystals formed from the stressinduced melting recrystallization process of the kebab crystals formed early at later stage of hotstretching process.Only few shish-kebab crystals formed at stretching temperature of 90 ℃ due to that few chain entanglements were disentangled to form shish-kebab crystals.But finally many shish crystals formed directly from the randomly oriented lamellae at 90 ℃.5.The UHMWPE fibers with different high spinning concentration were prepared by using industrial production line.The effects of high spinning concentration on the ultimate structure and performance of UHMWPE fibers were studied.The results indicated the increase of spinning concentration made the increase of entanglements,and worsened the crystal structure,finally caused the decrease of fiber performance.6.The effects of high spinning concentration on the structure and performance of UHMWPE fibers were further studied basing on the research theories of structural evolution of UHMWPE fibers during hot-stretching.The structure and performance of UHMWPE fibers were successfully optimized,and the spinning concentration was further improved.In a summary,we have studied the structural evolution of UHMWPE fibers during hotstretching process.We discovered the formation conditions of shish-kebab crystals have a great influence on their subsequent evolution.In addition,the evolution of shish-kebab crystals of UHMWPE fibers with low spinning concentration was smoother than that of fibers with high spinning concentration due to the effect of chain entanglements.