Study On Refinement And Mechanical Properties Of Macromolecular Material Lamellar

Nano-scale grain refinement is a mature and powerful tool to enhance the mechanical and physical properties of metals and alloys.However,for semicrystalline polymers with an annual consumption of hundred millions tons,the related practices and theories have been rarely reported.Introducing the concept of nano-grain refinement into polymer systems,the key is to realize the grain refinement of lamellar crystal which is the most basic structural unit.Starting from the features of polymer crystallization,the current thesis proposes a melt processing method of combining melt stretching and rapid quenching to achieve the lamellar refinement,and studies its influence on the mechanical behaviors of polymer materials.The main results and conclusions are summarized as follows:Crystallization behaviors of isotactic polypropylene(iPP)under the coupling external field of melt stretching and rapid quenching were studied by combining the scanning electron microscope(SEM),wide angle X-ray diffraction(WAXD),differential scanning calorimetry(DSC)and small angle X-ray scattering(SAXS)measurements.The results shown that the lamellar refinement was not only reflected in the prominent reduction of crystal thickness and lateral size,but also in the increase of lamellar distribution density by an order of magnitude.Such a crystalline grain refinement was further demonstrated to greatly enhance the tensile toughness of iPP material without sacrificing the modulus and strength.In addition,the fine-grained and toughened iPP displayed a rope-like fracture morphology,indicating that the lamellar refinement was conducive to the generation of deformation-induced fibrillation,which should be a key factor for the high toughness of material.This work provides a general method for mechanically toughening semi-crystalline polymers through crystalline grain refinement at the nano-lamellar scale.For high-density polyethylene(HDPE),the effects of holding time,cooling method and melt stretching strain on the lamellar morphology and mechanical property of material were investigated through the similar experiments.The results once again verified the feasibility of combining melt stretching and rapid quenching to achieve the lamellar refinement.It was also demonstrated that the lamellar refinement could greatly enhance the tensile mechanical properties of HDPE.Thus the melt processing method proposed in this thesis to realize the lamellar refinement is generalizable.At the same time,by comparing the results of rapid quenching,natural cooling and different melt stretching strains,it was proved that the lamellar refining effect in HDPE has a strong dependence on the cooling rate,while the melt stretching strain did not make a great difference.