Study On Transition Behavior And Mechanical Properties Of Mesophase Polypropylene During High Pressure Annealing

Isotactic polypropylene(iPP)is one of the most widely application plastics with excellent mechanical properties,good thermal resistance and processability.In fact,the polycrystalline structure of iPP material has a great influence on its performance,and the processing process is the process of structure formulation.Therefore,it is an important research to improve the performance of iPP products by optimizing the processing technology.iPP has rich polymorphism.Besides the three crystalline polymorphs of α,β and y forms,the preparation of mesophase iPP and the study of cold crystallization behavior have concerned widely.The research can not only help people understand the formation mechanism of metastable polymers,but also provide new ideas for improving the physical properties(especially transparency and toughness)of iPP material.In this work,the mesophase iPP was prepared by rapid pressurization method,and then subjected to high pressure annealing treatment.The annealing pressure was 1.25-1.75 GPa,and the annealing temperature was fixed at 200 ℃.Combined with wide-angle X-ray diffraction(WAXD)and small angle X-ray scattering(SAXS),atomic force microscopy(AFM)and differential scanning calorimetry(DSC)were used to study the phase transition of mesophase iPP under high pressure annealing.Subsequently,we used the stretching device to study the structural evolution of the y phase prepared by different methods(high pressure annealing and slow pressurization)during the stretching process,and analyze the relationship between crystal structure evolution and mechanical properties of the products.The main research results are as follows:1.Phase transition of iPP mesophase prepared by pressurization in high pressure annealing process(1)It was first discovered that high pressure annealing can induce the transition of mesophase iPP to γ phase,which is likely to be a solid-solid phase transition.and the phase transition kinetics is strongly dependent on the annealing pressure.The WAXD test results show that the phase transition rate is faster at lower pressures:the phase transition rate decreases gradually with the pressure increases,the onset time and finishing time increase;When the pressure is further increased to or above a certain critical pressure(between 1.65 and 1.75 GPa),the mesophase structure stability during the annealing process,and the phase transition is completely suppressed.(2)It was observed by AFM that the initial mesophase sample and the y phase sample after the phase transition were nodule structure.During the annealing process,the nodule size of the mesophase decreased from 8.2 nm to 7.2 nm.The SAXS results show that the size change of nodule structure during annealing process is related to the formation of γ crystal.When the mesophase to γ crystal transition,the transition of the straight chain structure to the unique cross structure of γ crystal causes a long period of reduction.Crystal perfection is related to annealing time.(3)The DSC was used for thermal analysis of samples under different annealing pressures.It was found that the devitrification of rigid amorphous fraction(RAF)occurred before the transition of the mesophase to the γ phase,and the devitrification temperature of RAF increased with the increase of pressure.When the devitrification temperature of the RAF reaches 200 ℃,the RAF structure is stable and phase transformation does not occur.2.Study on the relationship between structure and mechanical properties of γ-iPP prepared by slow pressurization method and high pressure annealing method(1)The γ phase prepared by the slow pressurization method has a typical spherulite structure,which is fomed by stacking a large number of lamellae.The γphase prepared by the mesophase high-pressure annealing method has no spherulitic structure,which is formed by nodule crystals,and there is no obvious periodic arrangement between the crystals.The experimental results show that the γ phase crystallinity prepared by the slow pressure method is higher,and the crystal perfection degree is greater than that of the high pressure annealing γ phase sample.(2)The high-pressure annealing γ phase sample has good toughness,and there is no obvious stress whitening phenomenon in the uniaxial stretching process.This is determined by the nodule structure of the γ crystal.The tiny nodule crystal can effectively disperse the stress and thus avoid stress concentration.At the same time.the structure facilitates the stress transfer between the amorphous region and the crystal region,thereby suppressing the generation of holes and crazing.