| As we all know,the microstructure of products determines their macroscopic performance.Meanwhile,processing method determines the microstructure.How to establish a processing-structure-performance relationship is the focus of research in the field of polymer materials.There are many factors that affect the final structure during processing.At the same time,pressure is an important physical parameter which used widely.Therefore,it is very important to explore the effect of pressure in the field of crystallization behavior of polymers.The preliminary work of the research showed that isotactic polypropylene(i PP)is a kind of representative polymer by studying the influence of pressure on the crystallization behavior of polymers.Four crystal phases of ?,?,? phase and smectic phase(mesophase)could exist in i PP products.In this work,the coexistence of multiple crystal phases in the product may greatly improved its comprehensive mechanical properties greatly by changing the processing methods to control the crystallization behavior of i PP and obtain crystalline products with multiphase structure is the focus of this work.Furthermore,researches show that high-pressure treatment can produce mixed polymorphic i PP materials,and different pressurized conditions can obtain products with different crystal structures of ? phase,? phase,mesophase,mixed polymorphic ?/?-i PP,mesophase and ? mixed phase.Moreover,it can even control the content of different crystal phases.In this work,we have studied the effect of pressurization on the crystallization behavior of i PP melts containing ? nucleating agents(?-NA),which is likely to provide new ideas for the structural design of i PP materials.In this study,the crystallization behavior of i PP/WBG-II melts at different pressurization rates was studied for the first time.The 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 microstructure and thermal properties of the products,and the influence of the coupling effect in the pressurization rate and ? nucleating agent on the crystal structure of i PP were discussed.Subsequently,several products were selected for mechanical property testing,and the polycrystalline structure of i PP on the mechanical properties of products was explored.The main results are as follows:1.Effect of pressurization rate on the crystal structure of i PP/WBG-II(1)Changing the pressurization rate can control the polycrystalline structure of the product.Two critical pressurization rates were found.When the pressurization rate R ? 15 MPa/s,a mixed polymorphic ?/? can be obtained.When R ? 75 MPa/s,pure mesophase i PP is obtained,and when R = 15-75 MPa/s,the mesophase and ?/? of three-phase mixed structure can be obtained.(2)When the pressurization rate is between 0.6-15 MPa/s,with the increase of the pressurization rate,the content of ? phase increases first and then decreases,while the content of ? phase decreases.The analysis of the crystallite size and long period found that within the pressurization rate,? crystal and ? crystal grow competitively.?-NA mainly promoted the formation of ? crystal.With the increase of the pressurization rate,the formation of ? crystals decreases rapidly,which provides more space for the growth of ? crystals and increases the content of ? phase.The change in the phase content,caused by the competitive effect,is likely to be the result of the space effect.The results show that the comprehensive influence of nucleating agent and pressurization rate on the crystallization behavior of i PP melt can be used to adjust and control the i PP multi-component crystal structure and phase content more accurately.(3)Comparing the data of samples with nucleating agent content of 0.2 wt% and 0.5 wt%,it is found that the critical pressurization rate is almost not affected by the content of nucleation agent in the range pressurization rate selected in this work.Meanwhile,the ? phase content reached the maximum value 23% and 25.1% at 1.9 MPa/s in the two systems,respectively.When the pressurization rate reaches 75 MPa/s,a complete mesophase is formed,and the final mesophase content is independent of the nucleating agent content.This result indicates that the formation of mesophase is a kinetic process,and its formation and growth process are not affected by ?-nucleating agent.2.Effect of polymorphic structure on the mechanical properties of i PP/WBG-IIUnder the combined action of nucleating agent and pressurization rate,mixed phase i PP can be obtained.At the same time,the addition of nucleating agent makes the crystal size decrease under the same conditions.The results of mechanical performance test show that(1)the two-phase mixed ?/?-i PP-0.2 exhibits higher elongation at break than ?-i PP-0.2 and higher yield strength and modulus than pure ? crystals due to changes in crystallinity and differences in crystal structure.(2)The three-phase mixed ?/?/ meso-i PP-0.2 improves the toughness by 238% compared to the two-phase mixed ?/?-i PP-0.2,which is attributed to the unique nano-nodular structure of the mesophase,which has excellent ductility and can greatly improve toughness of the products significantly. |
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