Research On The Phase Transitions Of Xylenes Under High Pressures

As the advent of organic synthetic materials,humans have escaped the history of relying solely on natural materials,and high pressure has been proved to be an effective tool for the synthesis of functional materials,which can make atoms tend to reacted with other atoms and then form bonds,thus forming a more saturated structure.Experimental and theoretical studies have shown that many organic compounds under high pressure will be transformed into compounds of different morphologies,such as crystals,solids and polymers,which are different from ambient pressure.In this paper,we focus on the properties of xylenes under high pressure,and using combined theoretical calculations and experiments(such as,Raman spectroscopy,infrared absorption spectroscopy,X-ray diffraction,mass spectrometry)methods to systematic study the crystallization,phase transitions,polymerization.Moreover,the high pressure properties were compared and analyzed between xylenes and other organic compounds.In the first chapter,we first introduce the high pressure physics and high pressure technology and their breakthroughs in the high pressure properties of materials.In addition,a brief introduction was made for organic materials and xylenes.Then,we give summarizes on the background and significance of the topics selected.In the seconed chapter,we introduce the experimental equipment and experimental methods used in high-pressure research,including DAC,in-situ high pressure IR,high pressure Raman,high pressure X-ray diffraction and mass spectrometry.In the third chapter,we observed the liquid para-xylene(p-xylene)formed crystal around 0.1 GPa and the changes of the Raman and IR spectra also indicated pressure-induced crystallization.The X-ray diffraction pattern of 0.84 GPa confirmed the structure of crystallized p-xylene are monoclinic structure with space group Cc(9).As the pressure continues to increase to 13 GPa,another crystalline transition occurs in p-xylene,producing a crystalline structure of another monoclinic structure of the C2(5)space group.Finally,the liquid chromatography-mass spectrometry(HPLC-MS)analysis of the sample after decompression confirmed that the phase transition of xylene under high pressure is the polymerization phase change,and the polymerization products are mainly the trimers and tetramers of p-xylene mixture.In chapter ?,the high-pressure investigations on ortho-xylene(o-xylene)are introduced.We study it using combined IR and Raman spectral measurements at room temperature and pressures up to 32.1 GPa,respectively.The original liquid state o-xylene visually crystallized at around 0.4 GPa.Upon compression,the spectra changed;including splitting,disappearing,broadening of modes,and the appearance of new modes.We found four more phase transitions in o-xylene to phases III,IV,V,and VI at around 1.4,3.9,12.3,and 16.8 GPa,respectively.The final product recovered from over 26 GPa was preserved at ambient conditions and the mass spectrometry experiment indicated phase transition are polymerization.In chapter ?,the high-pressure investigations of m-xylene are described.Here,we report the crystallization,phase transformation and polymerization of meta-xylene(m-xylene)under high pressure.The optical micrographic observations,infrared(IR)changes,and appearance of the lattice modes in the Raman spectra indicated the crystallization of m-xylene at ~0.7 GPa and has a monoclinic phase with the C2(5)space group.The in situ IR,Raman spectroscopy and XRD results revealed two other crystalline phase transitions at ~6.4 and 18.3 GPa,respectively.Moreover,transitions above 18.3 GPa were irreversible.The mass spectrometry experiment indicated the involvement of trimer and tetramer polymers in the final recovered product,thereby characterizing a polymerization phase transition.In chapter ?,we compared and analyzed the phase transitions of the three isomers of xylene under high pressure,it is concluded that the molecular structure stability is affected by the different positions of the methyl groups.Therefore,the phase transition pressure and the number of phase transitions under high pressure had a difference.This provides a new idea for effective separation of xylene mixtures with different structures and fills the gap in the high pressure study of xylenes.Through the comparison,analysis and discussion on the properties of high pressure crystallization and polymerization of xylenes and other organic compounds,the law of the high pressure property of organic compounds was opened.At the same time,it provides an important experimental method and theoretical basis for the study of phase transition of C and H at high pressure.In chapter ?,we summarized the research contents and made a rospects.