Polymorphism Of N-C₁₆-C₁₇ And N-C₁₇ In Nanoconfinement

Phases of materials in nanoscale space are the structural basis of the heated topic of nanoconfinement effect.However,polymorphism of the polymorphic materials in nanospace,different to the bulk system,is not easy to be predicted.Selectivity of the polymorphs thus becomes one of the bottlenecks to be broken in this field.Normal alkanes?n-alkanes?,having linear chain molecular structure,are among the most fundamental building blocks of organic and polymeric systems.The n-alkanes have some peculiar features in phase equilibriums such as odd-even effect and surface-freezing effect.Under the effect of pore size,interface interaction and pore morphology,the phase behavior of nanoconfinement n-alkanes in porous materials has the new change,especially in binary system.The investigations on the n-alkanes and their mixtures in confined system can provide a well-defined model for studying the complex compound such as liner polymers.Therefore,exploring the phase behavior of n-alkanes and their mixtures under nanoconfinement has great significance.In this dissertation,n-alkanes have been selected as model substance,adsorbing in porous materials such as SBA-15,KIT-6,controlled porous glasses?CPGs?,which have different dimensionalities and ordering of the pores so that the alkane molecules are posed in different molecular force fields.Using low-temperature differential scanning calorimetry?DSC?as a fundamental method,coupled with temperature-dependent powder X-ray diffraction?XRD?and the synchrotron radiation technology test the transition temperatures,enthalpies and other parameters,establishing the phase diagram of the single component and the binary.From the possible newly found phases and their stable and metastable properties,and the changed in the phase transition temperature a nd components,the relations among nanoconfinement effect,interface interactions and pore morphology,and molecular structures of the polymorphic materials and polymorph selectivity can be elucidated.The research may help understanding the properties of the nanoconfineme nt materials and the basic thermodynamic properties for related research such as synthesis,adsorption,freezing of the na nomaterials.Also provide useful model systems to understand the physical behaviors of linear and biological polymers.The main work of this dissertation is as follows.?1?Synthesis of the porous materials SBA-15,C/SBA-15 and the materials composite like n-alkanes/porous.According to the demand of the experimentation and the limitation of the laboratory conditions,the experiment project to preparation the silica-based SBA-15 with the pore diameter at about 15 nm and the carbon-based C/SBA-15 with the pore diameter at about 10 nm.The necessary parameters such as pore volume,specific surface area and ordering have been test before using the porous materials.Meanwhile,successfully prepared composite of n-alkane/SBA-15,n-alkane/CPG,n-alkane/KIT-6 in different pore diameters.?2?Phase behaviors of hexadecane-heptadecane(n-C₁₆H34–C₁₇H36,n-C₁₆–C₁₇)system in bulk and confined in the CPGs?300 nm,8.1 nm?have been investigated using the low-temperature DSC and temperature-dependent XRD.In CPG?300 nm?,the phase regions of n-C₁₆–C₁₇ system is similar to the bulk in the composition and temperature ranges,and it is understandable since the alkane solids in the nanopores still possess the lateral and lamellar ordering,the sign of complete crystals.However,There is a significant changing in the phase behavio r of the n-C₁₆–C₁₇ system in CPG?8.1 nm?in reference with the bulk.A new RII phase comes out in the mixtures with high C₁₇ concentrations.In diffraction analysis,the confinement gives rise to additional disordering in the packing of the chain molecules,which are caused by size effect and the pore wall.?3?Through the DSC and XRD,phase behaviors of the model substance(n-C₁₇)adsorbed in CPG,KIT-6,SBA-15 have been investigated by changing conditions of the nanoconfinement.Exploring the effect of nanoconfinement,interface interactions and pore morphology.The results show that the phase transition temperature has a great decrease in CPG,KIT-6,and SBA-15.At the same time,n-C₁₇ comes out a new rotator RII phase.We consider that the effect of nanoconfinement changes not only the packing of the alkane molecules,but also the relative free energy,leading to a new rotator phase.Meanwhile,an ordering structure and low-dimensional confinement condition has a strong effect on packing of the alkane molecules.