Polymorphic Liquid States In Ionic Type Liquids And Its Impact On Relevant Phase Transitions

Liquid is one of the three basic states of materials in nature.However,the understanding of liquid has long been limited to the basic cognition that liquid is homogeneous,ordered in short range but disordered in long range.Recently,growing evidences of polymorphic liquid states in various types of liquid have greatly challenged the conventional understanding.How to understand the origin of liquid polymorphism and the transformation process between different liquid states precisely are important scientific issues to develop the current theory of liquid.Further,it is of both practical and scientific significances to study the impact of liquid polymorphism on crystal polymorphism,glass transition process and liquid relaxation process.In the light of the above questions,two ionic type of liquids,molten Sodium Acetate Trihydrates?phase change material?and N-butyl-N-methylpyrrolidinium bis?trifluoromethyl sufonyl?imide?room temperature ionic liquid?,are investigated using Nuclear Magnetic Resonance?NMR?,Differential Scanning Calorimetry?DSC?,Vibrating-Tube Density Meter?VTD?,Raman spectroscopy?Raman?and Molecular Dynamics?MD?simulation in this dissertation.The primary results of this dissertation are as follow:?1?Sodium acetate trihydrate?SAT?is a widely used phase change material for energy storage in daily life and industry.A first-order liquid-liquid phase transition?LLT?in the homogeneous melt of SAT is observed.The change of structure and dynamics associated with this transition have been systematically investigated using NMR.It is revealed that the chemical environment of carbon atom of carboxyl group in acetate anion has significantly changed accompanied with a notable change of dynamics.The first-order nature of this transition is confirmed by remarkable undercooling phenomenon.To further probe the nature of this transition,VTD is employed to monitor the relevant density change.No detectable density discontinuity is observed within experimental error upon transition,which suggests that density is not the dominant order parameter.Subsequently,the liquid structure of molten SAT is studied via MD simulation.Two locally favored structures?LFS?of“tightly packed”and“loosely packed”are identified according to the orientation of carbon-carbon bond in acetate anion to sodium cation.And the transformation between the LFSs could be the nature of the LLT.These results are of crucial significance to both the understanding of the structure of liquid and the nature of first-order phase transition.?2?The cycle performance of SAT as a phase change material for nenergy storage is degraded due to the segregation of sodium acetate?SA?.Therefore,the effect of LLT on the segragation of SA is explored.The DSC measurements reveale that the crystallization temperature range of sodium acetate from high temperatrure liquid?HTL?ranges from 290K to 315 K.In contrast,the crystallization temperature range of sodium acetate from low temperature liquid?LTL?is significantly wider,ranging from 290 K to 340 K.The results that different liquids exhibit different undercooling degree could be of great significance to understand the nature of glass transition.Further,the composition of the crystallization products of different liquids are characterized using XRD method.The results show that crystallization products of SA from HTL is a mixture of less stable SA-I and stable SA-II.In contrast,the crystallization products of SA from LTL is completely SA-II.These results suggest that the liquid polymorphism and crystal polymorphism could be fundamentally related.This work may also be of crucial significance to industry applications.Through LLT,the unexpected crystal polymorphs can be inhibited to improve the rate of desired polymorph products.?3?As a new type ionic liquid with remarkable electrochemical performances,N-butyl-N-methylpyrrolidinium bis?trifluoromethyl sufonyl?imide(Pyr₁₄TFSI)is widely used as a new electrolyte in various modern electrochemical devices such as lithium batteries,fuel cells and supercapacitors.In contrast to the rapid advances in applications,fundamental researches on the evolution of its liquid structure are far from enough.Here,a liquid-liquid crossover in Pyr₁₄TFSI around 35?is identified using NMR and Raman.Above this temperature,the fast motion of alkane chain result in a homogeneous liquid state.However,the rate of motion dramatically decreases as temperature drops below the transition temperature,reducing the exchange rate between different conformation of cation.As a result,the liquid shows two distinct microstructures and exhibits strong heterogeneity.Further DSC measurements reveal a heat capacity anomaly accompanied with this transition.This research could be of significance to the understanding the origin of various thermodynamic anomalies in other liquid systems such as water and water-like liquids.?4?A slow relaxation from non-equilibrium liquid state to equilibrium liquid state after the melting of Pyr₁₄TFSI is identified.The NMR and VTD measurements reveal that the relaxation is ultra slow and the elaxation time is estimated to be 10⁴ s.The glass transition temperature is found to be highly related to the relaxation process.Further,the stable liquid shows a better electrochemical performance than unstable liquid.Understaning the relaxation process is of significance both to understating the evolution of liquid structure and a better application of this material in industry.