The Effect Of Liquid Complexity On The Melting Process And Glass Transition Process Of (S)-(+)-Ibuprofen

Solid-liquid transition includes melting and glass transition.The interpretations of the mechanism of solid-liquid transition have always been of great significance in both science and industrial application.The quest of understanding the nature of solid-liquid transition has led to new concepts and groundbreaking discoveries such as the topological phase transition,quasi-crystal and metallic glass.The studies on the transition pathway of solid-liquid transition have greatly contributed to the development of protein drugs,the progress of the next generation of phase contrast memory,the improvement of metallurgy technology and the invention of new energy materials.The traditional melting theories mainly focus on the breakdown of the crystal structure based on the role of lattice vibrations,shear modulus,surface and dislocations.The classical glass transition theories concentrate on the evolution of dynamics and thermodynamics of supercooled liquid with the help of potential energy landscape,density fluctuations,configuration entropy and free energy landscape.However,in these theories,liquid is assumed to be disordered and homogeneous.A single order parameter,such as density,is adequate for the description of the liquid state.Accordingly,the emergence of local structures in the liquid is ignored and the effect of liquid complexity on the solid-liquid transition is seldom discussed in the classical solid-liquid transition theories.However,recent studies suggest that the liquid is complex and heterogeneous due to the existence of different local structures.This updates the understanding of liquid.For example,a liquid of sufficient complexity could assume different liquid phases of the same composition,even in a single component system.It could also challenge the traditional solid-liquid transition theories.In this thesis,(S)-(+)-ibuprofen(SIBP),which has complex liquid structures,is chosen as a candidate to explore the effect of liquid complexity on the solid-liquid transition,including melting and glass transition.The change of structure and thermodynamics and dynamics of SIBP during melting and glass transition process are systematically measured by differential scanning calorimeter(DSC),nuclear magnetic resonance(NMR),high precision density measurements,Raman scattering(Raman)and numerical simulations and more details are list below:1.To systematically study the effect of liquid complexity on the SIBP melting process,the DSC,NMR and high-accruacy density measurements are applied to study the change of structure and thermodynamics and dynamics during melting of SIBP crystal.Unlike the one-step transition process predicted by classical melting theories,the transition pathway of SIBP melting is shown to be a two-step transition process.SIBP crystal firstly transforms into a metastable liquid.Compared with stable liquid,the metastable liquid contains larger populations of locally favored structures,and thus exhibits higher density.In addition,the metastable liquid exhibit a pretty slow relaxation process which only disappears at a higher temperature than melting point.The high-density metastable liquid rapidly change into low-density stable liquid through a spinodal transition accompanied by long-range structral fluctuations.This makes the melting of SIBP crystal not at a fixed point but within a temperature region,which ends at a spinodal temperature.These findings indicate that the locally favored structures of liquid have a significant influence on the melting process,and imply that three-dimensional melting may also be a continuous process.These results are of great scientific significance to the understanding of the liquid transition theory and provide new insight into the nature of melting.2.At present,there is still lack of systematic studies on the relationship between endothermic curve of glass transition and heating rates.In this chapter,the relationship between endothermic glass transition curve of simple liquid and heating rates is systematically studied by numerical simulation.(1)When the cooling and heating rates are equal,the shape of heat capacity curve remains unchanged and the peak temperature of heat capacity curve shifts with different heating rates.(2)Under the condition of constant cooling rate,the maximum value and the peak temperature of heat capacity curve increases with the increasing of heating rate.(3)The fragility plots exhibits a straight line.(4)The average structural relaxation time varies exponentially with temperature.3.To observe effect of the liquid complexity on the glass transition,the thermodynamic,dynamic and structural changes of SIBP during glass transition are carefully examined by DSC and Raman spectroscopy.A transient metastable supercooled liquid state(TMSLS)emerges during the glass transition of SIBP,which makes the aging process SIBP become a two-step relaxation process.The non-equilibrium glass state firstly relax to TMSLS,then into the supercooled liquid state(SLS),and transition from TMSLS to SLS exhibits characteristics of spinodal transition.The fragility plots and the temperature dependence of average structural relaxation time exhibit an abrupt jump at the transition temperature,and are accompanied by a sudden change of the liquid structure.These results indicate that there is a qualitative change in the free energy landscape of SIBP supercooled liquid at the transition temperature,which could intrinsically facilitate the glass transition.In addition,the glass transition of several organic molecular systems are also studied.Similar to the SIBP,a transition like behavior is observed in the fragility plot.This indicates that the effect of liquid complexity on glass transition process may be universal in organic molecular systems.These results may help to reveal the nature of glass transition.