Studies On The Kinetic Heterogeneity Of Dielectric Relaxation And Enthalpy Relaxation In Glass-Forming Molecular Systems

Due to the advantages of continuously adjustable composition and isotropic properties,glassy materials have been widely used as engineering and functional materials referring to various walks of life.However,the glass state with high energy and long-range disorder in structure is a metastable state,the understanding to the nature of which is still very limited.In particular,because of their heterogeneous dynamics,the stability and uniformity greatly affect the lifetime and application of glassy materials.Therefore,it is necessary to study the heterogeneity of glassy materials.It has been found that the properties of glassy materials are closely related to their relaxation kinetics.A variety of relaxation behaviors have been found in glassy materials using different analytical methods.For example,dielectric relaxation of monohydric alcohols detected structural relaxation corresponding to glass transition,faster secondary relaxation,and Debye relaxation related to hydrogen-bonded superstructure.These relaxations allow for some important parameters for relaxation kinetics related to the glass transition,which is significant to understand the structures and physical oringins of glassy materials.In this study,different small molecule systems with high glass forming ability were used to obtain the influence factors on structural and kinetic heterogeneity.Relaxation kinetic parameters were obtained firstly by analyzing their enthalpy and dielectric relaxation,which were used to build the relationship between molecular structures and heterogeneity,and explain their relationship with kinetic differences as well as glass heterogeneity.In addition,by comparing the relaxation kinetic of the binary or ternary mixed systems and the pure substances,and the effect of the concentration fluctuation of the mixed systems on the kinetic heterogeneity was further confirmed.Based on the heterogeneity of the glassy material,a quantitative relationship between normalized glass transition width and concentration fluctuation was established.This further broadens the application range of glass transition behavior.In order to obtain the parameters of relaxation kenitic in supercooled liquid and glassy state for pure or mixed small molecules,broadband dielectric,calorimetry and X-ray techniques were mainly used to study the origin of differences between enthalpy relaxation and dielectric relaxation in glass forming systems.The main research contents are as follows:Small-molecule systems were studied using broadband dielectric techniques under a wide range of temperature conditions.It is found that in ibuprofen and nidazole drug molecules,the chiral structure has no effect on kinetic heterogeneity.Therefore,abnormal non-exponential factor β changes were found in benzoyl ferrocene and aminomethyl dimethyl ferrocene,which were confirmed to be related to the abnormal movement pattern of the ferrocene structure through the discussion of the Kirkwood equation,which originating from the enhanced heterogeneity caused by the transient dipole moment.There is a positive correlation between the dipole moment and the non-exponential factor β,and molecular polarity is highly correlated with dielectric relaxation kinetic heterogeneity.In addition,it was further found that the rigid and flexible structures have the greater impact on the kinetic heterogeneity.In small molecular systems such as metal-organic molecule ferrocene,the flexible structure leads to a decrease in the non-exponential factor and an increase in the kinetic heterogeneity.Subsequently,the corresponding enthalpy relaxation kinetics and thermodynamic analysis of the above substances were also anylyzied by calorimetric techniques.The relationship between enthalpy relaxation kinetic heterogeneity and molecular chirality,polarity,other structures,physical/chemical properties is discussed.Further comparisons revealed differences between the two relaxation kinetic parameters,clarifying the link between the origin of the differences and the kinetic heterogeneity.The similarities and differences of the relaxation kinetics of binary mixed systems were studied by means of enthalpy relaxation and dielectric relaxation.A series of abnormal kinetic behaviors were found in the strongly asymmetric system triphenylethylene and picoline,such as the decoupling of glass transition temperature,the reversal of nonexponential and non-linear factors,the increase of normalized glass transition width.The origin of the kinetic heterogeneity in the mixing system was explored,and it is confirmed that the abnormal behavior originates from the concentration fluctuations in the mixtures.Subsequently,ageing experiments were also carried out in the extremely asymmetric mixed system of tripropyl phosphate and polystyrene with two glass transitions.It is found that the abnormality was not due to phase separation,but the existence of strong concentration fluctuations leading to the existence of a continuous concentration gradient.The prepeak produced by ageing also confirmed that the system was not only two glass transitions,but a wide range of glass transition region.By comparing the relaxation kinetic behaviors of pure and binary small molecules,it is found that there is a linear relationship between the abnormal behaviors and the normalized glass transition width.In addition,by further adding the active agent Phenanthridine to study the relaxation behavior of strongly asymmetric mixed systems,it is found that the decoupling between glass transition temperature and nonexponential factor obtained by different measurement methods is more related to the physicochemical properties(or polarity)of components,while the normalized glass transition width is more closely related to the concentration fluctuation.The small angle and wide-angle X-ray scattering of three binary mixed systems with different concentration fluctuation and mixing heat are studied.Through data processing and analysis,it is found that the invariant Q representing the partial square difference of electron density deviates from the value of ideal mixing,which can quantitatively judge the size of concentration fluctuation.By establishing the linear correspondence between the invariant Q and the normalized glass transition width,it further confirmed that the normalized glass transition width can quantitatively and accurately judge the concentration fluctuation.Compared with other methods,the measurement process is more time-saving and simple.