Explorations Of The Scheme To Measure Relaxation Time And Dielectric Spectrum Theory Of Small Molecular Liquids

Glass transition is an important issue in condensed matter physics,but up to now,no theory and model have been accepted widely.In terms of dynamics,glass transition is a crossover between the a relaxation time of glass-formers and observation one.And when the relaxation time is about 100s,it is considered that glass transition of the liquids takes place.Therefore,the measurements of relaxation time of liquids are the keys to explore the mechanism of glass transition.Meanwhile,the dielectric spectra,having to be described by the theory and model of glass transition,can provide the important information to glass transition researches.However,at present,the empirical formulas are mainly used to describe the dielectric spectra of liquid relaxation,so it still needs further study to establish the theory of dielectric spectra based on the microscopic models of glass transition.Therefore,the above problems in glass system are studied by focusing on dielectric spectroscopy experiment and theory in this paper,and the main works and results of this paper are as follows.The feasibility of obtaining a relaxation time of liquids by probe ionic conductance in the liquids is discussed according to Maxwell,Stokes-Einstein,and Nernst-Einstein equations as well as existing analyses of experimental data.Then a measurement scheme is proposed to detect ? relaxation time(?)of glass-forming liquids,which is based on the measured ionic conductivity of the liquids doped with probe ions by the low and middle-frequency dielectric spectroscopy.For the small molecule liquids with only single component,the relaxation times given by this scheme are well agreed with the ones obtained from the a relaxation peaks in this paper and other literatures.Moreover,the relaxation time data of 1,2-propanediol are given by this scheme in a larger range than that in literatures.And the results of the binary mixed liquids are similar to the ones of single component liquids.So indicate the reliability and accuracy of the scheme,which only bases on a physical mechanism and a device,avoiding the deviations in the combinations.Moreover,the applicability of the scheme in binary mixed liquids can help to gain the abundant and continuous-controllable relaxation time for the study of glass transition.Meanwhile,to investigate the influence of the change of liquid structure and interaction on the scheme,the diffusion coefficients of probe ions are studied,which are in the monohydroxy alcohols with a change of the liquid microstructure near 250K.The results show that the diffusion coefficients increase first and then decrease with the rise of temperature in the range above 250K,obviously different from the behavior of glycerol,a typical small molecular glass-former.The temperature corresponding to the maximum diffusion coefficient is the crossover one between the hindrance of the ion diffusion caused by the interspace reduction among the hydrogen bond molecular clusters due to the decrease of the cluster size and the promotion by thermal activation as temperature goes up.The results further verify the size change of the hydrogen bond clusters in monohydroxy alcohols with the increasing temperature and show that the scheme proposed cannot be used in monohydroxy alcohols.Based on the molecular string model of glass transition,the exact formula of complex permittivity of independent molecular strings,which have a spatial conformation of the free rotating chain,is inferred after the derivation of the orientation cosine correlation function in a arbitrarily given direction between two segments.The formula contains the information of dielectric relaxation dynamics of all molecular strings and modes,which is qualitatively consistent with the characteristics of the experimental results.In order to calculate expediently,the gained formula of complex permittivity is simplified.Then its numerical calculation scheme is given based on the analysis of the molecule distribution versus the string length and the influence of the string length on the complex permittivity.A large number of numerical calculations and comparisons are carried out to research the effects of system parameters,azimuth angle,effective coordination number and temperature,on the imaginary part of complex permittivity in the molecular string model,and the rules of the effects are presented.At last,the theoretical results are compared with the experimental data,which shows that the formula can describe the data with smaller width of relaxation peaks well,while describing the one with the larger width badly.On the basis of this paper,the issues needed to further research are applicability of the relaxation time measurement scheme proposed,existences of the anomaly of probe ion diffusion coefficients in other monohydroxy alcohols,and descriptions of the dielectric spectra when considering the intermolecular correlation.