Enthalpic Relaxation Studies Of Glass Forming Materials With Unusually Non-exponential Dynamics

Relaxation of glass-froming materials could be invesitigated by numerous measurement methods.Among them,the most fundamental and most important method is the enthalpy relaxation study by virtue of the calorimetric techniques.Enthalp y relaxations are useful to the studies of the non-Arrhenius,non-exponential,and non-linear relaxation behaviors of glass-froming materials.Non-exponential relaxation is a basic feature of relaxation which is typified by the non-exponential parameter ?.In generally non-exponential relaxation systems,numerous relaxation parameter correlations were suggested such as the correlation of non-linear parameter x<?,Bohmer correlation of ?and fragility m,and the correlation of glass transition width with m and ?.Whether these correlations are still applicable and whether there are new relaxation parameter correlations in unusually non-exponential systems remain to be clarified.Here,we utilize enthalpy relaxation method to thoroughly study the weakly non-exponential relaxation system metal-organic framework ZIF-4,strongly non-exponential relaxation system deoxycholic acid,and the exponential relaxation system monohydroxy alcohol isomersIn the studies of metal-organic framework ZIF-4,we obtained the fragility m=34 by the relation of fictive temperature and cooling rate established by the enthalpy difference method,and the low m is explained by the low thermal pressure coefficient.Tool-Narayanaswamy-Moynihan-Hodge(TNMH)equation is used to fit the normalized heat capacity curve,generating ?=0.71 and x=0.47,the high ?shows the weakly non-exponential relaxation feature.The obtained ?and m follow the correlation of x<?,Bohmer correlation,and the correlation of glass transition width with m and ?,imlying the applicability of earlier correlationsThe molecular pharmaceutical dexoycholic acid was used to study its relaxation behavior.Using the melt-quenching and ball-milling methods,we obtained the fully glassy materials.Differential scanning calorimetry was used to measure the thermodynamics features of the fully glassy deoxycholic acid.Compared to melt-quenched glasses,the milled glasses showed the absence of glass transition overshoot.Further analyses shown that in addition to the earlier report that the high fictive temperature Tf gives rise to low overshoot,? is also a crucial parameter to determine the overshoot.Based on the above results,for the first time we reported the linear correlation of glass transition overshoot height ACp-os with Tf and ?.Low ? of milled glass stems from the large numbers of specific surfaces generated in the milling process,which gives rise to the large relaxation rate dispersion between surface molecules and bulk moleculesEight hexanol isomers were selected to study the exponential Debye relaxation in terms of enthalpy and dielectric relaxation measurements.For the first time we found the prominent correlation between the ? relaxation strength and the normalized heat capacity increment at the glass transition temperature Tg,while for Debye relaxation strength no correlations with the normalized heat capacity increment were found.We also found that the activation energy of Debye relaxation is related with that of the ? relaxation at the relaxation time 100 s,together with the athermal behavior of Debye relaxation and the similar dynamics feature with Rouse mode,the entropy-driven process was suggested in the exponential Debye relaxation.In conclusion,we utilized the weakly non-exponential relaxation system metal-organic framework ZIF-4 to check the eariler correlation x<?,correlation of fragility m and ?,and the correlation of glass transition width with m and ?.For the milled dexoycholic acid glass we firstly utilized ?to explain the height of overshoot and established the relation of glass transition overshoot height ?Cp-os with the Tf and ?.In the studies of the exponential Debye relaxation of mo no hydroxy hexanol isomers,we firstly found the new correlation between the ? relaxation strength and normalized heat capacity increment as well as the correlation of Debye and ? relaxation activation energies at glass transition.Those results elevate our understandings of the systems with unusually non-exponential relaxation features,which facilitate to resolve the problems of glass transition and relaxation.