Studies On The Glass Formation Thermodynamics For Pure And Binary Molecular Systems

Scrutinizing critical thermodynamic and kinetic factors for glass formation and the glass stability of materials would benefit the screening of the glass formers for the industry of glassy materials.In this work,firstly,we study the glass transition behaviors of pure pharmaceutical molecules,aiming at elucidating the factors that contribute to the glass formation.Compared to their crystalline counterparts,glassy pharmaceuticals show greatly improved water solubility,chemical activity and bioavailability,and have high application values.Glass transition related thermodynamics and kinetics are performed on the pharmaceuticals using calorimetric,dielectric and viscosity measurements.The characteristic thermodynamic and kinetic parameters of glass transition are found to reproduce the relations established for small-molecule glass formers.The systematic comparison of the thermodynamic and kinetic contributions to glass formation reveals that the melting-point viscosity,?m,is the crucial quantity for the glass formation.Of more interest is the finding of a rough correlation between the melting-point viscosity and the entropy of fusion normalized by the number of beads of the pharmaceuticals,suggesting the thermodynamics can partly manifest its contribution to glass formation via kinetics.The glass transition behaviors in binary molecular systems are subsequently studied in this work.Compared to the pure systems,binary systems are featured by the interactions of unlike components.It is noted that the balance between interactions of unlike and like components determines the mixing thermodynamics,implying the enthalpy of mixing,?Hmix for the system.The effects of the ?Hmix on the glass transition behaviors are quantitatively studied by focusing on five binary systems of diverse ?Hmix in the signs(positive vs.negative)and amplitudes(small vs.large).The composition dependence of glass transition temperatures,Tg-x,for all systems are detected.Opposite shifts in the Tg-x deviations from the ideal mixing rule are observed that mixtures of positive ?Hmix usually show negative deviations while those of negative ?Hmix show positive deviations.In addition,systems of larger ?Hmix show more pronounced deviations despite the sign of ?Hmix.A basically linear correlation is revealed between ?Hmix and maximum Tg shifts.The empirical Gordon-Taylor and Couchman-Karasz equations are also applied to interpret the Tg-x deviations for all the systems,however,only a small fraction of the deviation is explained.Compared to the pure components such as Tg,glass transition heat capacity increment,?Cp,and density ?,which are strengthened by the two equations,the more significant importance of ?Hmix in the glass transition is manifested.The glass transition and dynamics for binary mixtures composed of benzene and five other chemicals via thermodynamic approach are studied in this work.The interactions between benzene and these chemicals can be divided into three groups,namely the strong,moderate,and weak interactions.The five systems are confirmed to have ?Hmix of both positive and negative signs.Based on the guidance of such ?Hmix data,the extrapolations of the Tg values in the benzene-rich regions for all the systems are made to obtain the pure benzene's Tg,producing a value ~142 K.Likely,a value around 80 is evaluated for the fragility m-index of pure benzene through extrapolations.The obtained Tg and m values allow for the construction of the activation plot in the deeply supercooled region of benzene.The poor glass forming ability of benzene is also analyzed and can be attributed to its extremely low melting-point viscosity,?m.