Studies On The Mixing Thermodynamics And Glass Transition Behaviors In Binary Molecular Systems

The investigation of the composition dependence of the glass transition temperaturein miscible systems has remained to be a controversial issue during the past decades.Though several empirical equations have been proposed to explain the issue, most ofwhich are based on the ideal cases, while non-ideal mixing would be more typical. Sincethe enthalpy of mixing (Hmix) generates during the mixing, and the interactions betweenthe groups in the mixture have been testified to be connected to Hmix, some equations aredeveloped to take such interaction into account. However, these equations require eitherthe molecular level interaction parameter, such as the equilibrium constants ofself-association and association between unlike groups. Unfortunately, the experimentaldetermination of such parameters has been rarely reported, and they are usually taken asadjustable parameters.A quantitative investigation of the contribution of the mixing thermodynamics toglass transition has been performed in several molecular binary miscible systems with theapplication of broadband dielectric spectroscopy, differential scanning calorimeter, andC80microcalorimeter. The composition dependence of Tgin different systems have beenobserved to deviate from the ideal mixing rule to generate either negative or positivedeviation, and the negative deviation corresponds to the positive Hmixwhile the positivedeviation corresponds to the negative Hmix. The Couchman-Karasz equation and Gordon-Taylor equation have been applied to analyze the Tgbehavior. However, only a small partof the deviation could be explained by the two equations. Hence, compared to the capacityincreases or the products of Tgand densities of pure components, the Hmixcan be a moresignificant parameter in determining the deviation.