Unveiling the Dependence of Glass Transitions on Mixing Thermodynamics in Miscible Systems

The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, ΔH(mix) with the change in sign (positive vs. negative) and magnitude (small vs. large). The effects of positive and negative ΔH(mix) are demonstrated based on two isomeric systems of o- vs. m- methoxymethylbenzene (MMB) and o- vs. m- dibromobenzene (DBB) with comparably small absolute ΔH(mix). Two opposite composition dependences of the glass transition temperature, T(g), are observed with the MMB mixtures showing a distinct negative deviation from the ideal mixing rule and the DBB mixtures having a marginally positive deviation. The system of 1, 2- propanediamine (12PDA) vs. propylene glycol (PG) with large and negative ΔH(mix) is compared with the systems of small ΔH(mix), and a considerably positive T(g) shift is seen. Models involving the properties of pure components such as T(g), glass transition heat capacity increment, ΔC(p), and density, ρ, do not interpret the observed T(g) shifts in the systems. In contrast, a linear correlation is revealed between ΔH(mix) and maximum T(g) shifts.