Glass transition study of eutectic mixtures for pharmaceutical applications

Several blends of PEG 6000 – SR 33557 (those with a glass transition detected on cooling: 20–30 wt% PEG 6000) were heated at different heating rates, β. Solidification depends on both the cooling rate (α) and the mixture composition. A glass transition was detected by differential scanning calorimetry (DSC) in mixtures containing drug (SR 33557) >60 wt%. The dependence of the glass transition temperature, Tg, on the heating rate is analyzed using an empirical relationship of the kind Tg=A+B lnβ and the activation energy concept. The parameter B is nearly a constant, 1.7±0.2 blends with drug content >60 wt%. Activation energies obtained were 370±35 kJ/mol for the drug and 250±20 kJ/mol for the mixture with 20 wt% excipient. Moreover, the effect on Tg of cooling rate is analyzed. In samples cooled at different rates and heated at the same rate, the glass transition temperature increases as previous cooling rate increases.