Self-diffusion of the amorphous pharmaceutical indomethacin near Tg

Secondary ion mass spectrometry has been utilized to measure self-diffusion coefficients D in supercooled indomethacin from 310 K to 350 K (Tg = 315 K). Over this temperature range, D varies from 10−17 cm2s−1 to 10−11 cm2s−1. The temperature dependence of D is significantly weaker than the temperature dependence of the viscosity or the structural relaxation time τα, and scales as τα−0.76. At Tg, D is enhanced by more than a factor of 100 in comparison to the viscosity and the expectations of the Stokes–Einstein equation. For deeply supercooled indomethacin, the temperature dependence of D matches the temperature dependence of crystal growth rates for all three polymorphs.