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 (T-g = 315 K). Over this temperature range, D varies from 10(-17) cm(2) s(-1) to 10(-11) cm(2) s(-1). The temperature dependence of D is significantly weaker than the temperature dependence of the viscosity or the structural relaxation time tau(alpha), and scales as tau(-0.76)(alpha). At T-g, 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.