Self-diffusion of the amorphous pharmaceuticalindomethacin near Tg

Abstract

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 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 τ_α, and scales as τ_α^−0.76. 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.