“Stabilization” of Amorphous Ketoprofen in Thin Films

It has been shown that depositing ketoprofen as thin films on glass substrates has a stabilizing effect on the amorphous state of ketoprofen. Polyethylene glycol (M = 6000 g/mol) was mixed with ketoprofen in a wide range of concentrations. Amorphous thin films were prepared by spin coating and subjected to storage conditions with different levels of relative humidity. The films were characterized by specular X-ray diffraction and atomic force microscopy to assess their stability in dry to wet atmospheres. In a dry atmosphere, the amorphous films remained stable for up to 4 months, although ketoprofen possesses a glass transition temperature of −6 °C. However, when subjected to a humid atmosphere (over 50% relative humidity), ketoprofen tends to crystallize in the amorphous films. At low solution concentrations (i.e., low film thickness and low ketoprofen loading) and high humidity, only nanometer-size crystals could be detected. Increasing the polymer mass ratio may favor or prevent crystallization of ketoprofen in the amorphous films depending on its own crystallization behavior in those films.