Vapor-Deposited Thin Films: Studying Crystallization and α-relaxation Dynamics of the Molecular Drug Celecoxib

Abstract

Crystallization is one of the major challenges in using glassy solids for technological applications. Considering pharmaceutical drugs, maintaining a stable amorphous form is highly desirable for improved solubility. Glasses prepared by the physical vapor deposition technique got attention because they possess very high stability, taking thousands of years for an ordinary glass to achieve. In this work, we have investigated the effect of reducing film thickness on the α-relaxation dynamics and crystallization tendency of vapor-deposited films of celecoxib (CXB), a pharmaceutical substance. We have scrutinized its crystallization behavior above and below the glass-transition temperature (T_g). Even though vapor deposition of CXB cannot inhibit crystallization completely, we found a significant decrease in the crystallization rate with decreasing film thickness. Finally, we have observed striking differences in relaxation dynamics of vapor-deposited thin films above the T_g compared to spin-coated counterparts of the same thickness.

Figure 1

Temperature evolution of α-relaxation times for vapor-deposited CXB films with different thicknesses. The red line represents the VFT fit for bulk material.

Figure 2

Crystallization rate k as a function of temperature (on the left) and the Avrami parameter (n) (on the right) for CXB films prepared using the vapor deposition technique as measured at 368 K for different thicknesses. In the insets, the crystallization rate and the Avrami parameter are presented as a function of the film thickness.

Figure 3

Optical microscopy images depicting the crystallization of vapor-deposited CXB films of various thicknesses on Si wafers with time as measured at room temperature. The scale bar represents ∼3 mm.

Figure 4

Temperature evolution of α-relaxation times for vapor-deposited CXB films with different thicknesses obtained while maintaining different substrate temperatures. The red line represents the best VFT fit for bulk material. T_room means the room temperature of the substrate for deposition, which is ∼0.9T_g.

Figure 5

Crystallization rate k (on the left) and the Avrami parameter (on the right) for CXB vapor-deposited films with different thicknesses and substrate temperatures during deposition as measured at 368 K. T_room means the room temperature of the substrate upon deposition, which is ∼0.9T_g. The inset plot thickness depends k and n.

Figure 6

Temperature evolution of α-relaxation times for vapor-deposited and SC CXB films. The red line represents the best VFT fit for the bulk sample.

Figure 7

Crystallization rate constant (k) characterizing the rate of the crystallization process, on the left, and Avrami parameter (n), on the right, at 368 K, for CXB films of the same thicknesses but prepared using different methods. The inset plot thickness depends on k and n.