Role of α-Relaxation on Crystallization of Amorphous Celecoxib above Tg Probed by Dielectric Spectroscopy

Abstract

In the present study, the role of α-relaxation toward isothermal crystallization of amorphous celecoxib was studied using dielectric spectroscopy (DES). The dielectric response of the α-relaxation was measured as a function of frequency (10⁻¹ to 10⁶ Hz), isothermally at every 4 K interval in the range of 303.15 to 443.15 K. The dielectric loss spectrum at each temperature was analyzed using the Havriliak Negami (HN) equation to extract the characteristic relaxation time, τ(HN). Two Vogel-Fulcher-Tammann (VFT) functions were required for representing the temperature dependence of τ(HN) across the temperature range of study. The VFT fit parameters obtained from the two regions varied drastically pointing toward the underlying differences in the dynamics of relaxation above and below the crossover. Later, in situ isothermal crystallization experiments were performed at 363.15, 368.15, 373.15, and 378.15 K. The conversion rate, obtained from the normalized dielectric strength, was modeled using the Avrami model, which indicated the possibility of different crystallization mechanism at higher crystallization temperatures. HN shape parameters, α(HN) and product of α(HN) and β(HN), were analyzed during the course of crystallization to understand the dynamics of amorphous phase when crystallites were being evolved. HN shape parameters indicated α-like motions were affected, whereas β-like remained unaffected by the crystallization temperature. Characteristic crystallization time, τ(cr), obtained from Avrami fits, showed Arrhenius type of temperature dependence (R² = 0.999). A plot between log τ(cr) and log τ(HN) show a linear regression with R² of 0.997 indicating the direct correlation between these two phenomena. However, the coupling coefficient was found to be varying within the temperature range of study, indicating tendency of crystallization to be more diffusion controlled at higher crystallization temperatures. With different crystalline solid phase crystallizing at higher crystallization temperature, complemented with direct correlation between log τ(cr) and log τ(HN), Avrami modeling of crystallization and HN shape parameter analysis, the role of α-relaxation in the crystallization of amorphous celecoxib at T > T(g) is emphasized.

Role of α-Relaxation on Crystallization of Amorphous Celecoxib above T_g Probed by Dielectric Spectroscopy

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