Publications

We employed dielectric spectroscopy to analyze charge conduction, glass transition temperature, and relaxation dynamics in six alginate (Alg) samples at low temperature. All underwent vitrification of a hydrated amorphous fraction: the as-stored Na-Alg powder at 220 K, the temperature-quenched Ca-Alg hydrogel and xerogel at 212 K, the frozen Na-Alg solution and the Na-Alg exposed to water-vapor saturated atmosphere at 194 K, and the slowly cooled hydrogel at 170 K. All samples displayed a Johari-Goldstein relaxation, and three of them displayed a structural relaxation; both processes originated from coupled Alg-H2O motions in the hydrated amorphous fraction. Charge conduction in all samples was dominated by Grotthuss proton shuttling (either in bulk water/ice or in hydration layers). H2O-rich samples displayed the H-bond network relaxation of ice, with an activation energy between 25 and 31 kJ mol-1, close to the energy of formation of one H-bond per H2O molecule. This relaxation was faster in the quenched hydrogel than in the slowly crystallized hydrogel and the frozen solution, likely due to a smaller average size of ice domains in the former; the latter samples display two separate ice relaxations, likely due to ionic impurities in the ice domains in contact with the Alg chains. The Na-Alg powder equilibrated in a water-vapor saturated atmosphere also displayed the local relaxation of bulk-like amorphous water with an activation energy of 51 kJ mol-1, equivalent to two H-bonds per H2O molecule. Most samples displayed spectral changes between 260 and 270 K due to ice melting. In the fast-quenched hydrogel, a premelting transition was observed at 220 K.

JTD Keywords: Absorption, Behavior, Dielectric-relaxation, Diffusion, Dispersion, Ice, Liqui, Nm pores, Spectra, Water

In order to exploit the pharmacological potential of natural bioactive molecules with low water solubility, such as curcumin, it is necessary to develop formulations, such as amorphous polymer dispersions, which allow a constant release rate and at the same time avoid possible toxicity effects of the crystalline form of the molecule under scrutiny. In this study, polymer dispersions of curcumin were obtained in PADAS, a biodegradable semicrystalline copolymer based on 1,12-dodecanediol, sebacic acid and alanine. The dispersions were fully characterized by means of differential scanning calorimetry and broadband dielectric spectroscopy, and the drug release profile was measured in a simulated body fluid. Amorphous homogeneous binary dispersions were obtained for curcumin mass fraction between 30 and 50%. Curcumin has significantly higher glass transition temperature Tg (≈ 347 K) than the polymer matrix (≈274-277 K depending on the molecular weight), and dispersions displayed Tg's intermediate between those of the pure amorphous components, implying that curcumin acts as an effective antiplasticizer for PADAS. Dielectric spectroscopy was employed to assess the relaxation dynamics of the binary dispersion with 30 wt% curcumin, as well as that of each (amorphous) component separately. The binary dispersion was characterized by a single structural relaxation, a single Johari-Goldstein process, and two local intramolecular processes, one for each component. Interestingly, the latter processes scaled with the Tg of the sample, indicating that they are viscosity-sensitive. In addition, both the pristine polymer and the dispersion exhibited an interfacial Maxwell-Wagner relaxation, likely due to spatial heterogeneities associated with phase disproportionation in this polymer. The release of curcumin from the dispersion in a simulated body fluid followed a Fickian diffusion profile, and 51% of the initial curcumin content was released in 48 h.Copyright © 2023. Published by Elsevier B.V.

JTD Keywords: antioxidant, bioavailability, dielectric spectroscopy, domain havriliak-negami, glass transition temperature, kinetic stability, molecular mobility, nm pores, phase-behavior, physical stability, release kinetics, temperature, thermodynamic quantities, time, Amorphous formulations, Dielectric spectroscopy, Glass transition temperature, Kinetic stability, Kohlrausch-williams-watts, Molecular mobility, Release kinetics