Molecular mobility in raffinose in the crystalline pentahydrate form and in the amorphous anhydrous form

The aims of the study are to characterize the slow molecular mobility in solid raffinose in the crystalline pentahydrate form, as well as in the anhydrous amorphous form (Tg = 109 degrees C at 5 degrees C/min), and to analyze the differences and the similarities of the molecular motions in both forms.

Thermally stimulated depolarization current (TSDC) is used to isolate the individual modes of motion present in raffinose, in the temperature range between -165 and +60 degrees C. From the experimental output of the TSDC experiments, the kinetic parameters associated with the different relaxational modes of motion were obtained, which allowed a detailed characterization of the distribution of relaxation times of the complex relaxations observed in raffinose. The features of the glass transition relaxation in raffinose were characterized by differential scanning calorimetry (DSC).

A complex mobility was found in the crystalline form of raffinose. From the analysis of the TSDC data, we conclude that these molecular motions are local and noncooperative. A sub-Tg relaxation, or secondary process, was also detected and analyzed by TSDC in the amorphous phase. It has low activation energy and low degree of cooperativity. The glass transition was studied by DSC. The fragility index (Angell's scale) of raffinose obtained from DSC data is m = 148.

TSDC proved to be an adequate technique to study the molecular mobility in the crystalline pentahydrate form of raffinose. In the amorphous form, on the other hand, the secondary relaxation was analyzed by TSDC, but the study of the glass transition relaxation was not possible by this experimental technique as a consequence of conductivity problems. The DSC study of the glass transition indicates that raffinose is an extremely fragile glass former.