Relation between configurational entropy and relaxation dynamics of glass-forming systems under volume and temperature reduction

Abstract

The structural relaxation dynamics of two molecular glass forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S C , as predicted by Adam and Gibbs (AG), was also investigated. As S C is not directly accessible by experiments, it was estimated, following a common procedure in literature, from the excess entropy S exc of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T-P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S exc and S C was found, with different proportionality factors in isothermal and isobaric conditions. Using such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature.

Combining Flash DSC, DSC and broadband dielectric spectroscopy to determine fragility

“…The reliability of these investigations is demonstrated by comparing results obtained from two polymeric systems selected for their amorphous structure: polystyrene (PS), considered as a reference material because extensively studied by F-DSC, and poly(ethylene terephthalate)-glycol (PETg) which is much rarely studied by F-DSC techniques. In a second time, the temperature dependence of the cooling rate obtained by F-DSC and DSC combination has been compared to the temperature dependence of the relaxation times classically obtained from broadband dielectric spectroscopy, experiment considered as the reference concerning the fragility measurements [22][23][24][25][26][27][28][29][30].…”

Study of molecular dynamics of the pharmaceutically important protic ionic liquid verapamil hydrochloride. II. Test of entropic models

“…In the recent research 33 it was found that the ratio g P ͑T͒ / g T ͑P atm ͒ is approximately a constant temperature function and its value is usually slightly bigger than one. ͑18͒, i.e., the ratio g p / g T , needs to be found.…”