Excess entropies and related quantities in glass-forming liquids

Abstract

Comprehensive analysis of calorimetric data for large number of low molecular weight and polymeric substances has shown that although neither excess entropy at the glass transition temperature ASg nor heat capacity jump ACP (calculated per mole of "beads") nor the ratio of the glass transition temperature to the second-order transition point, Tg/T2, can be regarded as "universal" constants for all glass formers, as implied in the "isoentropic state" concept or Tg, the value of ACP still appears to be an intrinsic constant for a family of substances with a similar structure in the disordered state prior to quenching. The amount of a residual disorder in the glassy state, as measured by the ratio of ASg to the melting entropy, ASm, was found to be predictable by the equation ASg

Cryogenic grinding of indomethacin polymorphs and solvates: Assessment of amorphous phase formation and amorphous phase physical stability

“…IMC may dissolve liquid methanol from which crystallization of IMC polymorphs could occur. With a T g of À 1708C, 22 methanol could act as a potent plasticizer of any amorphous IMC formed during grinding. Plasticization could lower the T g of IMC to below room temperature, resulting in rapid Standard deviation values are given in parentheses.…”

“…Using T g and DC p values of 458C and 0.41 J/g/8C for IMC, respectively, 25 and À 1708C and 0.81 J/g/8C for methanol, respectively, 22 predicted T g of an 8.2% w/w methanol in IMC mixture was 138C.…”

The old problems of glass and the glass transition, and the many new twists.

“…The theoretical connection may be made through the AdamGibbs theory for kinetic processes in cooperative systems (40), which leads to the relation ,q = A exp(C/TSc), [4] where Sc is the configurational entropy at temperature T. This yields Eq. 1 if the functional form assumed for excess heat capacity ACp in evaluating Sc [Sc = fTK(ACp/T)dT] is taken to be ACP = KIT, which is a good approximation to the experimental findings (41,42). The form the heat capacity would take at temperatures well below Tg, where laboratory studies are impeded by the long time scales, is an open question.…”

Thermodynamics and fragility of glass-forming alloys

“…It appeared, however, that this would not hold for substances of different families where diverse contributions to the total residual entropy could appear and be relevant. This was inferred from the location of points referring to various substances in a plot of S g versus ln(T g /T K ) [27] where several correlation lines were drawn each implying a constant, or effective, value of C p in the temperature range between T g and T K 1 . In this context, metallic glasses, for which a "bead" should correspond to every atom in the alloy, appear to align similarly to inorganic glasses, with slope of C p = 18 J/mol·K, higher than that of polymeric and organic glasses (12.5 J/mol·K) (Fig.…”