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Transitions, relaxations, and thermodynamics in the glassy state
Author(s) -
Simha Robert
Publication year - 1980
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760200114
Subject(s) - thermodynamics , glass transition , materials science , relaxation (psychology) , thermal , volume (thermodynamics) , degree (music) , thermal equilibrium , volume fraction , thermodynamic equilibrium , polymer , physics , acoustics , composite material , psychology , social psychology
The PVT properties of polymer glasses are discussed from two points of view. First, as a low frequency tool in the study of sub‐glass relaxations. Dilatometric results illustrate the sensitivity of the thermal expansivity and its temperature coefficient to dynamic processes occuring in the glass. The second point of view considers the quasi‐thermodynamics of the non‐equilibrium system, under conditions where the rates of relaxational processes are small in comparison with experimental rates, and time dependent processes are practically absent. The starting point is a theory of the equilibrium melt which describes the characteristic liquid disorder in terms of a temperature and pressure dependent hole or loosely, free volume fraction. This function is obtained by the minimization of a configurational free energy. Upon reaching the glass transition, this dependence is reduced, and further so in a sub‐glass relaxation region. However, it is not eliminated until temperatures of the order of 50 to 70 K are reached. While this picture is qualitatively universal, significant quantitative differences are observed with different chemical structures and different thermal and pressure histories. A comparison of thermal expansivities of high and low T g systems indicates that in the former the free volume retained upon reaching T g is comparatively large and the departure from equilibrium or degree of freeze‐in comparatively small. Similarly, it appears that the glassy densification generated by cooling the melt under pressure is more extensive in high T g glasses. Such results imply structural differences. These should be investigated by comparative studies of (a) time dependent processes in terms of the free volume functions, (b) sorption and transport, and (c) the temperature dependent dynamics of density fluctuations.