Relaxation Times during Isothermal Contraction near the Glass Transition

Changes in Relaxation Times during Isothermal Contraction near the Glass Transition 

Perhaps the most direct observation of the relation between relaxation times and free volume, as expressed in its most general form by equation 49, is obtained from viscoelastic (or viscosity or dielectric ) measurements repeated over a period of time during the spontaneous contraction of a glassy sample that occurs after quenching it to a temperature near or below Tg, as portrayed in Fig. 11 -7. As the volume decreases, the relaxation times increase. Since the contraction takes place at constant temperature and pressure, the change in ar can be entirely attributed to collapse of free volume as a function of elapsed time. Moreover, since the occupied volume should remain constant at constant T and P, experimental measurements of the decrease in v provide the decrease in ly directly. Thus, during such an isothermal contraction at temperature T, the fractional free volume at time t is 

Effects of Molecular Weig it and Other Variables on Free Volume and 

Although this chapter is concerned primarily with effects of temperature and pressure, it is convenient to mention here the effects of some other variables on 

Since ai2 is a ratio of relaxation times, the relaxation times are proportional to (equation 4 of Chapter 10), and in a comparison of different molecular weights a and T are constant. Substitution of equation 67 into equation 68 gives the molecular weight dependence of fo at constant temperature. 

---