Degradation phenomena physical properties

The extent of molecular motion depends on the free volume. In the glassy state, the free volume depends on the thermal history of the polymer. When a sample is cooled from the melt to some temperature below Tg and held at constant temperature, its volume will decrease (see Rgure 8.18). Because of the lower free volume, the rate of stress relaxation, creep, and related properties will decrease (29-33). This phenomenon is sometimes called physical aging (29,32), although the sample ages in the sense not of degradation or oxidation but rather of an approach to the equilibrium state in the glass. 

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