Physical ageing mechanisms and stabilization techniques

Physical ageing can result from the spatial reorganization of polymer chains or segments (relaxation of enthalpy, volume, orientation or stress crystallization etc.), transport phenomena (penetration of a solvent, migration of additives) and superficial phenomena (e.g. cracking in a tension-active medium). 

Transition from liquid to glass and crystallization are both phenomena responsible for polymer solidification at the end of a processing operation. Since they are kinetic phenomena, they lead to an out-of-equilibrium thermodynamic state glassy polymers present an excess of unstable conformations and free volume semi-crystalline polymers are not totally crystallized, their melting point being largely lower (usually some dozens of degrees) than the equilibrium value. 

in their use conditions, polymers are subjected to a residual molecular mobility (Ji motions in glassy polymers, a motions in the rubbery amorphous phase of semi-crystalline polymers), they will undergo a molecular reorganization towards the thermodynamic equilibrium, characterized by  

The variation against time of creep compliance, /( ), has been the subject of significant research, in particular in the case of organic glasses (Struik, 1978). As an example, the general shape of creep curves of samples aged for three different durations lOxig and 100 xtg, is presented in Fig. 12.2. 

The decrease in creep compliance (i.e. increase in Young s modulus), as well as the increase in yield stress, can be viewed as advantages for many industrial applications. Unfortunately, these changes are counterbalanced by a catastrophic decrease in ductility/toughness, as schematized in Fig. 12.3. Thus, the consequence of structural relaxation can be understood as an increase in the polymer brittleness. 

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