Equilibrium glassy state

The inplane shear stress-strain tests reported here have been well demonstrated to be a reliable test for matrix-dominated properties in composites 141). For the selected mechanical properties that were monitored, their sensitivity to the thermal history was well demonstrated. In particular, the embrittlement process during the sub-Tg annealing or physical aging has been clearly observed. This decrease in molecular mobility, which gives rise to an increase in relaxation time and hence a decrease in toughness, can be rationalized as a decrease in free volume in an approach towards the equilibrium glassy state. 

A priori, one would assume on the basis of crosslink density that a postcured system (presumably with a higher crosslink density) would have a higher density. This may actually be the case at the postcure temperature. At room temperature, however, Gillham et al.176) have indicated that the higher-crosslinked system may be quenched further from the hypothetical equilibrium glassy state, resulting in a higher value of free volume or lower density. 

For all of the polymers, the time required for relaxation of the enthalpy to that of the equilibrium glassy state at the various temperatures is extremely long, even at temperatures of the order of 15 C belov the corresponding glass-transition temperatures. The principal rate-determining factor Is the glass-transition temperature, as anticipated frcmi previous studies (1.2). 

Figure 4. Relationship between the relaxation time, r, and the enthtdpy displacement from the equilibrium glassy state, (Qe — QO, for PC at carious temperatures...

Experience has shown that these films must have a high glass transition temperature, Qrpically > 180 C, to insure thermal stability for many years. An important focus of current research is therefore directed to the enhancement of the thermal and temporal stability of x X The stability of polar order in the thermodynamically non-equilibrium glassy state varies from polymer to polymer. Chemical attachment of the chromophore to the polymer as a sidechain (2), as part of the mainchain (J), orinacrosslinkedstructure(4), has been found to retard the relaxaticm of alignment... 

In the study on enthalpy relaxation, we studied the response of polymers subjected to aging in a non-equilibrium (glassy) state. We monitored relaxation of... 

While at a temperature below Tg, some degree of molecular motion does occur. With time the molecular system does approach the true equilibrium state, i.e., the equilibrium values of volume, enthalpy or other state function variables. Physical aging represents a thermodynamic drive towards the equilibrium glassy state, and the recovery process involves a decrease in free volume as the material ages. 

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