Physical ageing structural relaxation

The kinetic character of the glass transition and the resulting non-equilibrium character of the glassy state are responsible for the phenomena of structural relaxation, glass transition hysteresis, and physical aging (Kovacs, 1963 Struik, 1978). 

Figure 4.4 shows a dilatometric or calorimetric experiment to show structural relaxation (physical aging) and glass transition hysteresis. The sample is cooled from T0 to T it is kept at Tj for a certain time and heated again to T0. During the cooling step, the material vitrifies at B, resulting in an abrupt decrease in both the expansion coefficient and the specific heat. 

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

To summarize, most of the experimental results on the yielding of thermosets may be interpreted by stating that the structure affects chemical structure, secondary relaxations, physical aging, etc., on the proportionality constant are still to be explored. 

The influence of temperature and strain rate can be well represented by Eyring s law physical aging leads to an increase of the yield stress and a decrease of ductility the yield stress increases with hydrostatic pressure, and decreases with plasticization effect. Furthermore, it has been demonstrated that constant strain rate. Structure-property relationships display similar trends e.g., chain stiffness through a Tg increase and yielding is favored by the existence of mechanically active relaxations due to local molecular motions (fi relaxation). 

Here J is a constant, and T2 Tg — 50 K. The above equation is not valid for T < Tg and has an apparent singularity in x as T - T2. This basically prevents us from following this line of thought in order to determine the low temperature structural relaxation and physical aging in glassy polymers. 

Equation (23), together with Eqs. (2), (15) and (19), provide the basic theoretical relationships for the prediction of the structural relaxation and physical aging... 

We have reviewed the recent development of a nonequilibrium statistical mechanical theory of polymeric glasses, and have provided a unified account of the structural relaxation, physical aging, and deformation kinetics of glassy polymers, compatible blends, and particulate composites. The specific conclusions are as follows ... 

A convenient way to analyze physical aging processes (structural recovery) is to use a relaxation function < t) defined as... 

Experimental data are presented to show that the JG relaxation mimics the structural relaxation in its volume-pressure and entropy-temperature dependences, as well as changes in physical aging. These features indicate that the dependences of molecular mobility on volume-pressure and entropy-temperature have entered into the faster JG relaxation long before structural relaxation, suggesting that the JG relaxation must be considered in any complete theory of the glass transition. 

Borde, B., Bizot, H., Vigier, G., and Buleon, A. Calorimetric analysis of the structural relaxation in partially hydrated amorphous polysaccharides, n. Phenomenological study of physical ageing, Carbohydr. Polym., 48, 111, 2002. 

Figure 3.1. Schematic illustration of temperature dependences of the specific volumes of amorphous materials. This figure also illustrates the effects of the nonequilibrium nature of glass structure, which results from kinetic factors. Glass 1 and Glass 2 are specimens of the same polymer, but subjected to different thermal histories. For example, Glass 1 may have been quenched from the melt very rapidly, while Glass 2 may either have been cooled slowly or subjected to volumetric relaxation via annealing ( physical aging ) in the glassy state.

One problem encountered with polymeric membranes is their physical aging which can alter the permeation and the mechanical properties. Sinko et al. [144] noticed a decrease in the water permeability and in the mechanical rate of relaxation with time, which was proportional to the aging temperature. A relaxation coupling model could be used to predict equilibrium in the structural change profile. 

The phenomenon described here is termed physical aging or structural relaxation. It can be detected through the time evolution of not only thermodynamic properties such as specihc volume or enthalpy but also mechanical or dielectric properties. 

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