Structural Relaxation and Physical Aging

FIGURE 12.7 Illustration of thermal history used in enthalpic relaxation experiments via an enthalpy vs. temperature plot. (Adapted from Cowie, J.M.G. and Ferguson, R., Macromolecules, 22, 2307, 1989.) 

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. 

Specific volrrme against temperature data for poly(vinyl acetate) are tabulated in the following (Meares, 1957). Locate the glass transition and estimate its width, and cq (the specific volume at the glass transition, Vg, is equal to 0.839 cm mol ). 

Consider the following polymer pairs and explain, giving reasons, whieh of the two polymers should display a glass transition at higher temperamre  

Which of the following polymers will have the highest and which the lowest glass transition (a) poly(methyl acrylate) (Rj = H and Rj = CH3), (b) poly(methyl methacrylate) (Rj = CH3 and Rj = CH3), and (c) poly (pentyl acrylate) Explain. 

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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... 

Hodge, I. M. in MRSSymp. Proc. Vol. 215, Structure, Relaxation, and Physical Aging of Glassy Polymers, MRS Pittsburg, 1991,11. 

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. 

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). 

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 ... 

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