Aging enthalpy recovery

FIGURE 5.16 Enthalpy recovery results for o-terphenyl aged at Tj — 11°C in (a) bulk state and (b) confined in an 11.6-nm pore diameter controlled pore glass material showing much smaller buildup of enthalpy overshoot upon aging of the confined material. Originally, similar data were interpreted [McKenna et al. [1992] to imply reduced aging in confined systems. (Data from Simon et al. [2002].)... 

The thermograms shown in Fig. 13.4a demonstrate clearly the development of two distinct enthalpy recovery peaks that increase with increasing aging time ta- Good separation of the peak maxima is obtained as tends to increase with aging time for both components but at different rates. This indicates that phase separation has occurred and the enthalpy recovery peaks are characteristic of each distinct phase in the mixture. A miscible blend of PVC and atactic PMMA was also treated in a similar fashion, but only one enthalpy recovery peak could be detected (Fig. 13.4b) indicating a single-phase system. 

The behavior of the two-phase systems is complex and responses on aging can be affected by the thermal history and aging temperature. This is well illustrated by a series of investigations, the two-phase blend of acrylonitrile-butadiene-styrene copolymer (ABS, Tg = 110 °C) and polycarbonate of bisphenol-A (BPAPC, Tg = 151 °C). Due to the phase-separated structure of the blend, two enthalpy recovery peaks are detected by enthalpy relaxation and attributed to the two components (Tang and Lee-SuUivan 2008). However, aging appears to have little effect on the ABS component even at temperatures close to the ABS glass transition. 

In this article on physical aging, the phenomena associated with structural recovery and physical aging are described, starting with discussion of volume recovery, enthalpy recovery, viscoelastic properties, and failure. There are more in-depth reviews of the phenomena and models associated with the glass transition... 

In a similar way, Kovacs [9] has invoked a distribution of relaxation times to account for multiple relaxation events in the aging polymer, where each relaxation time depends on the glass structure and the temperature. The difference is that rather than trying to fit the observed heat capacity data, the movement of the enthalpy recovery peak in the DSC curves was followed, as discussed by Hutchinson [10]. This approach obviated the need to define a specific distribution of relaxation times. 

Simon, S. L., Plazek, D. J., Sobieski, J. W., and McGregor, E. T., Physical aging of a polyether-imide volume recovery and its comparison to creep and enthalpy measurements, J. Polym. 

Cowie, J. M. G., Elliott, S., Ferguson, R Simha, R Physical aging smdies on poly(vinyl acetate) - enthalpy relaxation and its relation to volume recovery. Polymer Communications, 28(11), pp. 298-300 (1987). 

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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