Pressure Dependence of the Glass Transition

Vleeshouwers, S., and Nies, E., Stochastic theory for the glassy state 2. The pressure dependence of the glass transition. Colloid Polym. ScL, 274, 105-111 (1996). 

Atake, T., and Angell, C. A., Pressure dependence of the glass transition temperature in molecular liquids and plastic crystals, J. Phys. Chem., 83, 3218-3223 (1979). 

The site entropy is thus a sensible candidate for describing fluid relaxation outside the immediate vicinity of the glass transition. In a more precise language, is actually an entropy density, and the maximum in Sc T) derives from an interplay between changes in the entropy and fluid density as the temperature is varied. Explicit calculations demonstrate that the maximum in Sc T) disappears in the limit of an incompressible fluid, which is physically achieved in the limit of infinite pressure. The pressure dependence of Sc T) is described in Section X, where it is found that the maximum in Sc T) becomes progressively shallower and 7a becomes larger with increasing pressure. 

Fig. 6. Specific volume pressure curves for the l.c. polymer shown in Fig. 5. Thin dashed lines pressure dependence of the phase transformation temperatures l.c. to isotropic, Tc, and the glass transition temperatures, T , full line specific volume-temperature cut at 2000 bar (isothermal measurements)...

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. 

We now discuss the impact of this general property on theories and models of the glass transition. The primary concern of most theories is to explain the temperature and pressure dependences of the structural relaxation time ra. The dispersion (n or >kww) °f the structural relaxation is either not addressed or... 

The general experimental fact of constant frequency dispersion (or time dependence of the correlation function) of the a-relaxation at constant Ta for different combinations of T and P has an immense impact on glass transition. Although the data were mostly obtained by dielectric relaxation, the same effect was found in some glass-formers by photon correlation spectroscopy. The primary concern of most theories, including those mentioned in the NY Times article, is to explain the temperature and pressure dependences of the structural relaxation time Tq.. In these theories, the dispersion of the structural relaxation is either not addressed, or else considered separately with additional input not involved in arriving at r . Consequently, the frequency dispersion is unrelated to the relaxation time of the structural a-relaxation in these theories, and they are unlikely to be consistent with the T, / -superpositioning property by happenstance. 

The role of water in the conformation, the activity and the stability of proteins has been investigated with many experimental and theoretical approaches. Because of its importance it has been coined as the 21 amino acid . There is now sufficient experimental evidence for the fact that dry proteins do not unfold by increased temperature or pressure [21]. Low levels of hydration give rise to a glassy state and the temperature of the glass transition depends on the amount of water as observed for synthetic polymers. Water can therefore be considered as a plasticizer of the protein conformation. Whereas hydrophobic interactions have dominated the interpretation of the data, hydrogen bond networks of water may also play a predominant role in water-mediated interactions [48,49]. 

The temperature dependence of the JG j8-relaxation time, tjg, of the PVME component in a 25% PVME/75% PS blend at ambient pressure, Pa = 0.1 MPa, changes from VFT-dependence to Arrhenius T-dependence at the glass transition temperature,, of the PVME component. At the crossover, the relaxation time, tjg(Pj, Fg/), is approximately s. At any of the elevated pressures, P, up to 300 MPa, the same change was observed, and it occurs at temperature, Tp, where tjgCP.Tp) is about the same as xjG Pa,Tgf) 10 " - s. 

Experimental data from our laboratories will be shown for an extensive series of amorphous polymers with glass transitions between Tg = 200 and 500 K. We discuss the temperature dependence of the hole-size distribution characterized by its mean and width and compare these dependencies with the hole fraction calculated from the equation of state of the Simha-Somcynsky lattice-hole theory from pressure-volume-temperature PVT) experiments [Simha and Somcynsky, 1969 Simha and Wilson, 1973 Robertson, 1992 Utracki and Simha, 2001]. The same is done for the pressure dependence of the hole free-volume. The free-volume recovery in densified, and gas-exposed polymers are discussed briefly. It is shown that the holes detected by the o-Ps probe can be considered as multivacancies of the S-S lattice. This gives us a chance to estimate reasonable values for the o-Ps hole density. Reasons for its... 

Kilburn, D., Wawryszczuk, J., Dlubek, G., Pionteck, J., Hassler, R., and Alam, M. A., Temperature and pressure dependence of the free volume in poly isobutylene from positron lifetime and pressure-volume-temperarnreexperiments,Macromol. Chem. Phys., 207,721-734(2006b). Kim, S. H., Chung, J. W., Kang, T. J., Kwak, S.-Y, and Suzuki, T., Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positronium annihilation lifetime measurements. Polymer, 48, 4271-4277 (2007). 

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