Polystyrene hydrostatic

Relaxation Map. Figure 16 shows the relaxation times isolated in hydrostatic polystyrene. The elementary processes isolated between 94 C and 103°C are characterized by relaxation times following a compensation law, eq. (8) at the compensation temperature = 135 C, they all have the same value (t = 0.75 s). This mode corresponds to the dielectric manifestation of the glass transition. 

Fig. 14. TSC spectra of hydrostatic polystyrene after polarization at temperatures Tp < 150°C indicated by the arrows. 

Fig. 16. Arrhenius diagram of dielectric relaxation times of hydrostatic polystyrene. The arrows indicate the temperatures of the TSC maxima.

Relaxation Map. The Arrhenius diagram of rheomolded polystyrene represented in Figure 20 is qualitatively analogous with the one of hydrostatic polystyrene (see Fig. 16). The relaxation times isolated in the glass transition mode follow a compensation law with = 145 C and = 0.19... 

Fig. 1. Surface density of crazes versus time in polystyrene for several levels of hydrostatic tension or and deviatoric stress s. From Ref. , courtesy Taylor and Francis, Ltd.

It is possible to produce orientation at normal temperatures in a brittle thermoplastic like polystyrene, by the application of a sufficiently high hydrostatic pressure during extension. The hydrostatic pressure prevents brittle failure at small strains, and can be regarded as reducing Ty, below ambient temperature. Similar results may be obtained by hydrostatic extrusion, where a billet of the polymer is forced through a reducing die under the pressure of a hydraulic fluid. An example of the properties achieved in this way has been given in Section 1.2 above. 

Except for supercritical extraction conditions, hydrostatic pressure effects are typically of negligible importance for simple solvent vapors diffusing in polymers, since the saturation vapor pressure is low, <101.3 kPa (1 atm), in most applications. The predictive power of the approach is indicated by the results for the mutual diffusion coefficient of toluene in a toluene-polystyrene system (Fig. 14) (35). 

Based on equation-of-state concepts, how does the density of polystyrene vary (at 125°C) with hydrostatic pressure ... 

In several SPM studies, a Tg higher than the bulk value was observed, even for very thick bulk-like films. It has been suggested that the pressure the tip exerts on the surface could be interpreted as a hydrostatic pressure (22). At a macroscopic level, it is well known that Tg linearly increases with increasing hydrostatic pressure (0.3 K/MPa for polystyrene) (23). This is interpreted as the effect of subduing the thermal expansion of the material, therefore limiting the formation of free-volume necessary for the glass transition to take place. 

At the top of Fig. 4.35, polystyrene cooled under high pressures is analyzed. The polystyrene melt was vitrified at the slow cooling rate of 5 K/h under the various hydrostatic pressures indicated in the graph. The analysis was then carried out, after release of pressure at room temperature, far below the glass... 

Discussion. In hydrostatic and rheomolded polystyrene, the TSC peak associated with the glass transition is situated, like in reference polystyrene, in the temperature range 90-100 C. Its increasing magnitude has been attributed to conformational changes favoring microbrownian motion. The width of the distribution function of the relaxation time and also the order parameter is... 

In hydrostatic and rheomolded polystyrenes, the TSC peak associated with the liquid-liquid transition is shifted towards lower temperatures. Figure 21 illustrates this shift for rheomolded polystyrene (solid line spectra) in comparison with reference polystyrene (dashed line spectra). This evolution reflects a strong decrease in the critical temperature T down to 16 C and -22°C for hydrostatic and iheomolded polystyrenes, respectively. The spectacular decrease of T in rheomolded polystyrene, illustrated in the Arrhenius diagram of Fig. 21, is due to the liberation of molecular movements frozen in reference polystyrene. 

The short time mode corresponds to the glass transition. In polymers like polystyrene, a narrow distribution is observed. Ihe width of the distribution reflects the width of the distribution of the order parameter it is increased after mechanical orientation by addition of a dopant or additive, or under special glass forming conditions (hydrostatic pressure or rheomolding). The distributed relaxation times obey a compensation law, they are reduced to a single time at the compensation temperature T. The departure of from the glass transition is related to the kinetic aspect of the transition. Thermodynamic models are based on the linear relationship between the activation enthalpy and the activation entropy. 

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