Results And Discussion

Structure and Thermodynamic Properties at © >T>TC

The different theoretical approaches that have been applied to calculate Rg at the critical demixing point (Tc,^c) have led to conflicting conclusions. Starting from de Gennes' assumption that at the critical point polymer chains just begin to overlap and do not interpenetrate significantly [I6] it was shown that the polymer chains should be partially collapsed at T~=Tc and [24]. Conversely, the molecular theory [6,25] and computer simulations [26] show that conformation of individual chains below the © temperature should remain unperturbed and collapse to the globular state should occur only at T<TC. The temperature variation of the dimension of polymer chains at © > T> Tc was first explored using SANS along with the high concentration isotope labeling in [2] for solutions of PS in the © solvent CH-d. As is seen from Fig.3, Rg of interacting PS coils does not decrease over the whole temperature range below the © temperature. At the same time, the correlation length of the concentration fluctuations which is much smaller than Rg at T~ © diverges in the vicinity of Te These findings indicate that the deterioration of the quality of © solvents leads to the formation of microdomains of the size representing clusters of unperturbed, strongly interpenetrating polymer coils.

The region \ > Rg is realized in the vicinity of the critical demixing temperature and is thus associated with the poor solvent domain. The solvent quality can be usually improved by increasing the temperature or pressure, to move the solution away from the phase boundary. In © solvents (e.g. PS - CH) the solution is capable of reaching the © condition at some T and P whereas in poor solvents, e.g. PS - AC the condition cannot be reached at any accessible T, P. The effect of pressure on the thermodynamic state of PS/AC-d solutions is illustrated in Fig.4. Within experimental error, Rg remains independent of pressure down to the critical demixing PC. Although the © condition does not exist for PS-AC solutions, the values for Rg remain close to the unperturbed dimensions for Gaussian chains with Mw-11600 (i.e. Rg ~=0.27 Mw/2~29 A). The correlation length, diverges as P falls to P~PC, where the system exhibits pressure-induced phase demixing, but for solutions of PS in AC, the magnitude of remains generally > R, [9]. This delineates an important difference between the structure of the liquid © and poor solvents. The size of the concentration fluctuations in polymer solutions in © solvents can decrease below the dimension of the constituent chains and reach the value of ^(©) defined by Eq.8 (Figs.3, 5) whereas the value of in poor solvents always remains > ^(©) (Figs.4, 6) and levels off at some thermodynamic distance from the critical point.

Supercritical fluids in general and supercritical carbon dioxide in particular have emerged as an attractive alternative to the organic solvents used for polymer manufacturing and processing. One key advantage of SCFs is the possibility of continuously tuning the solvent quality by varying the pressure (or density) in addition to the temperature. Due to the high compressibility of SCF solvents the density may be

Figure 3. Rg(T) and \ (T) for PS-CH-d solution Figure 4. Rg(P) and Ç(P) for PS-AC-d solution (Mw-533000) at ambient conditions. Rg measured (Mw-11600)at T-30°C[9]. overtherange ofpressure 0.1<P<550MPa [9].

Figure 3. Rg(T) and \ (T) for PS-CH-d solution Figure 4. Rg(P) and Ç(P) for PS-AC-d solution (Mw-533000) at ambient conditions. Rg measured (Mw-11600)at T-30°C[9]. overtherange ofpressure 0.1<P<550MPa [9].

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