Experimental equipment and procedures for VLE-measurements

A number of problems have to be solved during the experiment. The solution is usually in an amount of some cm and may contain about 1 g of polymer or even more. Degassing is absolutely necessary. For example, Killmann et al. included special degassing units for each component of the entire equipment. All impurities in the pure solvent have to 

The measurement of polymer solutions with lower polymer concentrations requires very precise pressure instruments, beeause the difference in the pure solvent vapor pressure becomes very small with deereasing amount of polymer. At least, no one can really answer the question if real thermodynamie equilibrium is obtained or only a frozen non-equilibrium state. Non-equilibrium data ean be deteeted from unusual shifts of the %-function with some experience. Also, some kind of hysteresis in experimental data seems to point to non-equilibrium results. A eommon eonsisteney test on the basis of the integrated Gibbs-Duhem equation does not work for vapor pressure data of binary polymer solutions because the vapor phase is pure solvent vapor. Thus, absolute vapor pressure measurements need very careful handling, plenty of time, and an experienced experimentator. They are not the method of choiee for high-viseous polymer solutions. 

The polymer sample is put, after weighing, into the sample flask and the apparatus is evacuated. Degassed solvent is distilled into the measuring burette and from there a desired amount of solvent is distilled into the sample flask. The Hg-manometer is filled from the storage bulb and separates the polymer solution from the burette. Care must be taken to avoid leaving any solvent in the manometer. The apparatus is kept at constant measuring temperature, completely immersed in a thermostat for several days. After reaching equilibrium, the pressure is read from the manometer difference and the concentration is calculated from the calibrated burette meniscus difference corrected by the amount of vaporized solvent in the unoccupied space of the equipment. The pure solvent vapor pressure is usually precisely known from independent experiments. 

Difference/differential manometers have some advantages from their construction They are comparatively smaller and their resolution is much higher (modem pressure transducers can resolve differences of 0.1 Pa and less). However, there are the same disadvantages with sample/solution preparation (solutions of grams of polymer in some cm vol- 

Problems caused by the determination of the unoccupied vapor space were avoided by Haynes et al., since they measure the pressure difference as well as the absolute vapor pressure. Also, the concentration is determined independently by using a differential refractometer and a normalized relation between eoneentration and refractive index. Degassing of the liquids remains a necessity. Time for establishing thermodynamic equilibrium could be somewhat shortened by intensive stirring (slight problems with increasing polymer concentration and solution viscosity were reported). 

Difference/differential manometers have some advantages from their constraction They are com- 

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