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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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164 Christian Wohlfarth<br />

vent concentrations). Sorption- desorption<br />

hysteresis has never been observed when<br />

using piezoelectric detectors. Bonner and<br />

Prausnitz 85 reported some hysteresis results<br />

when applying their quartz spring sorption<br />

balance for polymer concentrations above<br />

85 wt%. This demonstrates the effect <strong>of</strong> reducing<br />

the amount <strong>of</strong> polymer from about<br />

50 mg for the quartz spring sorption technique<br />

by an order <strong>of</strong> 10 3 for the piezoelectric<br />

detector. However, measurements are<br />

limited to solvent concentrations well below<br />

the region where solution drops would<br />

be formed. On the other hand, measurements<br />

also can be made at higher temperatures<br />

and pressures. Limits are set by the<br />

stability <strong>of</strong> the electrical equipment and the<br />

construction <strong>of</strong> the measuring cell.<br />

(iv) Gas-liquid chromatography (GLC)<br />

In 1969 Smidsrod and Guillet 102 demonstrated that GLC could be used to determine the activity<br />

coefficient <strong>of</strong> a solute in a (molten) polymer at essentially zero solute concentration.<br />

This type <strong>of</strong> activity coefficient is known as an infinite-dilution activity coefficient.<br />

Smidsrod and Guillet also introduced the term “inverse” gas-liquid chromatography (IGC)<br />

because in IGC the liquid polymer in the stationary phase acts as a solvent for the very small<br />

amount <strong>of</strong> an injected solvent sample like the solute in this case. Methods and results <strong>of</strong> the<br />

application <strong>of</strong> IGC to polymers and polymer solutions have been reviewed continuously 13-25<br />

Figure 4.4.10. Schematic diagram <strong>of</strong> an isopiestic vapor<br />

sorption apparatus using a piezoelectric crystal detector.<br />

[Reprinted with permission from Ref. 101, Copyright<br />

1995, American Chemical Society].<br />

so that an extensive discussion is not required here. The equipment in principle does not differ<br />

very much from that used in analytical GLC. Figure 4.4.11 is a schematic <strong>of</strong> a simple<br />

IGC unit.<br />

Figure 4.4.11. Schematic diagram <strong>of</strong> an IGC apparatus: 1 - carrier gas, 2 - pressure reducer, 3 - gas cleaning unit (if<br />

necessary) , 4+5 - gas-pressure regulation and control unit, 6 - manometer, 7 - column, 8 - thermostat, 9 - mechanical<br />

mixer, 10 - inlet syringe, 11 - detector (the gas flows after the detector through a bubble flow meter that is not<br />

shown here), 12 - electronics, 13 - recorder.

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