A Polymer-Solution "Thermometer" Revisited Paul L. Dubin propaaed "a demonstration of the thermodynamic consequencesof specific polymer-solvent interactions" based on the decrease in solubilitv with increasine temoerature of oolvvinvlovridine in ethanol-toluene solution.' He did . . ... not specify whirh of the readily ava~lalrle1e.g.. from I'ulyscienrrs, Wnrringtcm, PA) pdywnylpyridmer should be used: poly-2-vinylpyridin.ur p$,ly.-l-\,inylpyridine.l'hefarr that puly-2-vinylpyrid~ne hecomes morewhhle methand-toluene with increasing rrmprratuw wherms pt,ly-4.vinyl1,yridine becwnes less soluble with inrredsing trmprrnturp in this solvent system leads to an interesting and informative extension of the demonstration proposed by Dubin. The modified demonstration may be performed as follows: Prepare 2% solutions of both polymers in ethanol (a good solvent). Add toluene (a poor solvent) to a point just short of phase separation. The resulting solutions, which should be clear at room temperature, will be approximately 7m vlv toluene for the poly-4-vinylpyridine and about 80%vlv toluene for the poly-2-vinylpyridine. Large test tubes containing the clear polymer solutions, labeled "2" and "4," are first shown a t r w m temperature (ZO'C), then placed in a beaker of cold water (lO°C) for a few minutes, allowed to return to room temperature, placed in a beaker of warm water (30°C) for a few minutes and finally allowed to return to room temperature again. When cwled, the poly-2-vinylpyridine solution becomes turhid indicating phase separation, but the poly-4-vinylpyridine solution remains clear. When warmed, the poly-Cvinylpyridine solution becomes turhid but the poly-tvinylpyridine solution remains clear. The slight difference in the structures of the two polymers leads to what appears to be a markeddifference in the temperature dependencies of their solubilities. ~~~~~~
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In general either raising or lowering the temperature of any polymer solution leads to phase separation. As the temperature is decreased phase separation is observed at the "upper critical solution temperature" and as the temperature is increased phase separation is observed at the "lower critical solution temperature." Phase separatian at temperatures both above and below the region of complete miseihility is associated with a minimum in the solubility parameter. Two opposing factors, a decrease in the polymer-solvent interaction with increasing temperature combined with an increase in the freevolume dissimilarity with increasing temperature are thought to account for this general behavior of polymer solution^.^ The oolv-2-vinvlovridine solution as oreoared for this demonstration has an uooer critical solution temoerature sliehtlv .. . . .. , b~lowrocm remprrature uherensthe p,lg-l-vinylpyridinrsolutiorl hasa lower oitital i.lutim temprraturerlichtl) nhwe nwm temperature. Thw dcmcmtrnr~cmimprwsrs upon studrnts thr importance ut rtnall rtructursl differences in molecules. I thank Paul Duhin and Ronald Starkey for helpful discussions.
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' Dubin, Paul L., J. CHEM.Ewc.,
58, 866 (1982).
Bilmeyer. Fred W.. Jr. "Textbook of Polymer Science." 2nd ed.,Wiley Interscience, New York, 1971, pp. 40-41.
Jack C. Norman College of Envmnmental Scences Un vsrsq of W rconson-Geen Bay Green Bay. W1 54302
1094
Journal
of Chemical Education
