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88 Ralf Steudel crease its yield by extraction of the quenched melt after the p-sulfur has de- composed which takes ca. two days at 20 C or one day at 40–60 C (see above). Since the quenching and extraction procedures may alter the molec- ular identity of the polymer it is convenient to use two different symbols for the polymer dissolved in the melt (S1) and for the isolated product (Sm). Be- low it will be shown that S1 is most probably a mixture of large rings and long chains at temperatures above 159 C. The most reliable determinations of Sm in the older literature are those by P.W. Schenk [19] and by J. Schenk [23] while the data reported by Koh and Klement [24] are probably too high, at least in the high temperature region. P.W. Schenk equilibrated liquid sulfur in an aluminum oven and allowed the melt to flow in a thin stream from the oven through a valve in the bottom. Using a jet of cold gas to blow the melt against a sheet of glass (P.W. Schenk) or copper or aluminum metal (Koh and Klement) the authors tried to rapid- ly quench the melt as a thin layer. In some experiments J. Schenk used liquid air to quench the melt, a coolant which is to be preferred over water since it yields very small particles of sulfur which are efficiently quenched. After extraction with CS2 at 20 C the Sm content was determined as insoluble residue. In general, the polymer content of liquid sulfur increases first slowly from the melting point to 155 C, but more steeply above 158 C although there is no indication for a sudden increase as dramatically as the viscosity increases in the region 159–190 C (see above). While the viscosity decreases above 190 C the polymer content increases further to a maximum of 40% [19]. Therefore, the claim [26, 34, 46, 47] that the sudden viscosity increase is caused by a precipitation of polymer from the melt and a sudden shift of the equilibrium from the monomer S8 to the polymer above 159 C is not justi- fied. For systems like liquid sulfur with highly entangled but not crosslinked chains the viscosity is proportional to the product N3·r3 where N is the chain-length and r is the concentration of chain-atoms [48]. The following polymer contents are representative examples for high-pu- rity sulfur melts quenched from the temperatures indicated: – 120 C: 0.5% [23]; 130 C: 1.0% [23]; 160 C: 4.5% [23]; 350 C: 37% [19] More recent data indicate that these concentrations are all slightly too high (see later). According to Koh and Klement who quenched the sulfur melts at room temperature the polymer content rises from ca. 1% at 135 C melt temperature, to 10% at 162 C, 36% at 200 C, and 56% at 250 C at which temperature a plateau is reached which extends to at least 300 C [24]. Since these authors did not purify the sulfur by the method recommended by Bacon and Fanelli [4] and since a quenching temperature of 20 C is probably too high the reliability of these data may be disputed. Sulfur melts containing various types of impurities show higher polymer contents after extraction (max. 65%), but this polymer is less stable at 20 C with regard to conversion to a-S8 [19].PDF Image | Topics in Current Chemistry
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