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Liquid Sulfur 89 Previous authors [19, 21–23, 34] also reported that the polymer concen- tration is nearly constant in the region 250–420 C although their absolute values in some cases differ from more recent results because of deficiencies in the quenching procedure or the impact of impurities including the water used for chilling the melt. Generally, it is assumed that the polymer is pres- ent as an ideal solution in a melt of small ring molecules. Ward and Myers [49] tried to determine the S1 content of the sulfur melt directly using Raman spectroscopy. The Raman spectra of hot sulfur melts are characterized by very broad and overlapping signals. The authors as- sumed identical Raman scattering intensities for the stretching vibrations of S8 and S1 and an identical temperature dependence of these intensities. In addition, they neglected the Sp content of the melt. The obtained concentra- tions scatter considerably but indicate that the S1 content increases from practically zero at 150 C to ca. 70% by mass at 260 C. From the tempera- ture dependence of the S1 content the enthalpy of formation of was estimat- ed as 19 kJ mol1 (for more recent data, see later). The occurrence of higher concentrations of S1 above 160 C results in a birefringence of sulfur melts [50] with a maximum between 175 and 185 C at which temperature the viscosity also attains its maximum value. If sulfur melts are quenched at temperatures below 30 C a material is obtained which initially is elastic and amorphous at room temperature but rapidly crystallizes to become a mixture of microcrystalline sulfur rings and polymeric molecules. The glass-transition temperature of the elastic material is ca. 30 C. Below this temperature the conversion to a microcrystalline mixture is very slow [51, 52]. For reasons of completeness it should be pointed out that polymeric sul- fur is also obtained if sulfur vapor is quenched from very high temperatures (e.g., 600 C) to very low temperatures (liquid nitrogen) followed by warm- ing the condensate to +20 C and extraction with carbon disulfide. Up to 60% Sm have been obtained in this way [53]. Commercially polymeric sulfur (trade name Crystex) is produced by a similar process (see the chapter on “Solid Sulfur Allotropes” in this volume). The glass transition temperature of Crystex is +75 C [51]. 2.2.2 Properties of m-Sulfur Polymeric or insoluble sulfur prepared from quenched sulfur melts by ex- traction is a yellow powder which slowly converts to S8 at 20 C. This reac- tion takes months to years but is accelerated by irradiation, by grinding, by heating as well as by catalysts like aqueous or gaseous ammonia. Heating to 100 C for 10 h destroyed all Sm in a quenched and aged melt which original- ly contained 2.8% of Sm [54, 55]. If Sm is treated with liquid carbon disulfide at 100 C (in a sealed thick-walled glass ampoule) it partly dissolves as a mixture of S8 and Sp, in other words by depolymerization [21]. The melting (depolymerization) temperature of solid Sm has been reported as 105–115 C [56, 57]. The molecules in the melt thus obtained rapidly equilibrate withPDF Image | Topics in Current Chemistry
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