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Liquid Sulfur 93 the same reason, some amines lower the viscosity because of reactions of the organic groups with sulfur producing some hydrogen sulfide [4]. The impact of the halogens chlorine, bromine and iodine on the viscosity of liquid sulfur has been studied most extensively. They all lower the viscos- ity dramatically, even in low concentrations, and the effectiveness in this re- spect decreases from chlorine to iodine. At the same time the maximum of the viscosity curve is shifted to higher temperatures. For example, the addi- tion of 0.02% of I2 gives a viscosity maximum of 5.7 Pa s at 225 C while 0.75% chlorine, added as sulfur chloride, give maximum viscosity values be- low 0.2 Pa s in the entire temperature region of 150–320 C [4]. It is generally assumed that the halogens as well as the sulfur chlorides react with liquid sulfur to chain-like dihalopolysulfanes SnX2. In this way the chain-length of the polymeric sulfur molecules is reduced and the viscosity thus lowered: SSþX2 Ð 2SX ð9Þ SSþSX2 Ð SXþSSX ð10Þ Since these reactions are reversible and since the elemental halogens as well as the low molecular mass sulfur chlorides are volatile they slowly es- cape from the hot melt if the sample is kept open to the atmosphere. In this case, the viscosity rises again with the heating time [4]. If selenium is added to liquid sulfur the transition temperature is some- what lowered since mixed selenium-sulfide chains and rings are formed [77] the dissociation energy of which is lower than for sulfur-sulfur bonds. The maximum viscosity is also lower compared to pure sulfur. Above a concen- tration of 70% Se the melt is highly viscous and obviously consists mainly of polymeric molecules at all temperatures as is the case with pure selenium [23]. The effects of phosphorus and arsenic are more complicated. These ele- ments are added as such or as P4S3 or As2S3, respectively, followed by heat- ing to 400 C for homogenization and cooling to the melting point. The maximum viscosity of these melts is lowered by approximately one order of magnitude if between 0.25% and 1.5% of arsenic are added but the increase in viscosity begins at much lower temperatures compared to pure sulfur [23, 78]. The polymer obtained from such melts is more stable thermally since the three-valent phosphorus or arsenic atoms obviously knit the sulfur chains together in a process similar to the vulcanization of organic polymers by elemental sulfur resulting in oligosulfane bridges of the type RSxR with x=1, 2,...[79]. Dilution of liquid sulfur with an inert organic solvent shifts the polymer- ization temperature to higher values.PDF Image | Topics in Current Chemistry
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