Plastic Sulfur

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Sulfur is a nonmetal main group element. It has four stable isotopes and it can be found in nature as a pure element, bright yellow crystalline solid, and in forms of sulfide and sulfate minerals. It is an essential element for living organisms, and is found in certain amino-acids (cysteine and methionine).

Sulfur exists in different molecular forms, it has more allotropes than any other element. These allotropic modifications, mostly rings of various sizes, differ from each other by individual properties.

At room temperature the most stable molecular structure is cyclic S8 molecule. Stable rhombic modification, also known as alpha-sulfur, consists of eight membered crown shaped rings of sulfur – S8, which stack up on each other building the crystal structure.


Rhombic sulfur melts at 113°C giving pale yellow molten sulfur with the same cyclic S8 molecules which becomes darker and more viscous as the temperature is increased. Only at 180°C this light red molten turns brown and becomes very viscous. At this point, eight-membered ring cleavage occurs, chains form giving a linear polymer of the general formula (S8)n

Molten sulfur easily ignites. Upon heating sulfur to its boiling point of 445ºC, obtained melt can be suddenly cooled by pouring a thin stream in a cold water, while a viscous mass is formed. This is plastic sulfur, also known as gamma-sulfur – a dark brown or even black, sticky elastic substance with no sharp melting point. If after one minute you remove the plastic sulfur with forceps and compare its flexibility with crystal sulfur, you can observe its rubber like consistency. Plastic sulfur has extraordinary low solubility in solvents. 


By measuring the viscosity of this melt at 170°C, it has been proven that one chain of the obtained polymer contains around one million atoms of sulfur. Plastic sulfur is unstable at all temperatures. Because rhombic sulfur is thermodynamically most stable sulfur modification, the plastic sulfur changes into this modification by the opposite process. On standing it slowly solidifies with time varying from a few minutes to a few hours and transforms to a brittle and bright rhombic form again, gaining the eight atom ring structure again. While covalent bonds prevail both in S8 chains and crown molecule, polymer mass is aligned and held by Van der Waal's forces and cross linking. S8 chains slowly return to S8 crown molecules.

Take care when handling plastic sulfur, as molten sulfur is still trapped inside if not cooled enough. The experiment should be performed within a fume hood or other well ventilated area, because toxic gas – sulfur(IV)oxide – is generated if the sulfur is lighted.


 
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