--- title: "Chalcogenide glasses and network polymers of sulphur" subject: "Engineering Chemistry" chapter: 4 key: "semester_2/engineering_chemistry:chalcogenide-glass-polymers" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chalcogenide-glass-polymers site: BCT Engineering --- # Chalcogenide glasses and network polymers of sulphur Engineering Chemistry · Chapter 4: Engineering Polymers - Asked in: 2075 Baisakh, 1+4 marks - Video: [Chalcogonide Glasses](https://www.youtube.com/watch?v=tW9PayLNmTk) — sab kuch - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chalcogenide-glass-polymers - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4 ## Question What are chalcogenide glasses? Give an account of preparation and uses of network polymers of sulphur. ## Short answer Chalcogenide glasses are amorphous solids containing S, Se, Te with As or Ge, prepared by melt-quenching; they are IR transparent and used in night vision and phase-change memory. Network polymers of sulphur include plastic sulphur from quenching molten sulphur, vulcanised rubber, and Thiokol polysulphide sealants. ## Answer #### Chalcogenide glasses **Chalcogenide glasses** are amorphous (non-crystalline) solids that contain one or more chalcogen elements (S, Se, Te; but not oxygen) combined with elements such as As, Ge, Sb, Ga or P. Examples: As₂S₃, As₂Se₃, GeSe₂, Ge–Sb–Te (GST), As–Se–Te. **Preparation:** the high-purity elements are weighed in the required ratio, sealed in an evacuated silica ampoule and melted at 600–1000 °C with rocking for a homogeneous melt. The melt is **rapidly quenched** (cooled quickly in air or water) so that it solidifies as a glass instead of crystallising. **Properties:** transparent in the infrared (about 1 to 15 µm), high refractive index, semiconducting, photosensitive, and can switch reversibly between amorphous and crystalline phases. **Engineering uses** - Infrared optics: lenses, windows, thermal-imaging and night-vision cameras. - IR optical fibres for sensing and laser power delivery. - Phase-change memory and rewritable optical discs (CD-RW, DVD-RAM) using GST alloys. - Photoreceptor drums of photocopiers (amorphous Se, As₂Se₃); photoresists and solar cells. - Switching devices, chemical sensors and waveguides. #### Network polymers of sulphur: preparation and uses **Plastic (polymeric) sulphur.** Ordinary rhombic sulphur has S₈ rings. When sulphur is heated above about 160 °C, the rings open and join into long chains (S)ₙ, and the liquid becomes very viscous. If this hot liquid (heated to near its boiling point) is poured into cold water, the chains are frozen as a brown, rubber-like, elastic solid called **plastic sulphur**. It slowly reverts to the S₈ form. S₈ → (heat above 160 °C) → ·S–S–S...S–S· (chain) → quench in water → plastic sulphur **Cross-linked sulphur polymers.** In vulcanisation, a small amount of sulphur forms –S– and –S–S– cross-links between rubber chains, making the rubber harder and stronger (used for tyres). Polysulphide rubber (**Thiokol**) is obtained from ethylene dichloride and sodium polysulphide: n ClCH₂CH₂Cl + n Na₂S₄ → [–CH₂CH₂–S₄–]ₙ + 2n NaCl. Cross-linking of sulphur chains into a network (for example with As or Ge, as in As₂S₃) gives chalcogenide glass networks. **Uses of sulphur polymers:** vulcanisation of rubber for tyres, Thiokol polysulphide rubber for sealants, fuel hoses and rocket binders, sulphur concrete and sulphur-based roofing and paving materials, and (SN)ₓ as a conducting polymer. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is plastic sulphur obtained? A. Cooling sulphur vapour slowly B. Heating sulphur above 160 C and pouring the hot liquid into cold water C. Dissolving sulphur in CS2 D. Burning sulphur in air Answer: B. S8 rings open into chains, and quenching in cold water freezes the chains as a rubber-like solid. (Key point: Plastic sulphur) 2. Why does liquid sulphur become very viscous above about 160 C? A. S8 rings open and join into long chains B. Sulphur oxidises to SO2 C. Sulphur crystallises D. Sulphur evaporates Answer: A. Ring opening and chain formation raise viscosity. (Key point: Viscosity change) 3. What is Thiokol made from? A. Phenol and formaldehyde B. Ethylene dichloride and sodium polysulphide C. Styrene and sulphur D. Silicon and CH3Cl Answer: B. ClCH2CH2Cl + Na2S4 gives [-CH2CH2-S4-]n + 2NaCl. (Key point: Thiokol) 4. Which step is vital in making chalcogenide glass? A. Slow annealing of a crystal B. Rapid quenching of the melt in a sealed evacuated ampoule C. Electrolysis D. Adding oxygen Answer: B. Elements are melted in an evacuated silica ampoule and rapidly quenched to avoid crystallising. 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