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Structure of Silica and Silicates

Structure of Silica and Silicates Silicate tetrahedron (SiO 4 ) 4  Vertex, Face and edge sharing Silicate structures CC Vertex > CC edge > CC face Si 4+ O 2  Silicate tetrahedron (SiO 4 ) 4 

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Structure of Silica and Silicates

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  1. Structure of Silica and Silicates Silicate tetrahedron(SiO4)4 Vertex, Face and edge sharing Silicate structures CCVertex > CCedge > CCface Si4+ O2

  2. Silicate tetrahedron (SiO4)4 • The excess (4) charge should be neutralized by the formation of primary bonds with other units or other cations • Due to the high cation-cation repulsion silica (SiO2) cannot form a close packed structure of oxygen anions with cations in tetrahedral voids • Effective number of silicon cation / unit = 1 Oxygen anion /unit = 2 (vertex sharing) • This arrangement maintains electrical neutrality of the structure

  3. Silica Non-crystalline (glass) Crystalline 1Dchain 2DSheet 3Dnetwork Upright tetrahedron Inverted tetrahedron

  4. Oxides can be added to silica to obtain a number of crystalline and non-crystalline silicates • Silica + Na2O + CaO = Soda lime glass►Alkali cations breakup the network of silicate tetrahedra ►The sodium ions stay close to the disrupted corner- electrostatic ►The network at the corner is bonded via Na-O bonds ►Na-O bond weaker →  in viscocity

  5. Vycor 96% SiO2, 4% B2O3 • Pyrex 80% SiO2, 14% B2O3, 4% Na2O • Window Glass 72% SiO2, 14% Na2O, 9% CaO, 4% MgO, 1% Al2O3

  6. POLYMERS • Most organic polymers based on covalent bonds formed by carbon • Covalent bonding  polymers are good thermal and electrical insulators  Inert • mer → polymer (marocmolecule)

  7. Tetrahedral bonding of sp3 hybridized C

  8. Degree of polymerization (D.P) → Number of repeating monomers in the chain • Molecular weight of chain = (M.W. of mer) x D.P. or • D.P. = M.W. chain / M..W. of mer • Typical M.W. of chain ~ 104 – 106 R1 R2 R3 R4

  9. Ethylene based long chain polymers

  10. POLYMERS Thermosets Thermoplasts • Increased plasticity with increasing temperature • Long chain molecules held together by secondary bonds • Low M.P. • E.g. Polyethylene, PVC, PTFE, PP, PMMA • 3D network of primary bonds • Hard and rigid • Do not soften on heating (become harder due to completionof left over polymerization reaction) • Degrade before reaching M.P. due to reaction with atmospheric oxygen • E.g. Bakelite, Melamine, Araldite, Aropol Plastics Randomly oriented molecules Fibres High alignment of molecules along fibre direction Elastomers Rubbery behaviour

  11. Other polymers not based on ethylene derivatives Polyamide fibres arecharacterized by thelinkage E.g. Nylon (6,6) NH(CH2)6NH . CO(CH2)4CO  6 Carbon atoms in the backbone of the chain between two NH groups Oxygen on the side group bonds with Hydrogen of an adjacent chain • Hydrogen bonds (stronger than Van der Walls bonds)  greater resistance to softening on increasing Temperature

  12. Polyester fibres arecharacterized by thelinkage E.g. Terylene • Oxygen in backbone instead of C • Provides flexibility → softening with temperature in spite of Hydrogen bonding

  13. Wood: main constituent is the cellulose chain Alignment of the long chains gives wood its directional properties: the elastic Modulus and tensile strength are 10-20 times more in the longitudinal directionas compared to the transverse direction

  14. ELASTOMERS • Using the example of RUBBER elastomers are illustrated • Monomer is the isoprene molecule • Both the side chains are on the same side of the molecule  tendency for bending and coiling ► Rubbery behaviour • Chain segments have translational mobility at room temperature • Long chain polymers • Very few cross-links • Raw rubber heater with sulphur (Vulcanization)→ Sulphur forms covalent bonds with Carbon → additional links between chains (cross-links) → increased stiffness of rubber • More vulcanization  more double bonds used up by sulphur bridges → e.g. ebonite is hard and brittle

  15. Polymers Non-crystalline Semi-crystalline Crystalline

  16. Can grow 1xal of polymers → folded chain structure E.g. Polyethylene has Orthorhombic unit cell • Chains can be aligned to some extent by mechanical working • Density increases with increasing alignment of chains Low density polyethylene (specific gravity 0.92) → 50 % crystalline High density polyethylene (specific gravity 0.97) → 80 % crystalline • Crsytallinity promoted by formation of hydrogen bonds instead of weak Van der Walls bonds → nylon and cellulose crystallize due to hydrogen bonds

  17. Crystallinity of long chain polymers • Long chain polymers are usually non-crystalline / semi-crystalline • Factors against crystallization ► Long chain ► Branching ► Random arrangement of side groups (large)► Polymerization of two (or more) different monomers (copolymers) ► Presence of plasticizers (additives which impede chains from coming together) Side group arrangement Atactic Random arrangement of bulky side groups Isotactic bulky side groups on same side Syndiotactic Alternate arrangement of bulky side groups

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