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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
Silicate tetrahedron(SiO4)4
Vertex, Face and edge sharing
Silicate structures
CCVertex > CCedge > CCface
Si4+
O2
- 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
Non-crystalline (glass)
Crystalline
1Dchain
2DSheet
3Dnetwork
Upright tetrahedron
Inverted tetrahedron
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
- Pyrex 80% SiO2, 14% B2O3, 4% Na2O
- Window Glass 72% SiO2, 14% Na2O, 9% CaO, 4% MgO, 1% Al2O3
- Most organic polymers based on covalent bonds formed by carbon
- Covalent bonding polymers are good thermal and electrical insulators Inert
- mer → polymer (marocmolecule)
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
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
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
Polyester fibres arecharacterized by thelinkage E.g. Terylene
- Oxygen in backbone instead of C
- Provides flexibility → softening with temperature in spite of Hydrogen bonding
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
- 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
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
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
