Explain following properties of polymer a) Electrical b) Optical.

a) Electrical properties – most polymers are good insulators because they have no free electrons or ions.

• Dielectric strength: the maximum electric field (voltage per unit thickness, kV/mm) that a polymer can withstand without breaking down electrically. High dielectric strength is needed for cable insulation and capacitor films. Examples: PE, PTFE, PVC.
• Insulation resistance: the resistance to current flow offered by the insulating material between two conductors. It is made up of volume resistance and surface resistance, and it falls with moisture, temperature and impurities.
• Surface resistivity: the resistance to leakage current along the surface of a polymer, in ohms per square. Polymers with high surface resistivity build up static charge, so antistatic additives are added for some uses.
• Polar polymers (PVC, nylon) have higher dielectric constants and losses than non-polar polymers (PE, PTFE, PS). Conducting polymers such as doped polyacetylene and polyaniline are exceptions.

b) Optical properties

• Refractive index (n = c/v): how much light bends on entering the polymer, typically 1.3–1.7. Examples: PMMA ≈ 1.49, polycarbonate ≈ 1.58. It is important for lenses and optical fibres.
• Transmittance: the fraction of incident light that passes through. Amorphous polymers (PMMA, PS, PC) are transparent, with up to about 92% transmittance. Crystalline polymers scatter light at crystallite boundaries and are translucent or opaque. Uses: glazing, lenses, displays.
• Photoelectric property: some polymers (e.g. conjugated polymers such as polyvinylcarbazole) produce charge carriers or a current when light falls on them. They are used in organic solar cells, photocopiers and photodetectors.
• Colour: it depends on light absorption. Most polymers are colourless, and colour comes from pigments, dyes or conjugated structures. It is important for appearance and for UV stability.