Answer: Tensile strength is the maximum stretching stress a polymer can bear before breaking; insulation resistance is its resistance to current leakage between conductors; refractive index is \(c/v\) for light in the polymer. They decide the use of polymers in load-bearing, electrical and optical parts.
1. Tensile strength
- Definition: the maximum tensile (pulling) stress a polymer specimen can withstand before it breaks: \(\text{TS}=\dfrac{\text{maximum load}}{\text{original cross-sectional area}}\), in N/m² or MPa. Measured on a universal testing machine with a dumb-bell shaped specimen.
- Significance: tells whether a polymer can be used for load-bearing parts such as ropes, fibres, gears, pipes. It increases with chain length (molecular weight), crystallinity, cross-linking and strong intermolecular forces (e.g. H-bonding in nylon), and falls with temperature and plasticiser content.
2. Insulation resistance
- Definition: the resistance offered by a polymer to the flow of leakage current between two conductors (electrodes) separated by it, when a DC voltage is applied. It includes volume resistance (through the bulk) and surface resistance (along the surface); the related property is volume resistivity in Ω m.
- Significance: most polymers (PE, PVC, PTFE) have very high resistivity, about \(10^{12}\)–\(10^{16}\) Ω m, so they are used for cable sheathing, switches, plugs and PCB bases. It decreases with moisture, impurities, surface dirt and temperature; non-polar polymers are the best insulators.
3. Refractive index
- Definition: the ratio of the speed of light in vacuum to its speed in the polymer, \(n=c/v\). E.g. PMMA about 1.49, polystyrene about 1.59, polycarbonate about 1.58.
- Significance: decides how a polymer bends and transmits light; transparent amorphous polymers with suitable \(n\) are used for lenses, spectacle glasses, optical fibres, light guides and display covers. Crystalline regions scatter light (opacity), so optical polymers are mostly amorphous.
