Give the definitions and significance of following properties of polymer. Tensile strength, insulation resistance and refractive index.

1. Tensile strength

Definition: the maximum tensile (pulling) stress a polymer specimen can withstand before it breaks: tensile strength = maximum load / original cross-sectional area (N/m² or MPa).

Significance: it tells us whether a polymer can bear loads in fibres, ropes, films and structural parts. It rises with molecular weight, crystallinity, cross-linking and strong intermolecular forces such as H-bonds (e.g. nylon, Kevlar). It is used to select polymers for engineering uses.

2. Insulation resistance

Definition: the resistance offered by an insulating polymer to the flow of current between two conductors when a DC voltage is applied: R = V / I (in ohms or megohms). It combines volume and surface resistance.

Significance: it measures how good an electrical insulator the polymer is. High insulation resistance is needed for cable sheathing, switches, plugs and PCBs (PE, PVC, PTFE, bakelite). It falls with moisture absorption, temperature and impurities, which indicates how reliable the insulation will be.

3. Refractive index

Definition: the ratio of the speed of light in vacuum to its speed in the polymer, n = c / v. For most polymers n is about 1.3–1.7 (PMMA 1.49, PS 1.59, PC 1.58).

Significance: it decides how light bends and how much is reflected at the surface. It is important for designing lenses, spectacles, optical fibres (core of higher n than cladding), light guides, displays and transparent glazing. Matching refractive indices keeps polymer blends and composites transparent.