Effect of molecular weight on the refractive increment of polyethylene and n-alkanes

As in the case with other polymers previously reported, the values of the refractive index increment dn/dc of polyethylene and the n-alkanes change with molecular weight. Most of the variation may be understood by examination of the role of density in the Lorentz-Lorenz mixing equation for specific refractivity, R₁₂ = p₁R₁ + P₂R₂ used to calculate dn/dc. It may also be shown that as the absolute refractive index difference between solute and solvent becomes smaller, dn/dc becomes more sensitive to density change of the solute.