Effect of molecular weight on the refractive index increment of polystyrenes in solution

The variation of refractive index increment $\text{d}\text{n}\text{d}\text{c}$ with molecular weight has been studied using solutions of monodisperse polystyrenes (2000<M_w<4×10⁶) in benzene. The results are in good agreement with those obtained by several authors using other systems.We have shown that the linear relationship:

$\text{d}\text{n}\text{d}\text{c}\text{=}\text{d}\text{n}\text{d}\text{c}{}_{\mathrm{m}}\text{+}\text{K″}\text{Mn}$

where $\text{(}\text{d}\text{n}\text{d}\text{c)}{}_{\mathrm{m}}$ is the refractive index increment of the repeating unit and K″ a constant, holds only for low molecular weights. With the assumption of volume additivity, we have estimated quantitatively the linear portion of the curve, $\text{d}\text{n}\text{d}\text{c}\text{=f(}\text{1}\text{M}\text{)}$, observed for molecular weights below 20 000 as a function of polymer composition. Beyond this limit, the increase of $\text{d}\text{n}\text{d}\text{c}$ with molecular weight may be probably related to segment-segment interactions within the coil.