steric factor in polymers

The ratio, $σ$ , of the root-mean-square end-to-end distance of a polymer chain with unperturbed dimensions, $< r 2 > 0 1 2$ , to that of a freely rotating chain with the same structure, $< r 2 > 0 , f 1 2$ , i.e. $< r 2 > 0 < r 2 > 0 , f$ , in the limit of infinite chain length. The steric factor reflects the effect of hindrance to free rotation.

Source:

Purple Book, p. 50