Small-angle neutron scattering from single arm labeled polybutadiene star polymers in dilute solution

Dilute cyclohexane solutions of polybutadiene star polymers with 3, 4, 8, a nd 12 arms wherein one arm is perdeuterated have been investigated by small -angle neutron scattering. The scattering cross sections obtained showed a characteristic peak at finite values of the scattering vector, Q. This maxi mum is not a structure factor arising from intermolecular interactions but is intrinsic to the scattering law of star molecules with one arm of a diff erent isotopic composition to the other arms. The theoretical scattering la w developed for such isotopic copolymers was found to fit the observed scat tering well and provided values of the radius of gyration of the labeled ar m. No dependence of the radius of gyration on star polymer concentration wa s noted, but the radius of gyration of the labeled arm increased as the fun ctionality of the star increased from 3 to 12, the increase in dimensions i n the 12-arm star being ca. 25% greater than the radius of gyration obtaine d for the three-arm star. In these dilute solutions the radii of gyration w ere larger than obtained for the bulk melt state of the stars. The data hav e been compared with theoretical predictions, but generally a much stronger dependence of the radius of gyration on star functionality is observed tha n is predicted.