Influence of architecture on arm dimensions and interaction parameters in polybutadiene star polymers

Star branched polymers of polybutadiene with 3, 4, 8, and 12 arms have been synthesized with one arm perdeuterated. Small-angle neutron scattering dat a have been collected for each of the star polymers and for the four-arm st ar polymer mixed with hydrogenous linear polybutadiene of the same molecula r weight either as a single arm or of the whole star. The labeled four-arm star polymer has also been mixed with the equivalent hydrogenous star polyb utadiene. For the pure labeled star polymer the scattering is dominated by a peak located at a scattering vector determined by the radius of gyration of the arm. As the labeled star polymer is diluted by adding hydrogenous po lymer, the peak declines in amplitude and the maximum scattered intensity s hifts to zero scattering vector. The scattered intensity for each sample ha s been fitted with expressions based on random phase approximation theory a nd the radius of gyration of the labeled arm was obtained. Two interaction parameters were needed to obtain acceptable fits of the theoretical express ions to the data. Indisputable evidence for increased stretching of the lab eled arm as the number of arms in the star polymer increases has been obtai ned, and the stretching factor has been compared to predictions of a liquid state theory of star polymers. The interaction parameters obtained are con sidered to be reflections of the differential screening of core and corona segments of the star molecules, an integral aspect of the liquid state theo ry, rather than true Flory-Huggins-type interaction parameters.