Steric Constraints in Fractal-Regime Star Polymers

Star polymers at high functionality, f, and high arm length, z$_{arm}$, display a collapsed core structure described by Daoud and Cotton in a colloidal regime (CR). At lower functionality (f$<\sim $8) and relatively low arm length, stars display a polymeric structure in a fractal regime (FR). For FR stars in good solvents the arms display steric interactions analogous to polymer chains tethered to a surface. We have used small-angle neutron scattering to quantify, for the first time, this steric interaction so as to understand the approach to the CR as a function of z$_{arm}$ and f as well as temperature and solvent type. Experimental data from model star polymers and literature data from polyurethane stars are considered as examples.