Diffusion by Constraint Release in Branched Macromolecular Matrices

Tracer diffusion coefficients of linear, deuterated polystyrene chains in three-arm symmetric and asymmetric star-branched polystyrene matrices were measured by forward recoil spectrometry. The stars are molecules that have a single branch extending from a longer linear segment. Increases in the diffusion coefficients for the lower matrix molecular weights were used to extract τ_w, the lifetime of a lateral constraint on a diffusing molecule. Comparison of τ_w for linear and branched matrices indicates that τ_w depends on the molecular weight of the longest linear span of the matrix chains and not on the total matrix molecular weight. These results indicate that polymer chain branching does not affect the relaxation of a polymer melt for branch molecular weights up to at least 4 times the entanglement molecular weight.