Polymer Diffusion in Linear Matrices: Polystyrene in Toluene

The translational diffusion coefficient of polystyrene, labeled with azobenzene derivatives and having a molecular weight range 10⁴ ≤ M ≤1.8 × 10⁶, has been studied in matrices of the same polymer in toluene with the technique of forced Rayleigh scattering. The independent variables were the matrix concentration C and molecular weight M of the labeled (tracer) polymer, over a wide range of the matrix molecular weight 10⁴ ≤ M_p ≤ 8.4 × 10⁶. The diffusion coefficient of the tracer polymer D_trat 20 °C is found to be sensitive to M_p until M_p≥ (3-5)M; when M_pexceeds this limit, D_trno longer depends on M_pand we call D_trat such a limit the asymptotic tracer diffusion coefficient D_tr ^∞. In the context of the reptation model, the M_pdependence of D_tris interpreted as evidence for a substantial extent of the tube renewal process in semidilute solutions. At the high ends of M (~(1–2) × 10⁶) and C (~ 10–20 wt %), D_tr ^∞ ∞ M^-3C^-3 is a better representation of the data than D_tr ^∞ ∞ M^-2C^-1,75. Further, D_tr ^∞is shown to scale with (C/C*) in semidilute solutions for 3.5 × 10⁴ ≤ M ≤ 1.8 × 10⁶.