Power laws in polymer solution dynamics

The dynamical screening length ξ_h in semidilute to highly concentrated polymer solutions of poly(methyl methacrylate) in propylene carbonate has been examined using photon correlation spectroscopy and pulsed field gradient nuclear magnetic resonance. A crossover between different concentration dependent regimes, ξ_h∼φ^-α where α is found to be ≈0.5, ≈1, and ≈2, is observed when the local viscosity is taken into account. Here φ is the volume fraction of polymer in the solution. Well-defined crossovers between α=0.5 and α=1 corresponding to a transition from a marginal solvent to a θ solvent behavior have been predicted to occur due to the reduction of excluded-volume effects between the spatially correlated polymer segments with increasing polymer volume fraction. However, a clear experimental validation of the crossover has never been presented before. The third regime (α≈2) is observed in the highly concentrated region where the static screening length is comparable to the persistence length of the polymer. The observation indicates that the rigid rod model previously used to describe concentrated solutions is an oversimplification valid only in the very high concentration limit. The obtained results at high concentrations are discussed in the frame of a simple physical model where segments at the persistence length scale are treated as flexible rodlike segments.