Onset of Excluded-Volume Effects in Chain Dynamics

The infinite-dilution oscillatory flow birefringence properties of five high molecular weight polybutadienes (M= 3.55 × 10⁵, 9.25 ×10⁵, 2.95 × 10⁶, 3.74 × 10⁶, and 1.6 × 10⁷) have been determined in dioctyl phthalate at 18.0, 21.5,25.0, and 30.0 °C. The θ temperature for this system has been estimated as 21.5 °C, and thus the results extend from the θ into the marginal solvent regime. For each temperature, the data for the five molecular weights are superposed to form master curves. The resulting plots are equivalent, via the stress-optical relation, to plots of the reduced intrinsic shear moduli, [G′p]_R and [G″p]_R, versus reduced frequency, ωτ₁, where τ₁ is the longest relaxation time. For 18 and 21.5 °C,the master curves are precisely described by the bead-spring model (BSM) with the hydrodynamic interaction parameter, h*,equal to 0.25. At 30 °C, however,effects attributable to excluded volume are apparent. At this temperature, the data are compared with five approximate but distinct approaches to incorporating excluded volume: the Gaussian BSM with variable h*; dynamic scaling; the non-Gaussian BSM with h* equal to 0.25 and incorporating chain expansion via the uniform expansion model, the blob model, or the renormalization group. All five methods can be used to describe the frequency dependence of the reduced intrinsic moduli very well but differ in their ability to describe the chain expansion concurrently. In particular, the renormalization group approach is as good or better than the other models, and thus the success of the BSM coupled with the renormalization group establishes an effective theoretical framework for describing quantitatively both the static and dynamic properties of isolated flexible chains.