Excluded-Volume Effects on the Transport Coefficients of Oligo- and Poly(dimethylsiloxane)s in Dilute Solution

Abstract

The translational diffusion coefficient D and intrinsic viscosity [η] of oligo- and poly-(dimethylsiloxane)s (PDMS) were determined in toluene at 25.0 °C in the range of weight-average molecular weight M w from 1.08 x 10 3 to 1.12 x 10 6 for the former and from 5.33 x 10 2 to 1.12 x 10 6 for the latter. The hydrodynamic- and viscosity-radius expansion factors α H and α η were then determined from the values of the hydrodynamic radius R H defined from D and those of [η] corrected for the specific interaction between polymer and solvent molecules, respectively, with the use of the corresponding values of R H,0 and [η] 0 for the unperturbed PDMS chain in toluene at 25.0 °C. Here, the latter values were obtained by multiplying the previous values of R H,Θ and [η] Θ in bromocyclohexane at Θ by respective proper factors that take account of the solvent dependence of the bead diameter and may be calculated by the use of the helical wormlike (HW) chain theory. The results show that both α H and α η become functions only of the scaled excluded-volume parameter z defined in the Yamakawa-Stockmayer-Shimada theory for the HW chain with excluded volume, being consistent with the previous results for atactic polystyrene. This implies that the quasi-two-parameter scheme may be valid for a H and α η irrespective of the differences in polymer species and solvent condition and, moreover, that there is no draining effect on α H and α η , although it is significant for the unperturbed PDMS chain. It is again found that the Barrett equation overestimates α H . This disagreement between theory and experiment may be semiquantitatively explained by the Yamakawa-Yoshizaki theory, which takes account of the possible effect of fluctuating hydrodynamic interaction on α H .