Diffusion coefficients and viscosities of linear atactic polystyrene (PS) with different molecular weight ($M_w = 5.75×10^5$, $9.0×10^5$, and $1.61×10^6$) are measured to examine the dynamic and mechanical properties of linear flexible polymers in a tetrahydrofuran (THF). The diffusion coefficients are measured by using the apparatus of a dynamic light scattering (DLS), and the viscosities by using a Haake viscometer for high shear rates and capillary viscometer for low shear rate.
It is verified that the THF is a good solvent for the PS because of the satisfaction of Huggins coefficient (0.32-0.37). Molecular weight distribution (MWD) analyzed by the histogram method from the diffusion coefficients is nearly consistent with gel permeation chromatography (GPC) data, and those results show that the PS samples have Wesslau distribution as well known.
In infinite dilute solutions, the hydrodynamic radius ($R_H$) is equal to the radius of gyration ($R_G$) according to the Stokes-Einstein hard sphere model, but the DLS data show that the $R_H$ is much smaller than the $R_G(R_G/R_H=1.7-1.9)$.
De Gennes suggested that the transition from the motion of one isolated polymer chain to that of blobs might be occurred at the overlap concentration[$C^*$]. However, the transitional concentration obtained from the DLS experiment, [$C^D$], appears over the $C^*$. In other hand, the transitional concentration obtained from the zero-shear viscosity ($η_o$) measurement, [$C^V$], is consistent with the entanglement concentration[$C^{**}$] predicted by Klein. Over the transition concentration, the zero-shear viscosity is shown as $η_0 ∝ C^{4.86}$.
As a result, it is concluded that the molecular model proposed by Klein, which incorporates the constraint release on a reptating chain, is more plausible than that by de Gennes.
