Mark-Houwink-Kuhn-Sakurada exponent at the Theta condition. Its invariancy with respect to the cross-sectional dimensions of polymer chains

The question raised by Fujita has been examined, namely why the intrinsic viscosity of many polymers at the Theta condition [eta](0) is proportional to M(1/2) over a broad range of molecular weights. An explanation is proposed, based on a combination of the theory of the wormlike cylinder model with reported data on cross-sectional dimensions of 30 polymers with the carbon backbone differing in both the size and type of side groups (vinyl polymers, polyacrylates, polymethacrylates). The behaviors of poly(dimethylsiloxane) and poly(ethylene oxide), which are not in line with this explanation, are briefly discussed.