TRANSPORT-COEFFICIENTS OF OLIGO(DIMETHYLSILOXANE)S AND POLY(DIMETHYLSILOXANE)S IN DILUTE-SOLUTIONS - THE DRAINING EFFECT

The translational diffusion coefficient D and intrinsic viscosity [eta] of oligo- and poly-(dimethylsiloxane)s (PDMS) were examined, especially regarding their molecular-weight dependence, in the range of weight-average molecular weight M(w) from 2.37 X 10(2) to 1. 12 x 10(6) in bromocyclohexane at 29.5-degrees-C (0). A study was also made of [eta] in methyl ethyl ketone at 20.0-degrees-C (theta). It is found that DM(W)1/2 decreases and [eta]/M(W)1/2 increases with increasing M(W) and none of them becomes a constant independent of M(W) even for such large M(W) that the ratio of the mean-square radius of gyration to M(W) is independent of M(W). This anomalous behavior may be considered the so-called ''draining effect''. This is believed to be the first observation of the effect for a long flexible polymer. For the oligomers of very low M(W), [eta] in bromocyclohexane becomes negative as in the case of polyisobutylene in isoamyl isovalerate and in benzene studied previously. Thus the data for [eta] were treated following the procedure proposed therein. The results for both D and [eta] may be almost quantitatively explained by the transport theory of the helical wormlike touched-bead model with values of the model parameters unambiguously determined. From the model parameters obtained, the PDMS chain is found to assume the local conformation with retention of large helical portions in dilute solution as in the case of the poly(methyl methacrylate) chain. The value of the static stiffness parameter lambda-1 is almost the same as that of atactic polystyrene, so that the PDMS chain may be regarded as stiffer than usually considered. The value of the hydrodynamic diameter d(b) of the bead is found to be appreciably small compared to those for the other flexible polymers studied previously. A rather detailed discussion of these results is given.