Random coil dimensions of poly(diethyl siloxane)

A sample of high molecular weight poly(diethyl siloxane), [Si(C₂H₅)₂O]_x, (PDES) was separated into a series of fractions by means of liquid-liquid precipitations. Four of the fractions were studied by osmometry and viscometry in toluene at 25°C. The resulting values of the number-average molecular weight, second virial coefficient, and intrinsic viscosity indicate that the unperturbed dimensions of PDES are essentially the same as those of poly(dimethyl siloxane) (PDMS) of the same chain length. Thus, differences in intramolecular interactions between these two chains have little effect on their unperturbed dimensions. Most pertinent in this regard are interactions between the side chains, which are favourable in trans conformations in both PDES and PDMS. These favourable interactions in trans conformations would be enhanced in the case of the ethyl groups in PDES, but this effect is apparently offset by a comparable increase in favourable interactions between ethyl groups and oxygen atoms in the competing gauche conformations.