The structure of polysiloxanes: Molecular size and shape of highly crosslinked polymethylsiloxanes in solution

Measurements of solution light scattering and solution viscosity have been made in ideal solvents on polymethylsiloxanes containing large percentages of crosslinking units. For the 1.5 Me/Si and 1.8 Me/Si polymers [η] = 0.326 M0.21 and [η] = 0.240 M0.28, respectively. Densities of the polymer molecules within their spheres of action have been calculated from the viscosity relationship of Flory. They are 0.10 and 0.05 g./cc. for the 1.5 and 1.8 Me/Si resins, respectively. These are to be compared with 0.007 g./cc. for a linear polydimethylsiloxane (Me/Si = 2.0). The high densities, the low viscosities, and the low viscosity exponents lead to the conclusion that these molecules are, to good approximation, spheres of constant density, a sort of micromicrogel.