Effect of Side Groups on the Conformation of a Series of Polysiloxanes in Solution

Abstract

The molecular conformation has been obtained for polysiloxane chains in which the pairs of substituents bonded at the silicon atom are CH3, CH3; C2H5, C2H5; CH3, C6H13; CH3, C16H33; and CH3, C6H5. Polysiloxanes dissolved in toluene and benzene were studied using a coupled system: Gel Permeation Chromatography/Light Scattering (GPC/LS). Attention was focused on the influence in the molecular conformation of the quality of the solvent and the type of substituent on the main chain. The results obtained were strongly dependent on the side group size and flexibility. Poly(dimethylsiloxane) samples of 273,400 and 97,600 daltons showed a semi-flexible conformation, but the sample of relatively low molecular weight (36×103 daltons) exhibited a slope of 0.6 in good solvents; i.e., demonstrated the behavior of dilute (non-overlapping) coils in a good solvent. With respect to the poly(diethylsiloxane), the source of the coil's expansion is the high mobility of the side chains. Poly(methylphenylsiloxane) exhibits a rigid rod conformation in both solvents. The orientation of the phenyl groups and the attractive interaction between these groups dominate the polysiloxane chain behavior in good solvents. For these polysiloxanes, the expansion factor value is analyzed and the influence of solvent on the polymer's unperturbed dimensions is discussed. Poly(methylhexylsiloxane) and poly(methylhexadecylsiloxane) show a spherical conformation in both solvents. This conformation may arise from strong interactions between the bulky side groups and the main chain.