Backbone stretching of wormlike carbosilane dendrimers

Wormlike dendrimers made of flexible and noninteracting segments were synthesized. Starting from a poly(methylhydrosiloxane) G(0) and using short propylsilane branches, the synthesis did not go beyond the second generation, as predicted from steric congestion. The starting polymer and the G(1) and G(2) dendrimers synthesized were studied by small-angle neutron scattering. The molecular-weight dependence of their radius of gyration (R-g proportional to M-nu, nu growing from 0.53 for G(0) to 0.94 for G(2)) showed the backbone conformation to go from very flexible for G(0) to nearly rodlike for G(2). This was supported by the growth of the persistence length from 12 to over 200 Angstrom, as deduced from an analysis of the data according to Benoit-Doty's law. The dendrimers being made of flexible parts, their stretching was attributed to the congestion of the peripheral branches. The absence of liquid crystallinity was imputed to the dynamical flexibility of the molecules.