How dendrons stiffen polymer chains:: A SANS study

The conformation of various polystyrene chains with first (G-1), second (G-2), and third generation (G-3) Frechet-type dendrons at the repeat unit has been investigated with small-angle neutron scattering. The increased density of the attached dendrons leads to a systematically greater cross-sectional chain diameter D. Bulky, high generation dendrons force the polymer backbone out of its all-trans conformation. The measured statistical Kuhn segment length initially increases in proportion to the chain diameter and then to a greater degree due to steric overcrowding and the concomitantly higher bending rigidity. The introduction of charges further leads to chain expansion and the development of interchain correlations. High molecular weight (G-2) chains develop fully excluded-volume chain properties with a Flory exponent of upsilon = 0.57 and a critical exponent gamma = 0.86 which is related to the enhancement of chain configurations with widely separated chain ends.