Solutions Of Stars Based On C-60. Structural Behavior As Revealed By Small Angle Neutron Scattering

The conformation in dilute solution and the structural behavior in semidilute solutions of polystyrene-substituted fullerene stars (StH) have been investigated using small-angle neutron scattering (SANS). Six-arm stars of low polydispersity and well-defined functionality (f = 6) have been synthesized by anionic polymerization with molecular weights covering nearly four decades (7600 < M-w,M-star < 2040000 g/mol). Stars with selectively H/D-labeled sequences linked to the fullerene core (StHD) have also been synthesized. SANS measurements have been performed in a wide range of scattering vectors (0.002 < q (A(-1)) < 0.2). The single star scattering form factor P(q) has been obtained from the scattering of highly dilute solutions by extrapolation at zero concentration. In the Guinier range of q, the measured molecular weight dependence of the radius of gyration R-g is found in close agreement with the results appearing in the literature on stars of different chemical structures and functionality. The form factor has been analyzed in the framework of scaling theories. Using the standard representation P(q) as a function of qR(g,) a good agreement has been observed in the Guinier and in the asymptotic ranges of q, but a significant discrepancy is noticed in the intermediate q range. The deviation is more and more pronounced as the arm length of the stars diminishes. That shows that, for stars with such a low functionality (six arms) the segment density profile suggested by the scaling theories is not really established. An identical behavior of the form factor is observed for selectively core-labeled stars in a solvent, matching the contrast of the outer sequences. Thus, it is shown that the variation of the arm length has no significant effect on the conformation of the arm segments attached to the core. A detailed analysis of the form factor of low molecular stars suggests that the scattering results from a shell of closely crowded extended chains. By matching the contrast of the outer arm sequences with the solvent, the structure factor S-int(q) of selectively H/D-labeled stars has been studied in the case of volume fractions below and above the overlap concentration C*. For stars with short arms, local ordering has been observed for low volume fractions (below C*), and the increase in concentration results in a more liquid-like inter-star spatial distribution. For stars with longer arms, a liquid-like spatial distribution is observed, whatever the concentration.