Polymer Dimensions in Good Solvents: Crossover from Semidilute to Concentrated Solutions

Using small-angle neutron scattering, we studied the variation of the polymer radius of gyration (R(g)) as a function of polymer concentration (phi) for solutions of a flexible-chain poly(methyl methacrylate) in chloroform. We observed for the first time a distinct crossover between swollen coils in the semidilute regime, where R(g)(2)proportional to phi(-0.26 +/- 0.03), and unperturbed coils in the concentrated regime, where R(g) is independent on concentration. The crossover occurs at phi(>)approximate to 0.15, a value that agrees reasonably well with phi approximate to 0.21 +/- 0.035, estimated with a scaling relationship between phi and the coil overlap concentration phi(*).