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Neutron coherent scattering techniques have been used for the determination of the conformation of polymer in bulk and experimental details are given about the application of this method to the study of polymeric systems. Measurements have been made for small and intermediate momentum ranges on a series of eight monodisperse deuterated polystyrenes of molecular weight ranging from 21,000 to 1,100,000. The results lead to the concluson that in amorphous state the conformation of the polymer molecule is indistinguishable from that in solvent and that the Debye scattering function which is valid for unperturbed chains applies for q ~1 as low as 10 Á.

We discuss theoretically and numerically the intramolecular form factor F (q) in dense polymer systems. Following Flory's ideality hypothesis, chains in the melt adopt Gaussian configurations and their form factor is supposed to be given by Debye's formula. At striking variance to this, we obtain noticeable (up to 20%) non-monotonic deviations which can be traced back to the incompressibility of dense polymer solutions beyond a local scale. The Kratky plot (q 2 F (q) vs. wavevector q) does not exhibit the plateau expected for Gaussian chains in the intermediate q-range. One rather finds a significant decrease according to the correction δ(F −1 (q)) = q 3 /32ρ that only depends on the concentration ρ of the solution, but neither on the persistence length or the interaction strength. The non-analyticity of the above q 3 correction is linked to the existence of long-range correlations for collective density fluctuations that survive screening. Finite-chain size effects are found to decay with chain length N as 1/ √ N .