A small-angle scattering study of the bulk structure of a symmetric diblock copolymer system

The bulk structure of a homologous series of symmetric polystyrene-polybutadiene (SB) diblock copolymers is investigated using small-angle X-ray and neutron scattering (SANS). The study focuses on the lamellar thickness, the lamellar correlation length and the concentration profile as a function of the chain length and the preparation method applied. The characteristic length, D, scales with the chain length, N, in the whole range studied, but with a clear change in scaling exponent near chi N = 29, in accordance with theoretical predictions of a crossover from an Intermediate-Segregation Regime (ISR) to the Strong-Segregation Limit (SSL). In the ISR (chi N similar or equal to 5 - 29), D is found to scale like D proportional to N-0.83 and in the SSL (chi N > 29) like D proportional to N-0.61. The temperature dependence of the SANS spectra is studied for a low molar mass sample in an interval around the order-disorder transition temperature (T-ODT) The peak position is found to vary more strongly with temperature than expected for Gaussian chains. Only a weak discontinuity of the peak position at T-ODT is observed. In summary, the phase behavior of symmetric SB diblock copolymers in the bulk spans three regimes: the Gaussian regime in the region chi N < 5, the ISR for 5 < chi N < 29 and the SSL for chi N > 29.