J. Phys. II France
Volume 7, Numéro 12, December 1997
Page(s) 1829 - 1854
DOI: 10.1051/jp2:1997217
J. Phys. II France 7 (1997) 1829-1854

A Small-Angle Scattering Study of the Bulk Structure of a Symmetric Diblock Copolymer System

Christine M. Papadakis1, Kristoffer Almdal2, Kell Mortensen2 and Dorthe Posselt1

1  IMFUFA (Institute of Mathematics and Physics), Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark
2  Condensed Matter Physics and Chemistry Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark

(Received 24 April 1997, received in final form 5 August 1997, accepted 1 September 1997)

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 ( $\chi N \simeq 5-29$), D is found to scale like $D \propto N^{0.83}$ and in the SSL ( $\chi N > 29$) like $D \propto N^{0.61}$ . 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 $_{\rm ODT}$). 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 $_{\rm ODT}$ 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 $\chi N > 29$.

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