Investigation of the Microphase Separation Transition in Low Molecular Weight Diblock Copolymers

The microphase separation transition in three diblock copolymer systems is studied by differential scanning calorimetry and small-angle X-ray scattering analyses. The experimental absolute scattered intensity profiles for a symmetric poly(styrene-block-isoprene) in the homogeneous state compare quantitatively to the theoretical predictions of Leibler. The scattering profiles for homogeneous poly (styrene- block-1,2-butadiene) and poly (styrene-block-butene) copolymers exhibit a mismatch in the critical scattering vector, q, compared to the theoretical prediction. These two diblock copolymers are asymmetric as a result of a dissimilarity in the statistical segment lengths of the two components. The theoretical scattering profiles may be brought in agreement with experiment by empirically adjusting the copolymer chain dimensions assumed in the calculation. These comparisons are used to evaluate the interaction parameters and their temperature dependence for the three copolymer systems. In all three systems, the observed conditions for the apparent spinodal are found to be in good agreement with the predictions of Leibler's theory. © 1989, American Chemical Society. All rights reserved.