Dynamic light scattering from the oriented lamellar state of diblock copolymers: The undulation mode

Using a fully oriented sample of a diblock copolymer, poly(dimethylsiloxane )-block-poly-(ethylene-co-propylene), we have observed, by dynamic light sc attering, the undulation mode of the block copolymer lamellae in the ordere d phase. This mode is observable only in a particular geometry where the sc attering vector is parallel to the lamellar structure. From the relaxation frequency of the undulation mode, the splay elastic modulus, K, of the lame llar structure is derived. It is shown that the elastic modulus K extrapola tes to zero near the order-to-disorder temperature where the translational long-range order disappears. The "fast" mode usually observed in block copo lymer melts on disoriented samples presumably corresponds to the undulation mode in an oriented sample, averaged over all possible orientations.