Abstract
Block copolymers (bcps) constitute a fascinating class of soft materials which have received considerable attention. The remarkable features of bcps lie in their capability in self-assembling to form a variety of long-range ordered nanostructures and serving as the model system to test the fundamental theories of soft matter physics. Using small angle X-ray scattering (SAXS), here we reveal a new characteristic of the diblock copolymer exhibiting lower critical ordering transition (LCOT), where the amplitude of the periodic composition variation of the lamellar structure formed by poly(ethylene oxide)-block-poly(4-vinylpyridine) (PEO-b-P4VP) was found to depend strongly on temperature. In the cooling process, the primary scattering peak diminished quickly with decreasing temperature, while the intensity of the second-order peak remained largely unperturbed. Construction of the electron density profiles along the lamellar normal from the SAXS curves indicated that the lamellar structure at high temperature (e.g. 200 oC) can be described by a two-phase model with diffuse boundary. As the effective interaction parameter decreased on cooling, the long-range order of the lamellar structure was still well preserved, signaling that the junction points were still highly localized at the interfacial region. The reduction of the primary peak intensity was due to the decrease of the electron density contrast between PEO-rich and P4VP-rich domains arising from the mixing of PEO and P4VP blocks. The fact that the junction points were always spatially constrained implied that the mixing between the block chains occurred via chain flipping or exchange. The constraint of the junction points and the chain flipping were evidenced by the presence of a density dip in the electron density profiles at the lower temperature; in this case, the lamellar structure is described by a three-phase model.