Interface densification in a microphase-separated diblock copolymer resolved by small-angle X-ray scattering

In block copolymers exhibiting lower critical ordering transition behavior, the segregation strength increases with temperature due to the significantly different thermal expansivities of the constituent blocks. Using synchrotron small-angle X-ray scattering, we reveal a phenomenon of segmental densification at the microdomain interface in this type of system, where the local electron density in a specific region of the interface surpasses that of the denser domain. As a result, the interfacial electron density profile deviates significantly from the hyperbolic tangent function expected for an incompressible melt. This unique densification effect can be attributed to the negative volume of mixing at the interface between the dissimilar segments and emerges when the electron densities of the constituent blocks are similar.