Surface-induced ordering and continuous breaking of translational symmetry in conjugated polymers
Surface-induced liquid crystalline phase transitions evoke fundamental interest and can provide deeper insight into the nature of low-dimensional phases. Board-like conjugated polymers are of particular interest because they exhibit novel sanidic liquid crystalline mesophases that have not been widely studied. Furthermore, films of these polymers often exhibit preferential molecular orientation near the free surface, suggesting that the surface influences the ordering process. However, the underlying mechanism of this surface-induced ordering remains unclear. Here, we use surface-sensitive grazing-incidence X-ray scattering to monitor the formation and disappearance of positional order in situ in thin films of two board-like conjugated polymers representative of two classes: polythiophenes and polydiketopyrrolopyrroles. The results show that the free surface induces the formation of a highly oriented, sanidic disordered mesophase at the surface of both conjugated polymers, thus marking the emergence of positional order. This ordering process continues upon cooling and involves multiple transitions into more ordered liquid crystalline mesophases, both at the surface and in the bulk. The positional smectic-like order parameter exhibits continuous temperature dependence near the transition, signifying that the breaking of translational symmetry by the surface occurs continuously. Theoretical analysis allows us to accurately describe the temperature-dependent order parameter when critical behavior is considered.