One-Dimensional Crystallization of Siloxane-Based Block Molecules

The self-assembly of block copolymers (BCPs) and liquid crystals (LCs) as a bottom-up approach towards well-defined nanostructures has been extensively studied for applications in nanotechnology. Industry demands for faster and smaller devices and therefore a continued decrease in feature size of supramolecular nanostructures is required. A new class of molecules has become of great interest to further explore the miniaturization of feature sizes. These molecules are block molecules in which “hard” aromatic blocks are covalently connected to a “soft” flexible polymer chain of discrete length. Such well-defined block molecules form perfectly organized nanostructures as a result of the phase-separation combined with intermolecular, crystalline interactions. Recently, monodisperse oligo(dimethylsiloxane) (oDMS) based triblock molecules were studied in our group. As a result of the discrete design of these molecules, they formed very small lamellar structures with feature sizes down to 2.2 nm. For these lamellae forming molecules, the crystalline end-groups appear to determine the morphology, like in LCs. In this work, we add another oDMS-based block molecule to the library in order to really distinguish between phase-separation and crystallization of these type of block molecules. For this, we synthesize dinitro-biphenylhydrazone-oDMS-dinitro-biphenylhydrazone (hydz-oDMS-hydz) triblock molecules with an oDMS spacer length of 8, 16, 24 or 40 repeating units. In contrast to the previous reported block molecules of this type, these hydz-oDMS-hydz show extremely fast melting and crystallization behavior with almost no super cooling. This is indicative for a nucleation controlled mechanism. Furthermore, these hydz-oDMS-hydz block molecules prefer to form cylinders instead of lamellar nanostructures. In these long-range ordered cylinders, the hydrazones crystallize and form a one-dimensional crystal within a siloxane matrix. Therefore, we can conclude that there is a strong cooperation between crystallization and phase-separation and they work together to create these molecules’ ideal morphology.