Design, synthesis and properties of molecular composites of rod‐like polymers

Processing and performance of fiber reinforced polymers suffer from problems related to the heterogeneous nature of the composites. The strong impact of the nature of the interface between fibers and matrix adds to the complication of the field. Consequently many attempts have been made to reduce the cross‐section of the reinforcing fibers to molecular dimensions and to increase the compatibility between the rod‐like molecules or bundles of molecules and the isotropic matrix of ordinary flexible polymers. All such attempts have failed, mainly for reasons of thermodynamics which predict immiscibility of rods and coils on the molecular level. A possibility to create structures in which molecular rods are embedded in a continuous matrix of flexible chain segments exists nevertheless. 1 Rod‐like backbone structures decorated with a skin of flexible side chains of moderate length in terms of the number of carbon atoms (typically C 6 ‐ C 18 ‐side chains) have been synthesized and tested for the spontaneous formation of molecular composites. Some of the materials built along this principle can be processed by solution casting or even melt extrusion. The academically more interesting materials can be built up layerwise by a modified Langmuir‐Blodgett technique. The latter process gives rise to monodomain textures which allow for straight‐forward testing of the mechanical properties by optical means (Brillouin‐spectroscopy) and by piezoquartz‐techniques. Hairy‐rod macromolecules can also be synthesized which contain crosslinkable side chains. Thus, after formation of layered, oriented structures, these can be crosslinked photochemically or by curing. The result is a network, which is unidirectionally reinforced by the rod‐like macromolecules. These novel types of networks can serve as membranes by which size exclusion is achieved via control of the distance between adjacent backbone molecules in the matrix. Recent synthetic advances in the field have concentrated on hairy‐rod macromolecules based on cellulose alkyl ethers and on derivatives of poly‐(p‐phenylene). The latter materials serve as examples of systems which can be processed by casting and extrusion processes. Due to their excellent thermal stability relevant data on the rheological and mechanical‐dynamical data have become available. These data serve to document the unique behavior of the hairy‐rod macromolecules as bulk materials.