Rod-coil diblock copolymer systems

Due to the conformation asymmetry in rod-coil diblock copolymer systems, the packing is expected to be totally different from conformationally symmetric coil - coil block copolymers. Semenov and Vasilenko [71] have predicted that a N-SmA transition can be either a first-order transition (in the case of large coil fraction) or a second-order transition (in the case of small coil fraction) and a SmC phase in a rod-coil system is also expected for f <036. 

Figure 40 Schematic representation of self-assemblies in the rod-coil diblock copolymer system by Lee et al. (a) the lamellar smectic A, (b) the bicontinous cubic and (c) the hexagonal columnar mesophase (n represents the degree of polymerization of the poly-isoprene coil segment). (Reprinted with permission from Ref. 120. Copyright 1998, American Chemical Society.)...

Liquid crystallinity and block microphase separation both compete during the minimization of free energy of the system. As we will show later in this review, in the case of a rod-coil diblock copolymer, liquid crystallinity plays a very important role in the microphase separation process and leads to morphologies distinctly different from the conventional spheres, cylinders and lamellar microstructures and include the arrow head, zig-zag, and wavy lamellae phases [40, 41], In the case of SGLC-coil... 

These materials represent the first observation of the SmC (zig-zag) and SmO (arrow head) structure in rod-coil diblock copolymers [41] in contrast to the homopolymer of poly( -hexyl isocyanate) which only form a nematic mesophase (both lyotropic [65] and thermotropic [66]). This confirms the idea by Halperin [60, 69] that rod-coil systems are a microscopic model for smectic liquid crystals in general. Although the SHIC rod-coil system has a relatively broad polydispersity, a smectic mesophase over a size scale of as much as 10 xm has been observed (Fig. 4B). This indicates that microphase separation plays a very important role in determining the self-assembly of the liquid crystalline process of these blocks. The existence of only a nematic phase in the rod homopolymer system is probably due to its broad polydispersity in contrast to the fact that a smectic meso-... 

The last reported diblock copolymer family that formed tubular aggregates in block-selective solvents was poly(phenylquinoline)-fc/ock-polystyrene or PPQ-PS, where PPQ was a rigid-rod block [47]. Such tubes are not discussed further for the following reasons First, the tubes had diameters of several micrometers and were not nanotubes. Second, the formation mechanism and chain packing in such tubes were not well understood at all. While Halperin [48] has developed a scaling theory for micelle formation from rod-coil diblock copolymers with the rod block forming the core, the theory did not apply to the PPQ-PS system as the block-selective solvents used were good for the rod PPQ block rather than the coil PS blocL... 

---