Copolymers rod-coil

Another key point is selective chemical functionalization at one or both ends, or inside the chain (see scheme 2).m Thus, thiolo functions can serve as clips to create contact with metal surfaces or particles. Quantitative end functionalization of the rigid-rod on one end is a key step toward rod-coil copolymer synthesis (see scheme 3),131 and such a covalent coupling of incompatible polymer blocks is relevant for supramolecular organization.141... 

Rod-coil copolymers can form unprecendented bulk morphologies, as illustrated by the work of Thomas, Ober, and coworkers on PHIC-PS copolymers [243,244], Micelle formation from rod-coil copolymers seems therefore to be a very promising route for the formation of nonspherical morphologies. 

Block copolymer micelles in which the core-forming polymer blocks are able to crystallize are relatively similar to rod-coil copolymers. A significant part of these crystalline-core micelles is actually resulting from the self-assembly of rod-coil block copolymers. 

Rod-coil copolymers are good candidates for the formation of rodlike or vesicular structures and can be built from polypeptide-block-containing copolymers, as previously discussed in Sect. 6. 

Recently, Stupp and co-workers have reported the synthesis and structural characterization via microscopy of rod-coil copoly mers with a short (60 A) meso-genic block end-attached to carboxylated poly(isoprene) of varying molecular weight (Radzilowski and Stupp 1994 Radzilowski ef al. 1993). A representation of such a rod-coil copolymer is shown in Fig. 2.36. The mesogenic block was shown... 

Early work predicted smectic (or lamellar) ordering in rod-coil copolymers (Semenov 1991 Semenov and Vasilenko 1986). In liquid crystals, a smectic A phase is a lamellar phase where the molecules are, on average, parallel to the layer normal. In a smectic C phase, the molecules are tilted with respect to this direction. The imbalance in interfacial area per chain for a rod or coil can lead to tilting of chains to maintain uniform density. Semenov (1991) constructed a phase diagram for rod-coil copolymers in which second-order phase transitions... 

Figure 4.27 Illustration of the synthetic approach to producing rod-coil copolymers...

Figure 2 Klok et al. s model of the self-assembly of rod-coil copolymer (styrene)10-b-(y-benzyl-L-glutamate)2o at 120°C. Small cylinders represent the peptide rods. Styrene coils, not shown for clarity, are proposed to protrude randomly from both sides of the oligopeptide clusters (Klok et al., 2000).

The covalent linkage of these different classes of molecules to a single linear polymer chain (rod—coil copolymer) can produce a novel class of self-assembling materials since the molecules share certain general characteristics of diblock molecules and rodlike liquid crystalline molecules.12 15 The difference... 

Many of the syntheses of rod—coil diblock and triblock copolymers as well as their interesting supramolecular structures and the intriguing properties of rod—coil copolymers are discussed in excellent books and reviews that have been published by several experts in the field.16 19 Here, we do not want to present a complete overview on reported rod—coil copolymers. Instead, we have highlighted the most recently synthesized rod—coil copolymers and their supramolecular structures. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Figure 6. TEM images for (a) zigzag lamellar morphology of rod-coil copolymer with Toa = 0.90 and (b) arrowhead morphology of rod-coil copolymer with Tod = 0.98. (Reprinted with permission from ref 52. Copyright 1996 American Association for the Advancement of Science).

Figure 9. Schematic diagrams of (a) strip morphology of rod—coil copolymer with Trod = 0.36 and (b) hexagonal superlattice of rod—coil copolymer with fmlj = 0.25. (Reprinted with permission from ref 57. Copyright 1997 American Chemical Society).

Rod—coil copolymers are a type of amphiphile that can self-assemble into a variety of ordered nanostructures in a selective solvent.36-37-71 In solvents that selectively dissolve only coil blocks, rod—coil copolymers can form well-defined nanostructures with rod domain consisting of the insoluble block. This results in an increase of the relative volume fraction of the coil segments relative to the rod segments, which gives rise to various supramolecular structures. Particularly, poly(alkylene oxide) as the coil block of rod—coil molecule has additional advantages due to complexation capability with alkali metal cation, which can provide an application potential for solid polyelectrolytes and induce various supramolecular structures.72-75... 

Wu et al. reported on a rod—coil diblock copolymers based on mesogen-jacketed liquid crystalline polymer as the rod block and polystyrene as the coil block (Scheme 6).82 Styrene was polymerized by TEMPO mediated radical polymerization, followed by sequential polymerization of 2,5-bis[4-methoxyphenyl]oxy-carbonylstyrene (MPCS) to produce the rod—coil diblock copolymer (20) containing 520 styrene and 119 MPCS repeating units. The rod—coil copolymer was observed to self-assemble into a core—shell nanostructure in a selective solvent for polystyrene... 

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