Rod-coil diblocks

Fig. Z36 Representation of the energy-minimized conformation of a rod-coil diblock prepared by Radzilowski ef al. (1993). The rod is a mesogenic moiety 60 A long, the coil is low molecular weight PI. The volume fraction of rod is about 0.2.

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

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. 

Ober and Thomas et al. reported on rod—coil diblock copolymers consisting of poly(hexyl isocyanate) as the rod block and polystyrene as the coil block (Scheme 2).51 53 The polymers (2) were synthesized... 

Figure 8. Morphology diagram for rod—coil diblock copolymers (2).

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... 

The authors also reported on the supramolecular self-assembly from rod—coil—rod triblock copolymers prepared by copolymerization of 5-acetyl-2-aminob-ezophenone with diacetyl functionalized polystyrene with low polydispersity (Scheme 12).110 In contrast to the rod—coil diblock copolymers which exhibit multiple morphologies, the triblock copolymers were found to spontaneously form only microcapsules or spherical vesicles in solution as evidenced by optical polarized, fluorescence optical, and scanning electron microscopies (Figure 33). 

Wan et al. used TEMPO-mediated polymerizations to prepare liquid crystalline (LC) polymers [147,148]. pSt-TEMPO was chain extended with a mesogen-jacketed LC monomer, 2,5-bis [(4-methoxyphenyl)oxycarbonyl] styrene (MPCS, Fig. 9) to form a rod-coil diblock copolymer. The resulting copolymer had an Mn=19,500 with an Mw/Mn=1.48. There was tailing to lower molecular weights, indicating the presence of some unreacted macroinitiator, but after extraction with cyclohexane, the remaining macroinitiator was removed, leaving pure block copolymer. H and 13C HMR analysis indicated the presence of both blocks, as did DSC analysis, which showed two Tgs,one at 117.2 °C (pMPCS) and... 

Rod-Coil Diblock Copolymers Based on Perfectly Monodisperse Rods. . . 65... 

Fig. 2 Mesophases of the rod-coil diblock molecules by the increasing volume fraction of coil segments...

In contrast to the rod-coil diblock copolymer consisting of perfectly monodisperse rods, the liquid crystalline morphologies of rod-coil diblock copolymer containing polydisperse rods seem to be studied in less detail. In certain cases, the polydisperse nature of the rod-segments could hinder self-assembly into regularly ordered supramolecular structures. However, due to relatively simple synthetic procedures, liquid crystalline polymer can be of benefit for new materials with controlled internal dimensions ranging from the nanometer to macroscopic scale. 

In the case of symmetric coil-rod-coil molecule, the rod segment is connected with coil segments at both ends. This gives rise to the formation of the liquid crystalline structure with higher interfacial area in comparison with rod-coil diblock systems at similar coil volume fraction. For example, the... 

Well-defined rod-coil diblock or triblock copolymers can be obtained from conjugated macro-initiators by CRP. In each case the synthetic route requires the mono- (for diblock) or di-functioimalization (triblock) of a previous conjugated polymer. For this purpose, this last one needs to be properly end capped with a high conversion and further transformed into a macro-initiator. Many examples can be found in the literature, such as poly(para phenylene vinylene)... 

Figure 2. Structure of liquid crystalline block copolymers (LC-BCPs) (A) rod-coil diblock copolymer (B) rod-coil diblock copolymer with flexible spacer in the rod block (C) side group liquid crystal-coil (SGLC- coil) diblock copolymers (D) coil -rod-coil ABC triblock copolymers (predicted to be novel ferroelectric fluid by R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett., 1987, 59(3), 343-346) (E) rod-rod diblock copolymer (one example of well-defined po-ly(n-hexyl isocyanate-fc-n-butyl isocyanate) rod-rod diblock copolymer was given by Novak et al. [68], however, no morphology studies were reported) (F) dendritic liquid crystal-coil (DLC-coil) diblock copolymer (not reported).

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... 

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