Block copolymers containing liquid crystalline structures

Block copolymers containing liquid crystalline structures... 

Liquid crystal block copolymers are a recently explored group of polymers which combine microphase separation and liquid crystallinity. Yet, as early as 1963, Gratzer and Doty [37] reported the first block copolymers containing two polypeptide blocks in which one of the blocks, poly(y-benzyl-L-glutamate) (PBLG), was already well known to be liquid crystalline. In principle, all the LCP structures shown in Fig. 1 can be incorporated into block copolymer structures with a second flexible coil or LC block. Some of the possible LC-BCP structures are shown in Fig. 2. Due to the limited nature of this review, structures such as the grafted LC copolymers [38] and various multiblock LC copolymers [39] will not be covered. 

For all copolymers studied (copolymers containing between 18 % and 83 % of polypeptide] the liquid crystalline structures are always lamellar and are very similar to the structure of saccharide-peptide block copolymers. In the lamellar structure of copolymers with a polyvinyl block and a hydrophobic polypeptide block, each sheet of thickness d results from the superposition of two layers one of thickness d/ formed by the polyvinyl chains in a more or less random coil conformation, the other of thickness dg formed by the polypeptide chains, in an a helix conformation, arranged in an hexagonal array, and generally folded ( 7, 8]. ... 

Block copolymers exhibit unique characteristics in that they are able to self-order to multiphase domain structures of submicron scale with various morphologies because of the relative incompatibility of the different blocks. On the other hand, the liquid crystalline (LC) mesophases provide an additional example of a state of matter characterized by non-crystalline order. The combination of these two different aspects into one single macromolecular architecture leads to block copolymers containing LC blocks (7). These materials can be valuable in elucidating specific aspects of polymer physics and, in addition, may be employed as highly versatile interfacially active additives and viscosity improvers potentially capable of providing enhanced optimization of material processing and performance. 

The synthesis and some thermal properties of three series of block copolymers comprising both main-chain and side-chain liquid-crystalline (LC) blocks in the same macromolecular structure are described. The former block is a semiflexible LC polyester (block B), and the latter is an LC polymethacrylate (block A) containing a variously substituted mesogenic unit. The two structurally different blocks were partly phase-separated within the glassy and LC states and underwent distinct phase transitions. Significant deviations of the transition enthalpies relative to those of the corresponding homopolymers suggest the occurrence of a more or less diffuse interphase which may depend on the nature of the mesophase formed. 

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