Calamitic Liquid Crystals as Nanoscale Analogues of a Block Copolymer

Abstract

The micro-segregation picture of a block copolymer was applied to a low molecular weight liquid crystal (LC) to clarify the similarities between them with respect to structured mesophase formation. Calamitic LC molecules were regarded as block compounds composed of a rigid anisometric core terminated by flexible chains. Then, the incompatibility parameter core-chain and volume fraction f core were considered analogous to AB and f A in A-B diblock copolymers. Fedors's method, which was originally used for polymers, was utilized to estimate core-chain and f core . We found good correspondence in the versus f phase diagram between block copolymers and block LCs.Liquid crystal (LC) materials that form complex nanostructured mesophases (e.g., cubic phase) have attracted increasing interest recently. 1 In these nonconventional LC materials, some individual molecular characteristics (e.g., hydrogenbonding, lateral dipole interactions) have been pointed out for the origin of such complex mesophase formations. 2,3 In addition, nanoscale segregation of incompatible molecular parts are considered to play an important role in their self-assembly. 4,5 Calamitic LC molecules can be regarded as block compounds composed of a rigid anisometric (mesogenic) core terminated by flexible chains. Polyphilic block molecules 1 consisting of a chemically incompatible core and chains, e.g., a hydrocarbon mesogenic core with oligo(ethylene oxide) chains, 6 siloxane chains, 5 or perfluoroalkyl chains 3 etc., have been found to form not only smectic but also columnar, cubic, and other threedimensionally structured mesophases. Such structured mesophases are popularly found in a block copolymer system, since even minor chemical differences are sufficient for such mesophase formation in large polymer molecules. 1 More precisely the parameters which govern these mesophase formations for A-B diblock copolymers are the total degree of polymerization N ) N A + N B , the Flory-Huggins parameter AB , and the volume fraction of the components f A . The Flory-Huggins parameter AB is a measure of the incompatibility between components A and B, and an order-disorder transition (e.g, formation of lamellar phase) occurs when AB N exceeds a critical value (∼10 at f ) 0.5). 7,4 Hence, low molecular weight LC molecules must possess a strong incompatibility between their different parts to form highly structured mesophases. 1 As the volume fraction f governs the interfacial curvature of the micro-segregated system, the phase diagram of the diblock copolymer can be represented in an f versus N diagram. 7 In this study, we try to adapt the f -N diagram approach to a structured mesophase forming a LC molecule to clarify the similarities to the copolymer. The Flory-Huggins parameter AB can be estimated according to the following equation:where δ A and δ B represent the Hildebrand solubility parameter and V R is a reference volume. 1 The solubility parameter δ is defined as the square root of the cohesive energy density and can be written as follows:where ∆E is the ...

Toward rational design of complex nanostructured liquid crystals

“…While working toward the rational design of nanostructured LCs, we have applied the block copolymer analogy to block LC molecules 12. The analogy of mesophases of block copolymers and amphiphilic molecules as LC molecules was noted over 30 years ago 13…”

“…Calamitic LC molecules can be regarded as block compounds composed of a rigid anisometric mesogenic core terminated by flexible chains. We applied the χ N ‐ f diagram approach to a linear block LC molecule to clarify similarities to the copolymer 12. In the case of diblock copolymers, χ AB can be estimated from [Eq.…”

“…(2)] can be applied to LC phases as well as to isotropic liquids. We extend this approach to block LC molecules 12. In this case, we assume the LC molecule is composed of two parts, a core and a chain, as described in Equation (5).…”

Microphase-separated multicontinuous phase in low-molecular-mass thermotropic liquid crystals

“…That is, at least some of the thermo-and lyotropic liquid crystals share properties and mechanism responsible for the formation of liquid-crystalline order, plausibly. The quasi-binary picture was quantitatively assessed later by Yoneya et al [40].…”

Experimental assessment of quasi-binary picture of thermotropics: Induced smectic A phase in 7CB–n-heptane system

“…The QB picture has been very useful in understanding cubic phases of ANBC(n). 16,24 Yoneya et al 25 independently applied a model of micro phase separation to cubic mesogens, which was based on their amphiphilic nature and had originally been developed for melts of amphiphilic block copolymers. 26 Although the validity and usefulness of the QB picture are certain in a limited class of mesogens such as ANBC(n), the range of its applicability has not yet been studied.…”