Side-Chain Liquid Crystal Block Copolymers with Well-Defined Structures Prepared by Living Anionic Polymerization I. Thermotropic Phase Behavior and Structures of Liquid Crystal Segment in Lamellar Type of Microphase Domain

Abstract

ABSTRACT:We have synthesized the A-B block copolymer with polystyrene as A segment and side-chain liquid crystal (LC) polymer as B segment by a sequential living polymerization. The copolymers were successfully prepared with quantitative yields, possessing the predictable M. values, compositions, and narrow molecular weight distributions. The thermotropic phase behavior and structures were examined for nine copolymers with the fraction of segments around 50 w% and with the various molecular weights ranged from 8000 to 34000. All the polymers exhibit the well-defined lamellar type of segregation and the side-chain LC segment in the microdomain forms crystal, smectic A (SA) and isotropic phases with increasing temperature. The lamellar segregation is maintained over the whole temperature region from crystal to isotropic phases, and the crystal and SA structures are formed with a preferential orientation to the microdomain interface such that the mesogenic side-chain groups lie parallel to the interface. The lamellar thickness depends on the SA temperatures. It gradually decreases from the value of crystal phase to that of isotropic phase. The reduction is around 2(}--25%, which can be explained in terms of the change in main-chain conformation of the LC block.KEY WORDS Block Copolymer I Liquid Crystal I Smectic Phase I Microphase Separation I Lamellar Structure I Living Polymerization I Liquid crystal (LC) polymers with mesogenic groups in the side chains (side-chain LC polymers) have been extensively studied. 1 • 2 More recently, the research interests have focused on a role of the main-chain backbone on liquid crystal structures and properties. The various parameters with respect to the main-chain backbone such as a degree of polymerization, molecular weight distribution, tacticity, and monomer unit distribution in copolymers can be considered. An essential breakthrough can be performed by controlled polymerization such as group-transfer polymerization and living polymerization, since these permit the control of several parameters given above because of the synthesis of welldefined structures. 3 -8 In the previous paper, 9 we succeeded to prepare a series of side-chain LC polymers with a well-controlled structure, by living anionic polymerization. Polymerization was attempted at an elevated temperature of -40oC since precipitation occurs during the polymerization at a usually applied temperature of -78oC. Irrespective of this unusual condition for the living anionic polymerization, the polymers were obtained with unimodal distribution of less than 1, 1 when the molecular weights are less than 12000. Their thermotropic phase behavior and structures were examined as a function of the molecular weight.

Influence of liquid crystalline formation on the phase behavior of side-chain liquid crystalline block copolymers

“…The isotropization temperature of BC-77 is nearly within experimental error of its ODT temperature. This correlation confirms our hypothesis that the isotropization of the LC domain can trigger the ODT [51]. This phenomenon can be explained on the basis of differences in smectic elastic free energies above and below the isotropization temperature.…”

Microphase‐Separated Morphology and Liquid Crystal Orientation in Block Copolymers Comprising a Main‐Chain Liquid Crystalline Central Segment Connected to Side‐Chain Liquid Crystalline Segments at Both Ends

“…LC orientation affects the microdomain orientation because of the anchoring of mesogenic units on the microdomain interface. Side‐chain (SC) LC segments repel amorphous segments to form microdomains and stretch the backbone perpendicular to the interface accompanied by arrangement of the mesogens in the SCs parallel to the interface . A magnetic field can align amorphous microcylinders as well as the LC director of the matrix along the field direction throughout the specimen .…”

Influence of Smectic Liquid Crystallinity on Lamellar Microdomain Structure in a Main‐Chain Liquid Crystal Block Copolymer Fiber

“…Block copolymers incorporating a liquid crystal (LC) segment are designated as LC block copolymers and have attracted attention because their microdomain orientation correlates to the LC orientation owing to the anchoring of mesogenic units on the microdomain interface. Most LC block copolymers have the side‐chain‐type LC segment, which tends to stretch the backbone perpendicular to the interface and arrange the mesogens in the side chains parallel to the interface . Owing to the homogeneous anchoring of mesogens at the interface, a magnetic field can align the amorphous cylinder as well as the LC director along the field direction throughout the specimen .…”