Rigid-rod polyesters with flexible side chains ix. phase behavior including nematic, layered, and hexagonal columnar phases in poly(p-biphenylene terephthalate) with alkoxy side chains

Abstract

In this study we prepared a series of poly(p-biphenylene terephthalate) with alkoxy side chains attached to terephthalate moiety. The carbon number of alkoxy side chains, n, is ranged from 8 to 18. These polymers exhibited well-defined thermotropic phase behavior with a nematic phase in the higher temperature region and two ordered mesophases in the lower temperature region. One of the lower temperature mesophases was found to be a hexagonal columnar phase in the polymers with smaller values of n (8 to 12), while the other is the layered mesophase preferentially formed from the polymer with n = 18. In the polymers with n = 13, 14, and 16, the columnar and layered mesophases coexist. In these intermediate polymers, the relative amount of the layered phase to the columnar phase increases with the increase of n. Further, it increases when the mesophase temperature decreases, indicating that the layered phase is thermodynamically lower temperature mesophase than the columnar one. While the layered phase is occasionally observed in this kind of polymers with long alkyl side chains, the columnar phase is new and interesting since many chains are included in its hexagonal unit cell. The number of chains increases from 7 to 12 with an increase of n from 8 to 14. To achieve the reasonable packing of the aromatic main chains, we proposed the special hexagonal structure where the fully extended main chains are closely packed with lateral spacing of around 4.5 Å to form a honeycombed network and the molten side chains are accommodated within a resulting cylindrical domain.