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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.KEY WORDS Aromatic Polyesters / Alkyl Side Chain / Liquid Crystal / Hexagonal Columnar Phase / Layered Phase / There have been several recent reports in the literature on thermotropic phase behavior of the rigid-rod polymers with long flexible alkyl side chains. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The most interesting property of these materials is an ability to form layered structures in crystals and liquid crystals when the alkyl side chains reach a critical length. The layered mesophases are characterized by a closed packing of the rigid-rod main chains into layers with the molten side chains occupying a space between the layers. The driving force for an adoption of such a layered structure is a type of microphase separation of the aliphatic and aromatic domains, 2,5,6,12 and the liquid crystallinity is the results of a partial or total lack of positional order with respect to the main-chain packing within a layer and of the molten side chains between the layers. 12,13 In a previous paper, 17 we studied the thermotropic phase behavior and structures of the rigid-rod poly(pbiphenylene terephthalate) main chain with flexible dodecyloxy side chains attached to terephthalate moiety, It is interesting that this material forms a hexagonal columnar phase. This is the first example of a hexagonal columnar phase in this type of aromatic polyesters with flexible side chains. Although several examples of the columnar phase have so far ...