Phase identification in a series of liquid crystalline TPP polyethers and copolyethers having highly ordered mesophase structures. 8. Phase and structural evolution in a series of copolyethers containing odd-numbered methylene units in both comonomers

A series of liquid crystalline copolyethers have been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)propane with, 1,7-dibromoheptane and 1,11-dibromoundecane (coTPPs-7/11). This represents the copolyethers containing only odd numbers of methylene units in both comonomers. The molar ratio between these two comonomers in this series ranges from 1/9 to 9/1. The coTPPs-7/11 exhibit multiple phase transitions during cooling and heating in differential scanning calorimetry experiments. In each of these thermal transitions, only a small undercooling and superheating dependence is observed upon cooling and heating at different rates. All of the coTPPs-7/11 possess similar phase behavior based on the structural analyses using wide-angle X-ray diffraction (WAXD) on both powder and fiber samples and electron diffraction experiments on thin-film samples in transmission electron microscopy. The low-temperature phase is identified as a tilted hexagonal columnar (Φ_TH) phase with multiple tilt angles between molecular axes and the hexagonal lattice normal. However, the hexagonal lateral dimensions in a Φ_TH phase with various tilt angles are identical. This Φ_TH phase transforms to another columnar (Φ′) phase during heating at approximately 140°C, as evidenced by the temperature-resolved WAXD fiber patterns. In this transition, the laterally ordered reflections from the (110) and (200) planes in the quadrants move toward the equator, corresponding to the decreased tilt angles. The discontinuous change in both the lateral d spacings and tilt angles represents a first-order transition. The columnar-columnar phase transition may be associated with the conformational disorder of the methylene units according to the infrared spectroscopic observations. Further heating of the samples leads to transitions from the Φ′ to a nematic (N) phase and from the N phase to the isotropic melt. Combining the copolymer phase behaviors observed with the corresponding homopolymers TPPs(n = 7 and 11), a phase diagram describing transition temperatures with respect to the composition can be constructed. Their transition properties are also discussed.