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Twist-Bend Nematic Phase in Biphenylethane-based Copolyethers

Abstract

The main-chain liquid crystal (LC) copolyethers in which the nematic-nematic phase transition was first experimentally observed were revisited and re-characterised. Grazing incidence X-ray scattering revealed that the lower temperature nematic (Ntb phase) could be highly aligned by shearing, even more so than in previously studied bent LC dimers. This was evidenced by a four-point wide angle X-ray scattering pattern, which originates from the convolution of two tilt distributions inside the Ntb helical structure. Through intensity simulation the orientational order parameter associated with each of the distributions, as well as the conical angle of the Ntb phase, were calculated. Additional information regarding the polymer chain conformation was obtained using polarised infrared spectroscopy. The findings suggest that the average conformation of the polymer chains is that of a helix and that the bend angle between mesogenic units is inversely related to temperature. All experimental evidence, including a jump in birefringence at the Ntb-nematic (N) phase transition, shows that copolyether samples mirror the behaviour of bent LC dimers over the transition. This confirms our long held view that the low-temperature nematic phase, reported nearly three decades ago,1 is indeed the same as that in the LC dimers, now known to be the twist-bend nematic. In addition, the unusual broadening of transition peaks in complex heat capacity, obtained by modulated DSC experiments, is discussed.

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Publication details

The article was received on 23 Dec 2017, accepted on 07 Feb 2018 and first published on 08 Feb 2018


Article type: Paper
DOI: 10.1039/C7SM02525D
Citation: Soft Matter, 2018, Accepted Manuscript
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    Twist-Bend Nematic Phase in Biphenylethane-based Copolyethers

    W. D. Stevenson, J. An, X. Zeng, M. Xue, H. Zou, Y. Liu and G. Ungar, Soft Matter, 2018, Accepted Manuscript , DOI: 10.1039/C7SM02525D

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