Influence of the mesogenic shape on the molecular dynamics and phase-biaxiality of liquid crystal main-chain polymers

Ever since the first theoretical phase diagram that was published by Straley in 1974, extensive theoretical efforts have been made on predicting the molecular shape and properties of mesogens that can form a biaxial-nematic phase. The theories suggest that mesogens, whose form is board-like rather than rod-like, are more likely to form biaxial phases. In this study we show that the onset of phase-biaxiality for nematic main-chain polymers can indeed, without significantly changing the glass transition temperature, be shifted to considerably higher temperatures by laterally broadening the mesogenic moieties. The investigations were performed by means of 2H-NMR spectroscopy on directly deuterated polymers and yield results that differ considerably from what could hitherto be observed in spin-probe measurements on side-chain polymers with similarly broadened mesogens.