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  • Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic, and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry [Elektronski vir]
    Thoen, Jan ...
    High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of ... providing simultaneous information on the temperature dependence of the specific enthalpy h (T) and on the specific heat capacity c p (T) , has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3′,4′,5′-tetrafluoro[1,1′-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (NF) and a high-temperature paraelectric nematic phase (N) . However, in DIO these two phases are separated by an intermediate phase (Nx). From the detailed data of h (T) and c p (T) , we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (xDIO). The xDIO dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the Nx−N transition of DIO was as low as L = 0.0075 ± 0.0005 J / g and L = 0.23 ± 0.03 J / g for the NF−Nx transition, which is about twice the previously reported value of 0.115 J/g for the NF−N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing xDIO . At all the Nx−N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the NF to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c p (T) resulted in an effective critical exponent α = 0.88 ± 0.1 for DIO and this value decreased in the mixtures with decreasing xDIO toward α = 0.50 ± 0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.
    Vir: Physical review. E. [Elektronski vir]. - ISSN 2470-0053 (Vol. 107, iss. 1, Jan. 2023, str. 014701-1-014701-12)
    Vrsta gradiva - e-članek ; neleposlovje za odrasle
    Leto - 2023
    Jezik - angleški
    COBISS.SI-ID - 137590531
    DOI

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