# Crystallization Kinetics of an Equimolar Liquid Crystalline Mixture and Its Components
## 1 Introduction
The phase sequence upon cooling given in [29] is Iso (421 K) SmA* (395 K) SmC\({}_{\text{A}}\)* (394.1 K) SmC* (392.4 K) SmC\({}_{\text{Y}}\)* (391.6 K) SmC\({}_{\text{A}}\)* (337.7 K) SmC\({}_{\text{A}}\)* (304 K) Cr, where Iso is the isotropic liquid, Sm denotes various smectic phases, and Cr is the crystal phase. In the SmA* phase, the average tilt angle of the molecules is zero, and this phase has paraelectric properties. The next three phases, present in a narrow temperature range, are interpreted in various ways in the literature. Resonant X-ray diffraction study of (S)-MHPOBC films, with a small admixture (up to 5% molar percentage) of the selenophene PB237, leads to the identification of these phases as ferroelectric SmC*, four-layer SmC\({}_{\text{F}\text{I}}\)*, and three-layer SmC\({}_{\text{F}\text{I}}\)*, in order of decreasing temperature [14]. However, it is underlined in [14] that the PB237 dopant, necessary for the resonant X-ray experiment, might alter the structure of the smectic phases. The presence of optical impurities also changes the phase sequence. According to [13], in pure (S)-MHPOBC, these are SmC\({}_{\text{A}}\)*, SmC\({}_{\text{F}\text{I}}\)*, and SmC\({}_{\text{F}\text{I}}\)*. Meanwhile, for (S)-MHPOBC with a small admixture of (R)-MHPOBC, the SmC\({}_{\text{F}\text{I}}\)* phase is replaced by SmC*, and SmC\({}_{\text{A}}\)* is interpreted as a phase with a short helical pitch, both for optically pure (S)-MHPOBC, and (S)-MHPOBC with a (R)-MHPOBC admixture up to 12% [13]. The sequence SmC\({}_{\text{A}}\)*, SmC\({}^{\star}\), SmC\({}_{\text{F}\text{I}}\)* is assumed in a recent paper [30]. The SmC\({}_{\text{A}}\)* phase has a wide temperature range. After supercooling, it transitions to the monotropic SmX\({}_{\text{A}}\)* phase by the appearance of a hexatic bond-orientational order within the layers [31]. The SmX\({}_{\text{A}}\)* phase is either SmI\({}_{\text{A}}\)* or SmFA\({}^{\star}\) (in [29] denoted as SmI\({}_{\text{A}}\)*), which are difficult to distinguish, as the only difference in their structures is the direction of the molecular tilt in respect to the local hexagonal order within the layers. This usually cannot be inferred from powder X-ray diffraction (XRD) patterns [31]. We did not find any XRD study of the oriented MHPOBC sample in the literature, which would distinguish SmI\({}_{\text{A}}\)* and SmFA\({}^{\star}\); therefore, we prefer to use the SmX\({}_{\text{A}}\)* notation for the hexatic phase. The second investigated compound, (S)-4\({}^{\prime}\)-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-[2-(2,2,3,3,4,4,4)-heptafluorobutoxy)etyl-1-oxy]-2-fluorobenzoate, is abbreviated as 3F2 HPhF6 (Figure 1), and exhibits only two smectic phases, with a sequence Iso (377.2 K) SmC* (364.6 K) SmC\({}_{\text{A}}\)* (307 K) Cr1 (299.5 K) Cr2 on cooling [32]. The SmC\({}_{\text{A}}\)* phase of 3F2 HPhF6 is orthoconic, meaning that the molecules' tilt angle reaches almost 45\({}^{\circ}\)[32]. This property improves the quality of the dark state in an LC display [33; 34]. The results for one LC mixture containing 3F2 HPhF6 have been published [4]. The polymorphism of the MHPOBC/3F2 HPhF6 system has not been investigated yet. The paper aims to investigate the crystallization process of MHPOBC, 3F2 HPhF6, and their equimolar mixture by differential scanning calorimetry (DSC), for various cooling rates and in isothermal conditions at different temperatures. This is part of the broader study of crystallization kinetics in antiferroelectric smectic liquid crystals, which has been focused on pure compounds up until now [35; 36; 37; 38]. The sequence of the smectic phases in the mixture was also determined by DSC and polarizing optical microscopy (POM). X-ray diffraction (XRD) was used to measure the smectic layer spacing in the mixture, as a function of temperature in comparison to the values for the pure components. Broadband dielectric spectroscopy (BDS) facilitated the identification of the smectic phases based on the relaxation processes observed in the mixture.