Effect of Alkyl Chain Length on the Phase Situation of Glass-Forming Liquid Crystals

Abstract

The phase behaviour of the latest synthesised compound belonging to a family of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[‘m’-(2,2,3,3,4,4,4-heptafluorobutoxy) ‘m’alkoxy]-benzoates (where ‘m’ means 3, 5 or 7 methylene groups) is described by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and Fourier-transform infrared absorption spectroscopy. It has been shown that as the length of the alkyl chain increases, a given liquid crystal possesses a greater number of mesophases and at a higher temperature it goes into the isotropic liquid phase. All examined compounds form a chiral smectic phase with antiferroelectric properties (SmC_A* phase), in which the temperature range of occurrence increases with the length of the molecule. The number of methylene groups also affects the glass transition. The compound with the shortest alkyl chain (‘m’ = 3) is vitrified from the conformationally disordered crystal phase. For the compound with five -CH₂- groups (‘m’ = 5), a glass transition from the monotropic high-order hexatic smectic SmX_A* phase is observed. In the case of the liquid crystal with the longest carbon chain (‘m’ = 7), the vitrification from the less ordered SmC_A* phase is visible. Differences in the crystallization kinetics, e.g., the nucleation-controlled mechanism for the compound with the shortest carbon chain vs. the complex phenomenon for its longer homologs, are discussed.

1. Introduction

One of the best-known applications using liquid crystals (LCs) are displays dedicated to various electro-optical devices. Currently in display technology, mainly nematic LCs are used; however, chiral smectic phases with ferro-, SmC*, or antiferroelectric, SmC_A*, properties are becoming more and more promising, because devices with short switching time, large contrast and many scales can be realized [1,2]. Furthermore, smectic glassformers are more suitable components of eutectic mixtures, useful in LCs display technology, than the compounds that crystallize easily. Thus, although the LCs forming the SmC_A* phase have been known since the 1980s [3], the study of their properties is still highly desirable [4,5,6].

The most promising compounds for practical use in display technology are those with a molecular structure based on that of the MHPOBC liquid crystal; the first published compound with a SmC_A* phase [3] and with a fluorinated terminal chain. These types of compounds possess the orthoconic SmC_A* phase with a tilt angle close to 45°, which enables the good quality of the dark state in LC displays [1]. The last extensive research showed the significant influence of the molecular structure of such compounds on their physical properties [7,8,9]. For instance, the fluorosubstitution of the aromatic molecular core affects the occurrence of the smectic phases and phase transition temperatures [9]. In this paper, we analyze the impact of the length of the alkyl chain on the phase behaviour of other MHPOBC-based compounds. This work is dedicated to the liquid crystals belonging to a family of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[‘m’-(2,2,3,3,4,4,4-heptafluorobutoxy) ‘m’alkoxy]-benzoates (where ‘m’ means 3, 5 or 7 methylene groups), abbreviated as 3FmHPhH7 compounds. The general formula of these molecules is presented in Figure 1. These compounds are composed of a partially fluorinated chain, a phenyl and a biphenyl moiety with a -COO- spacer, and a chiral alkyl chain linked by a second -COO- group with a rigid core.

The physical properties of the 3F5HPhH7 and 3F7HPhH7 compounds are the subject of several publications [10,11,12,13,14]; however, the study of the phase situation and cold crystallization (i.e., crystallization occurring during heating of the previously vitrified material) kinetics of these compounds has not been reported yet. While studying compounds exhibiting the SmC_A* phase, it is important to check if there is any detail of the molecular structure (in this case—the length of the oligomethylene spacer), which may affect their phase behaviour. So, the aims of this paper are: (i) to characterize the phase situation of the latest synthesised compound belonging to the 3FmHPhH7 family, i.e., the 3F3HPhH7 compound, (ii) to determine the cold crystallization kinetics under non-isothermal conditions of the 3F3HPhH7 compound, and (iii) to compare the influence of the carbon chain length (‘m’) on phase behaviour and kinetics of the crystallization of 3FmHPhH7 compounds (especially to check whether 3F3HPhH7 undergoes the glass transition in the smectic phase, as with its counterparts with longer alkyl chains) [10,11,12,13,14]. There are several studies where the influence of the chain length on the physical properties of homologs was discussed [15,16]. However, a systematic investigation to determine which features of the molecular structure are optimal for the occurrence of the glass transition may affect the planning of the syntheses of the liquid crystals with the desired properties for specific applications.

2. Materials and Methods

The 3FmHPhH7 compounds were synthesized in the Institute of Chemistry, Military University of Technology, Warsaw, Poland (details of synthesis are given in [17]). The purity of the samples was determined to be over 99.4% using the ASTM E928 Standard Test Method for Determining Purity by Differential Scanning Calorimetry with the use of TA DSC 2500.

POM textures were observed using a Leica DM2700P polarizing light microscope equipped with a Linkam LNP96-S heating/cooling stage and a Linkam T96-S temperature controller. The 3F3HPhH7 sample was placed on cover glass plates in the isotropic liquid phase and was cooled to 173 K at a rate of 10 K min⁻¹ and subsequently heated at a rate of 2 K min⁻¹. The thermooptical analysis (TOA) of textures was conducted in the TOApy program [18].

DSC measurements were performed using a TA DSC 2500 calorimeter under a nitrogen atmosphere. The indium and sapphire standards were used for calibration. The mass of the 3F3HPhH7 sample was 5.86 mg, and the molecular mass was 700.680 g/mol. The masses of other homologues are given in [10,12]. The sample was cooled and heated at several rates (±1, ±2, ±3, ±4, ±5, ±8, ±10, ±15, ±20, ±25 and ±30) K min⁻¹, in a temperature range from 173 K to the transition temperature to the isotropic phase.

X-ray diffraction patterns were registered using an X’Pert PRO (PANalytical, Malvern, UK) diffractometer with TTK-450 (Anton Paar, Graz, Austria) non-ambient stage in Bragg–Brentano geometry with CuKα radiation. Temperature was controlled with a TTK 450 (Anton Paar, Graz, Austria) attachment. The registration of one pattern took approx. 12 min plus 2 min to stabilize the temperature and the diffraction patterns were collected sequentially every 2 K or 5 K. The study was conducted using the FullProf program [19].

FTIR spectra were obtained using a Bio-Rad Digilab FTS 3000 Excalibur spectrometer, under a nitrogen atmosphere, in the wavenumber range of 4000–400 cm⁻¹, with a resolution ca. 0.4 cm⁻¹, with a number of scans of 64. The 3F3HPhH7 sample in the form of a thin film was placed between two zinc selenide ZnSe window discs. The sample was heated at the rate of 2 K min⁻¹ after fast cooling at a rate of 10 K min⁻¹, in the temperature range from 173 K to the transition temperature to the isotropic phase. The infrared spectra were taken in the aim to analyse band parameters: wavenumbers ν, full width at half maxima FWHM’s, areas S and intensities I.

3. Results and Discussion

3.1. Characteristics of Thermodynamic States of 3F3HPhH7 Compound

To determine the phase sequence of the 3F3HPhH7 compound, the DSC thermograms are registered with cooling/heating rates in a 5–30 K min⁻¹ range (Figure 2). For all applied cooling rates, we observe an anomaly at approx. 383 K with an enthalpy change of ∆H = 8.6 kJ mol⁻¹ and an entropy change of ∆S = 15.6 J mol⁻¹ K⁻¹ (values for the rate of 10 K min⁻¹), corresponding to the transition from the isotropic phase to the SmC_A* phase (Figure 2a). Due to the transition between the highly ordered chiral tilted smectic phase and isotropic phase, the ∆H of the clearing points is relatively high. The next anomaly, at 309 K, is related with the SmC_A*—Cr phase transition and it is characterized by very large thermal effects (∆H = 12.6 kJ mol⁻¹ and ∆S = 40.6 J mol⁻¹ K⁻¹, values for the rate of 10 K min⁻¹), Figure 2a. The entropy changes between the crystal and the SmC_A* phase is similar to the value obtained for the structurally similar compounds, for which the conformationally disordered (CONDIS) crystal phases were indicated [4,10,13,20]. To prove that the Cr phase in the 3F3HPhH7 is the CONDIS crystal type, we characterize the change in entropy $\mathrm{\Delta }{S}_{p.t.}$ of the first order phase transition as:

$$\mathrm{\Delta }{S}_{p.t.}=\mathrm{\Delta }{S}_c+\mathrm{\Delta }{S}_o+\mathrm{\Delta }{S}_p,$$

(1)

where $\mathrm{\Delta }{S}_c$, $\mathrm{\Delta }{S}_o$ and $\mathrm{\Delta }{S}_p$ correspond to the increase/decrease in entropy at the phase transition temperature given by freezing/activation of various degrees of freedom of molecules: conformational, orientational and positional, respectively. Usually, 7n $\le \mathrm{\Delta }{S}_c\le$ 12n J mol⁻¹ K⁻¹ (where n is a number of rotatable parts of the molecule), 20 $\le \mathrm{\Delta }{S}_o\le$ 50 J mol⁻¹ K⁻¹, and 7 $\le \mathrm{\Delta }{S}_p\le$ 14 J mol⁻¹ K⁻¹ [21,22]. A registered entropy change $\mathrm{\Delta }{S}_{p.t.}$ = 40.6 J mol⁻¹ K⁻¹ of the SmC_A*—Cr phase transition seems to be the sum of $\mathrm{\Delta }{S}_o$ = 30 J mol⁻¹ K⁻¹ and $\mathrm{\Delta }{S}_p$ = 10 J mol⁻¹ K⁻¹. Due to the estimated zero value of $\mathrm{\Delta }{S}_c$, the crystal phase in the 3F3HPhH7 is the CONDIS crystal type.

Further, the characteristic kink is observed at 247 K, which is the signature of the glass transition (Figure 3a) [23]. Many orientationally disordered (ODIC) as well as CONDIS crystal phases have a tendency to vitrification [24,25]. Upon heating, a wide anomaly with the onset temperature of 308 K is interpreted as the cold crystallization process (Figure 2b). The melting of Cr phase occurs at 357 K with the largest changes of enthalpy ∆H = 24.1 kJ mol⁻¹ and entropy ∆S = 67.2 J mol⁻¹ K⁻¹ (values for the rate of 10 K min⁻¹), (Figure 2b). The last anomaly, at 383 K, is associated with the SmC_A*—Iso phase transition (Figure 2b).

To confirm the phase behaviour of the 3F3HPhH7 compound, the representative textures characteristic for a given thermodynamic state are observed using the polarizing optical microscope (Figure 3a). These POM textures can be interpreted by numerical analysis, using ‘rgb’ and ‘gray-scale’ methods (Figure 3b); the procedures are described in [18]. The first method is useful when during the phase transitions, the POM textures change in colour. The textures collected at 173 K and 250 K upon heating of the sample (after previous fast cooling) belong to the glassy state of the crystal phase. The significant change in the light intensity in the range of 260–270 K may be interpreted as the softening of the gCr state. The occurrence of the local minimum on the ‘gray-scale’ curve is identified as a cold crystallization process. The melting of the Cr phase (the exampled texture is collected at 314 K) is associated with the decrease in the light intensity (with the texture at 358 K). The textures collected at 361 K and 375 K belong to the SmC_A* phase. The observed change in colour is caused by the continuous ordering of molecules in the smectic layers. The changes visible in the plots of the thermooptical analysis correlate well with the anomalies on the DSC curves. Comparing both ‘rgb’ and ‘gray-scale’ algorithms, one can see that the ‘rgb’ method provides much more sensitivity than the ‘gray-scale’ approach.

According to the DSC results obtained upon slow cooling/heating with a rate of 2 K min⁻¹, the 3F3HPhH7 compound under the given conditions does not undergo vitrification; moreover, during its cooling, two anomalies related to the crystallization of the sample are visible below 320 K (Figure 4a). The XRD patterns of the compound under study collected upon similar conditions confirm the existence of one crystal phase in this compound; the peak shifts are caused by a change in the unit cell parameters with temperature, and an increase in the intensity of thhe peak upon heating is due to a decreasing number of defects in the crystal phase (Figure 4b).

Figure 5 shows the temperature dependence of the smectic layer spacing of the 3F3HPhH7, determined from the position of the low-angle diffraction peak at 2θ ≈ 2.8°, according to the Bragg equation d = 0.5λ/sin(θ) [26], where λ = 1.5406 Å is the CuKα wavelength. The low-angle peak is absent in the isotropic liquid phase and in the crystal phase. In the SmC_A* phase, the layer spacing slowly increases with temperature, the values of d are in the range of 30.7(4)–31.2(5) Å.

The infrared spectra are analysed to elucidate the changes in the molecular interactions accompanying the phase transitions of the 3F3HPhH7 (Figure 6). The most sensitive bands to changes during the cold crystallization process are bands related with C–H vibrations. The asymmetric (above 2900 cm⁻¹) and symmetric (below 2900 cm⁻¹) C–H stretching vibrations exhibit an increase in intensities and high-wavenumber shift. The change in the band shape is also observed for the bending out of plane and wagging C–H vibrations (between 1500 and 1300 cm⁻¹) as well as rocking C–H mode (at 1100 and 800 cm⁻¹). These effects may be explained by the increase in the short-range order of molecules during the cold crystallization. The Cr–SmC_A* phase transition is generally associated with the decrease in intensity for all bands (except the symmetric C–C–O stretching vibrations at 1000 cm⁻¹). In addition, after the transition to the mesomorphic phase, a change in the shape of the rocking C–H band as well as the appearance of a new bending-in-plane C–H vibrations, both related with methyl groups, is observed. The mentioned effects may be caused by partial translational melting and the occurrence of changes in the motional freedom and intermolecular forces [27] due to the biggest values of enthalpy and entropy changes in the Cr–SmC_A* phase transition.

The bands associated with the stretching vibrations of the carbonyl group in the rigid core, ν(C=O)_core, and in the vicinity of the chiral centre, ν(C=O)_{chiral_c.}, are sensitive to various intramolecular interactions. Due to their good separation from other bands (Figure 6b) and a considerable distance from fluorine atoms, we discuss the vibrational dynamics of the 3F3HPhH7 as variations of these band parameters, e.g., positions ν, full width at half maxima FWHM’s, areas S and intensities I (Figure 7). The mentioned bands are fitted by the Lorentz function. The cold crystallization process is related with the high wavenumber shift of both bands and the increase in I, while the values of FWHM and S gradually decrease. The observed changes may be explained by the sensitivity of carbonyl groups to the short-range order of molecules, which is first built up during the crystallization process. In the Cr phase, an increase in ν is visible for both bands; however, the other parameters behave differently: (i) the FWHM factor shows the minimum for the ν(C=O)_{chiral_c.}, while it decreases for ν(C=O)_core; (ii) the opposite situation is seen for the I parameter; (iii) the S factor exhibits inflection points at the same temperature, at which the I parameter has the maximum for the ν(C=O)_{chiral_c.}. The slight fluctuations in the parameters of both bands in the crystal phases may result from an incomplete ordering within the crystal structure with the rotation of whole molecules still active. In the SmC_A* phase, a sudden growth in the values of all band parameters is registered (only the intensity of the ν(C=O)_{chiral_c.} decreases). The bands are broadened in the mesomorphic phase, as the molecules recover partial rotational and translational degrees of freedom. The C=O groups play a significant role in the discussed phase transitions not only of the 3F3HPhH7, but also for its homologues [10,28].

3.2. Kinetics of Non-Isothermal Cold Crystallization of 3F3HPhH7 Compound

Many substances do not tend to crystallize, even on slow cooling, but rather undergo supercooling to a glassy state. Around the glass transition temperature, T_g, the stochastic movements are slowed down or frozen [29]. Such vitrified material can subsequently crystallize upon heating at the cold crystallization temperature, T_cc. The cold crystallization process is usually described under non- and isothermal conditions. The isothermal cold crystallization process will be the subject of another paper.

The non-isothermal cold crystallization of the 3F3HPhH7 is analyzed based on DSC thermograms obtained for different heating rates of the sample (Figure 2b). This process depends on the thermal history of the material due to the shifting of characteristic wide exothermic anomalies toward higher temperatures and increasing their intensities as the heating rate increases. The cold crystallization degree vs. temperature D(T) for each heating rate dT/dt can be calculated from the DSC curves according to the formula [30]:

$$D\left(T\right)=\frac{{{\displaystyle \int }}_{T_{start}}^T\left(\mathrm{d}T/\mathrm{d}t\right)\mathrm{d}T}{{{\displaystyle \int }}_{T_{start}}^{T_{end}}\left(\mathrm{d}T/\mathrm{d}t\right)\mathrm{d}T},$$

(2)

where T_start and T_end are the beginning and ending temperatures of the crystallization process. Knowing the degree of crystallization, it is possible to obtain more information about the cold crystallization mechanism, according to the logarithmic form of the Ozawa model [31]:

$$\log \left(-\ln \left(1-D\left(T\right)\right)\right)=\log \left(Z\right)-n_O\log \left(\mathrm{d}T/\mathrm{d}t\right),$$

(3)

where Z is the Ozawa crystallization rate and n_O is the Ozawa exponent depending on the crystal size. Ozawa plots demonstrate a linear behaviour, in which the slope corresponding to the n_O parameter is reduced from n_O = 1.09 ± 0.08 (T = 317 K) to n_O = 0.61 ± 0.09 (T = 322 K); the intercept with the vertical axis is the log(Z) parameter and it decreases with increasing temperature (Figure 8a). According to the characteristic of the log(Z) parameter, we conclude that the non-isothermal cold crystallization depends mainly on the nucleation [32]. For the homologues of the 3F3HPhH7, e.g., for 3F5HPhH7 and 3F7HPhH7 compounds with longer alkyl chains, this process was found to be driven by two different mechanisms (diffusion-controlled for slow heating and nucleation-controlled for fast heating) [11,12].

To confirm that for the 3F3HPhH7 cold crystallization occurs according to one mechanism, regardless of the heating rate, we use the logarithmic form of the Mo model [33]:

$$\log \left(\mathrm{d}T/\mathrm{d}t\right)=\log \left(F\right)-a·\log \left(t-t_o\right),$$

(4)

where a is the ratio of Avrami n_A exponent to Ozawa n_O exponent, F is related to the heating rate and t₀ is the induction time of crystallization. Linear dependences of Mo plots over the entire range of the heating rate indicate the one mechanism of the non-isothermal process (Figure 8b). The values of log(F) necessary to achieve the given crystallization degree increase with increasing D(T), while the a parameter is almost independent of the degree of crystallization. The ratio between the Ozawa and Avrami exponents suggests that the shape of crystallites obtained in the non-isothermal conditions is more isotropic resembling growth reported in polymers, liquid crystals and other low-weight organic compounds [34,35].

The activation energy E_A of the cold crystallization process under non-isothermal conditions may be determined based on the Kissinger formula [36]:

$$\ln \left(\frac{\mathrm{d}T/\mathrm{d}t}{T_p^2}\right)=-\frac{E_A}{R{T}_p}+const,$$

(5)

or the Augis-Bennett equation [37]:

$$\ln \left(\frac{\mathrm{d}T/\mathrm{d}t}{T_p-T_o}\right)=-\frac{E_A}{R{T}_p}+const,$$

(6)

where T_p is temperature corresponding to the maximum of the exothermic anomaly in a thermogram, and T_o is the initial cold crystallization temperature. Since in the Kissinger and Augis–Bennett plots only one linear dependence is observed, we can assume that for all applied heating rates the mechanism of the cold crystallization is the same (Figure 8c). The value of the activation energy is (265 ± 11) kJ mol⁻¹ for the first model and (247 ± 28) kJ mol⁻¹ for the second. The obtained differences may result from the specificity of the used methods (the Kissinger model is derived for first order reactions, while the Augis–Bennett model is more general). An equally high value of the activation energy was obtained for the non-isothermal crystallization of the ordered crystalline smectic B phase in BBOA compound [38].

3.3. Impact of the Carbon Chain Length on Phase Behaviour and Kinetics of the Crystallization of 3FmHPhH7 Compounds

Recent research showed that the phase situation of the 3F5HPhH7 compound upon fast cooling is as follows: isotropic phase (Iso)—389 K—SmC*—380 K—SmC_A*—275 K—hexatic smectic SmX_A* phase—244 K—gSmX_A* state, while slow cooling leads to the melt crystallization (i.e., crystallization observing during cooling) of the sample at 270 K [10,11]. Upon heating after fast cooling, additionally the cold crystallization process and the melting of two co-existing crystal phases (Cr2 and Cr1) are observed: gSmX_A*—265 K—Cr2—305 K—Cr1—335 K—SmC_A*—380 K—SmC*—389 K—Iso. The carried out results for the 3F7HPhH7 compound reveal three enantiotropic smectic phases: SmA, SmC* and SmC_A* [12]. Upon fast cooling, the SmC_A* phase vitrifies: Iso—390 K—SmA—388 K—SmC*—374 K—SmC_A*—259 K—gSmC_A* state. Additionally, the broadband dielectric spectroscopy (BDS) measurements show the existence of two glass transitions at T_g,1 = 259 K and T_g,2 = 239 K [13]. The melt crystallization upon slow cooling is observed at 286 K. The phase behaviour upon heating after fast cooling is as follows: gSmC_A*—269 K—Cr—331 K—SmC_A*—380 K—SmC*—389 K—SmA—390 K—Iso.

The investigation reported here and [10,11,12,13,14] allows to demonstrate that the physical properties of the studied mesogens depend significantly on the carbon chain length. The diagrams in Figure 9 show the phase sequence of three 3FmHPhH7 homologues. It is observed that as the length of the alkyl chain increases, a given liquid crystal possess a greater number of mesophases: (i) the 3F3HPhH7 has only the SmC_A* phase, (ii) the 3F5HPhH7 exhibits the SmC_A* and SmC* phases as well as the monotropic SmX_A* phase, (iii) the 3F7HPhH7 possesses the SmC_A*, SmC* and SmA phases. The length of the (-CH₂-)_m changes the phase transition temperatures, for example, as the ’m’ parameter increases, a given compound goes into the isotropic liquid phase at a higher temperature. All examined 3FmHPhH7 samples form the SmC_A* phase; the temperature range of occurrence increases with the length of the molecule. The number of methylene groups also affects the glass transition. The compound with the shortest alkyl chain is vitrified from the ordered crystal phase. For the compound with five -CH₂- groups, a glass transition from the monotropic high-order hexatic smectic SmX_A* phase is observed. In the case of the liquid crystal with the longest carbon chain, the vitrification from the less ordered SmC_A* phase is visible. On the other hand, slow cooling of all samples leads to theirs melt crystallization. These results allow to conclude that the length of the (-CH₂-)_m flexible chain influences not only the polymorphism of the smectic phases, but also the vitrification and crystallization processes.

As aforementioned, the alkyl chain length of the 3FmHPhH7 compounds influences the occurrence of the cold crystallization. For the compound with ‘m’ = 3, this process depends primarily on the nucleation with the activation energy above 250 kJ mol⁻¹ (Figure 10). For the samples with ‘m’ = 5 and 7, the non-isothermal cold crystallization is a complex phenomenon, controlled by diffusion rates for slow heating and by nucleation for fast heating. The values of the activation energies are much smaller than for the 3F3HPhH7. An interesting observation is made regarding the length of the molecule vs. the cold crystallization temperature, T_cc, as well as the glass transition temperature, T_g, and the melt crystallization temperature, T_mc (Figure 10). In each case, the lowest temperature value is recorded for the 3F5HPhH7. The distinctive properties of the homologues with ‘m’ = 5 are also described in Refs. [8,15,39], where the influence of the oligomethylene spacer on the physical properties and on the helical twist sense in the SmC_A* phase of the 3FmHPhF6 compounds was investigated. The above implies that further investigations of the series of the 3FmHPhH7 compounds are necessary to check which factors are responsible for the abnormal behaviour of compounds with five -CH₂- groups. Presumably, the cause lies in the intermolecular interactions, and more information about these interactions and the ordering of molecules in smectic layers can be obtained from measurements using synchrotron radiation.

4. Summary and Conclusions

Investigations of the compounds from the 3FmHPhH7 family (where ‘m’ = 3, 5 or 7) prove that the length of the oligomethylene spacer has a significant impact on some physical properties, such as phase behaviour or crystallization kinetics. All compounds crystallize upon slow cooling with the lowest melt crystallization temperature for the 3F5HPhH7. However, fast cooling of the samples leads to the glass transition: (i) the vitrification of the 3F3HPhH7 is visible at 247 K from the CONDIS crystal phase, (ii) the glass transition of the 3F5HPhH7 is observed at 244 K from the monotropic high-order hexatic smectic SmX_A* phase, which can be either SmI_A* or SmF_A* phase [10], (iii) the 3F7HPhH7 is vitrified from the less ordered SmC_A* phase at 259 K (the second glass transition temperature, determined from BDS measurements, is 239 K) [13]. The increase in the number of methylene groups also affects a greater number of mesophases: (i) the SmC_A* phase for the 3F3HPhH7, (ii) the SmC_A* and SmC* phases as well as the monotropic SmX_A* phase for the 3F5HPhH7, and (iii) three SmC_A*, SmC* and SmA phases for the 3F7HPhH7. The temperature range of the SmC_A* phase occurrence increases with the length of the molecule.

The vitrified 3FmHPhH7 compounds can subsequently crystallize upon heating at the lowest cold crystallization temperature for the 3F5HPhH7. This process under non-isothermal conditions was found to be driven by only nucleation-controlled mechanisms for 3F3HPhH7, while for 3F5HPhH7 and 3F7HPhH7 it is a complex phenomenon, controlled by diffusion rates for slow heating and by nucleation for fast heating [11,13]. The activation energies of the non-isothermal cold crystallization decrease as the length of the alkyl chain increases. For all homologues, the C–H, C=O and C–C–O absorption bands are the most sensitive region on infrared spectra to the structural changes characteristic of appropriate changes between thermodynamic states, especially during the cold crystallization [10,28].

In general, the investigations show the correlation between the alkyl chain length ‘m’ and the phase behaviour as well as crystallization kinetics. However, some deviations are observed for the 3F5HPhH7, e.g., the lowest values of T_mc, T_g and T_cc temperatures compared with other homologues. Presumably, it is important to take into account different molecular conformations, intermolecular interactions and dielectric properties for a complete discussion of the compounds’ physical properties. The above implies that further investigations in the series of the 3FmHPhH7 compounds are necessary.