Comparison of short range interactions in nematic liquid crystals A²H-NMR study of 5CB-d₁₉as a solute

Abstract

Previous studies using dideuterium as a solute have demonstrated the importance for orientation of the interaction between the solute molecular quadrupole moment and the average electric field gradient present in liquid crystals. With the aim of learning about additional orientational mechanisms, we have studied the temperature dependence of the 'H-NMR spectra of the liquid crystal 5CB-di9 as the solute in three liquid crystal mixtures: 55wt% 1132EBBA, 56.5 wt % 1132EBBA and 70 wt % SCBEBBA. In these mixtures, the contribution from the environment to the average electric field gradient at the *H nucleus of dideuterium is zero. The spectra of 5CB-di9 in the mixtures 55 wt % 1132EBBA and 56.5 wt % 1132EBBA are equivalent, but are different from those in 70 wt % SCBEBBA. The spectra of 5CB in 55 wt % 11 32EBBA and 70 wt % 5CBEBBA are analysed using two different models for the short range potential, and parameters of the models are used to compare the potentials in the different mixtures. For a given spectral splitting of the chain CI deuteron, the reduced short range potential is the same in all three mixtures studied. The spectral differences observed are a consequence of different nematic-isotropic phase transition temperatures combined with the effect of transfgauche-isomerization in the hydrocarbon chain. IntroductionA detailed understanding of anisotropic intermolecular interactions leading to orientational ordering in liquid crystal systems is not available. The situation is complicated by the size and flexibility of the liquid crystal molecules. In many cases, the intermolecular interactions are described by a mean field theory [ 1-31 in which the interaction potential is expressed in terms of model parameters.The anisotropic interactions lead to an orientational ordering of the liquid crystal molecules. This orientational ordering can be described in terms of order parameters. Second rank order parameters can be measured by various techniques, including NMR (nuclear magnetic resonance) spectroscopy. NMR measurements can also be used to obtain information about order parameters of solutes in liquid crystal solvents [4,5].The use of small solutes in liquid crystal solvents has provided a useful route for investigating the intermolecular interactions in these systems. In the past, our group has proposed two important mechanisms for the intermolecular interactions acting between the solute and the liquid crystal molecules. First, the electric quadrupole moment of the solute interacts anisotropically with the average electric field gradient that the solute experiences in the liquid crystal environment. The second mechanism involves short range interactions that depend on the size and shape of the solute [6,7].

A temperature dependence study of small solutes in zero electric field gradient nematic liquid crystal mixtures

“…These E tg values are about 0´5 kJ mol -1 smaller, and the rms value about 0´15 kHz larger, than obtained in [32] when x   was used as a ® tting parameter.…”

“…A previous 2 H NMR temperature dependence study of the perdeuterated liquid crystal molecule 5CB as solute in the same`zero efg mixtures' as used in this study indicates that the orientational potential can be described by a short-range mechanism for these mixtures [32]. The experimental order parameters S a ,b of a solute in the`zero efg mixtures' are related to the shortrange potential U sr (X ) by Solutes in zero efg nematic mixtures 307 …”

“…In the published analysis of the temperature dependence of the deuteron NMR of the liquid crystal molecule 5CB as solute in the same`zero efg mixtures' as used in this study, the parameters x   and E tg were found to be heavily correlated [32]. The extra parameter, the trans± gauche energy di erence E tg , is needed owing to the¯exible chain part of 5CB.…”

“…In our calculations on 5CB, the ® rst e ect raises the value for E tg by about 6%, while the second lowers it by about 5%. Hence, the e ects of these two factors which were not included in the original calculations for 5CB [32]cancel. The ® tted E tg value for the 55 wt% 1132± EBBA mixture is 3´59 6 0´04 kJ mol -1 and that of the 70 wt% 5CB± EBBA mixture is 3´55 6 0´04 kJ mol -1 .…”

“…Thus it is useful to use the x   value obtained from our study and to reanalyse the 5CB data to estimate a value for E tg . The 5CB calculation was performed by varying the parameters k s and E tg , where the variation in k s with temperature is given by the same functional form as in [32]. The results are presented in table 6.…”