Correlated NMR and EPR studies in a nematic liquid crystal

Solid State Communications,

Vol.8, pp. 1577—1581, 1970.

Pergamon Press.

Printed in (Ireat britain

CORRELATED NMR AND EPR STUDIES IN A NEMATIC LIQUID CRYSTAL* Ronald Y. Dong Department of Statistics, University of Waterloo, Waterloo, Canada and M. Maru~i~ and C. F. Schwerdtfeger Department of Physics, University of British Columbia, Vancouver, Canada (Received 24 July 1970 by R. Barrie)

The proton spin-lattice relaxation time T~has been studied as a function of temperature and frequency in a nematic liquid crystal which has its mesophase from room temperature to ~85°C. It is found that a change in the dominant mechanism of T~occurs at -~‘69°C. This correlated with EPR measurements on vanadyl acetylacetonate dissolved in the same liquid crystal which indicates that at this temperature the molecules pass from weakly to strongly hindered motion. These results are discussed in light of the recent suggestion of a possible uniaxial-to-biaxial nematic phase transition.

SEVERAL papers have now appeared ~ reporting measurements of proton spin-lattice relaxation times in nematic liquid crystals. The general conclusion of these investigations is that the temperature dependence of T1 does not agree

the spin relaxation in the rotating frame, but gives a relaxation rate which again increases with increasing temperature in the laboratory frame.

with that predicted by a theory based on the mechanism of the dipolar interactions between spins6 being modulatedfrequency by small dependence thermal fluctuatThe predicted ions. v “~ofT’ is also not observed, the actual ‘value in Para-azoxyanisole (PAA) reported earlier 3being closer to v~’~’.5It has been suggested3 that better agreement on the temperature

The present T measurements in 4-methoxy benzylidene-4-amino-a--methyl cinnamic acid-n-propyl ester support above conclusions in the high temperature range (69—85°C). However, as the temperature is decreased, a definite change in the relaxation mechanisms occurs and at room temperature the frequency dependence of the relaxation rate (T 1~)has the form of A + B v 1 A preliminary report of some of these results has already appeared in j~4 Owing to the unusual behavior of T1, the transition temperature from the to thetaken in isotropic liquid phases wasnematic mistakenly Ito be ~69°C. The true clear point of an evacuated sample is, however, ~85°C. The radical change in T at ~69°C is believed to be due to a change in the dominant relaxation

dependence might be obtained by assuming diffusion to be the most effective mechanism causing the spins to relax. Recent spin relaxation studies 5onindicated a partially crystal thatzone-refined translationalliquid diffusion of PAA in the nematic phase contributes appreciably to ___________

* Research supported by the National Research Council of Canada.

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STUDIES IN A NEMATIC LIQUID CRYSTAL

mechanism. Possible mechanisms involved are suggested by the interpretation of the EPR signal of vanadyl acetylacetonate dissolved in the present liquid crystal ~ and of the signals temperature dependence. These are reported here, At higher temperatures (T > 69 °C), the rod-like molecules are free to diffuse along each other and have free rotation about the long molecular axis. Here the dominant mechanisms for spin relaxation are most likely translational diffusion and critical fluctuations of the order parameter S. As the temperature is lowered, the EPR results indicate that the moleculus become hindered in their motion, thus giving rise to a fixed or ‘glassy’ type EPR spectrum. This hindered motion suggests an increase in the order of the medium and thus an increase in the spin relaxation rate. A possible explanation of the observed behavior at about 69°C could be a second order phase transition from an uniaxial to a biaxial mematic phase. Such a transition has been predicted recently8 for nematic liquid crystals and effects thought to be due to such a transition have been observed in PAA ~ 8, ~ at 128°C. In a biaxial nematic phase, occurring at lower temperatures, one defines the degree of order as S 2 6~— 1 > where 1/2 K 3 cos = 1 and 12 denote fluctuations parallel and perpendicular to the optic axis respectively, Hence, the intra-molecular dipolar reiaxation, previously arising from fluctuations in one order parameter6, should be replaced by a process involving fluctuations in two order parameters. The biaxial nematic phase would have then an additional degree of freedom in thermal fluctuations causing a faster relaxation to occur while at the same time, reducing the degree of freedom in translational diffusion. The glassy type EPR spectra would support this latter conclusion, The experimental arrangement for measuring

T1 has been described in 1. The spin relaxation measurements in the rotating frame (T1~)were made with a conventional coherent pulse spectrometer at 18.2 MHz. The spin-locking field was set at six gauss. The EPR results were obtained at 9.1 kMHz using 100 kHz field modulation. Field positions were measured to ±0.1control G with a proton magnetometer. The temperature

Vol. 8, No. 19

apparatus was similar to that used in the NMR measurements. The cavity was placed in a glass sleeve with a heater coil wrapped around the sleeve and this combination was placed in a glass dewar. The modulation coils were mounted on the cavity. The power dissipated by the 10 G modulation employed raised the temperature noticeably and the system required about one half hour to come to equilibrium after the temperature was changed. The temperature measuring thermistor was placed in the sample liquid just outside the cavity The thermistors were very magnetic but in this arrangement did not affect the large linewidth spectrum (~“25 G). The temperature could be held constant to within ±0.1°C and the difference across the sample was less than 0.5°C. The liquid crystal was used as obtained. For an unevacuated sample the melting point was measured to be 54 °Con a Perkin—Elmer DSC—1B direct scanning calorimeter and the clear point to be B 83°C. The values quoted by Demus’°are 54°Cand 89°C respectively. Upon the addition of 10”3M vanadyl acetylacetonate the clear point dropped to 78°Cwhile the melting point remained the same as before within errors. A microanalysis was made on the liquid crystal. The analyzed percentages for C, H and N were 74.5, 6.75 and 4.0, which compare favourably with the theoretically expected values of 74.8 per cent, 6.8 per cent and 4.2 per cent respectively. The.proton spin-lattice relaxation time T~ at 13 and 24.7 MHz vs. reciprocal of temperature are shown in Fig. 1. In the temperature range from 25°C to about 55 °C, T 1 increases linearly. Then there is a steeper rise in T1 until about 69°C where it again assumes a more or less linear dependence with almost the same slope as that at the low temperature e~x1.This suggests that between 55°C and 69°C a different mechaniSm for spin relaxation is introduced. The T1pdata show a temperature dependence similar to that of T~except that the increase in T1~ between 55°C and 69°C is not noticeably sharp and the values of seem to decrease near the clear point. This temperature dependence of T1~near5and the clear point been observed recently perhaps is has a manifestation of the in PAA fluctuation of the order parameter. critical

Vol. 8, No. 19

STUDIES IN A NEMATIC LIQUID CRYSTAL

.

That at least two mechanisms are operative in the nematic liquid phase is evident from the frequency dependence of Tç~’ at room temperature plotted in Fig. 2. The experimental data are fitted to a straight line of the form A + Bv’. Thus the relaxation rate is composed of a frequency independent component A which represents

component a relaxation white erature. spectral Br’ rate region ‘. whose Bothand Aspectral and a frequency B depend density dependent onistempin its

20

7~

1579

-

05/

.

300~

0

ISOTROPIC L

2OoQUID

40 I

~I

60I

80

FIG. 2. Frequency dependence of

~~TIc~ASE

T

experimental points at 298°K. The 1.

•,

straight line is a plot ~ 103 = MHz). 2.4 + 1sec~( ii in unitsof of 220~

loor 0

2.7

I

2.9

+

0.30-

3.3

3.1

I0~ (‘K)~ 0.25

-

-

FIG. 1. Proton relaxation times T

1 and T1~vs. reciprocal of the temperature. The T1 measurements are at 13 MHz, x;

o.~o

and 24.7 MHz,.. The T1~measurement was made at 18.2 MHz, o, with a spin-locking field of 6 gauss.

o.I5f_

• •

The EPR results for vanadyl acetylacetonate dissolved in the liquid crystal as a function of temperature are shown in Fig. 3. The order 7 parameter was obtained asmotionally previouslyaveraged described. The order parameter for a

S

60

I

I

70 T ‘C

50

•~

90

FIG. 3. Order parameter ~Seff versus temperature for iO~molar vanadyl acetylacetonate dissolved in the liquid crystal.

spectrum is given by S

.~

<3 cos2 8— 1>

.~

(

—

a)/(a —A 1)

(1)

where is the measured hyperiine coupling parameter, a = 1/3(A,, + 2A~~ )and A,1, A1 are the parallel and 7perpendicular components the The discontinuity in the of slope hyperfine tensor.

of the curve at

66°C shows the onset of

hindered molecular motion. The temperature is slightly less than that observed in the T1

measurements liquid crystal and is very likely dueoftothe thepure 10 3molar paramagnetic

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STUDIES IN A NEMATIC LIQUID CRYSTAL

impurity. Equation (1) is strictly true only for complete motional averaging. In the present liquid crystal even at the clear point of 78 °C the viscosity is large enough to cause incomplete averaging. This is the reason that in Fig. 3 one does not notice an abrupt change in S at the clearing temperature. We have, however, plotted the values of S calculated from equation (1) to emphasise the change in molecular motion which occurs at 66°C but we have relabelled the parameter S effective. That this discontinuity in slope at 66°C is more than a viscosity effect has been checked by measuring the hyperfine interaction of vanadyl acetylacetonate dissolved in an oil of comparable viscosity to the liquid crystal as a function of viscosity (i.e., temperature). A complete study of this effect in different liquid crystals is in progress. The curve in Fig. 3 is very similar to that recently observed in another viscous liquid crystal ~ and the remarks made here could possibly pertain to that liquid crystal as well. We would suggest

Vol. 8, No. 19

caution in obtaining

the order parameter from

equation (1) in viscous liquid crystals. In conclusion, we find that a definite change in the nematic mesophase of the liquid crystal takes place at about 69°C. We suggest that this change may be the second order transition 8) Such from a uniaxial to a biaxial state. a transition would introduce a second order parameter. The errors inherent in obtaining S from a glassy type EPR spectrum and from NMR line moment measurements would make it extremely difficult to observe this second order parameter using these techniques even under the application of an electric field. Recent optical methods,’2 however, have proven to be very sensitive and could possibly show this second order parameter in a direct way. Acknowledgements — We would like to thank Drs. M. Bloom, R. Barrie and D. Balzarini for n~anyfruitful discussions.

REFERENCES 1.

WEGER M. and CABANE P., J. Phys. 30, C4—72 (1969).

2.

BLINC R., HOGENBOOM D.L., O’REILLY D. E. and PETERSON E.M., Phys. Rev. Lett. 23, 969 (1969).

3.

DUANE

4.

DUNG R. Y. and SCHWERDTFEGER C. F., (Henceforth designated I).

5.

DUNG R. Y., FORBES W. F. and PINTAR M. M., Solid State Commun. (to be published).

6.

PINCUS P., Solid State Cominun. 7, 415 (1969).

7.

SCHWERDTFEGER C. F. and DIEHL P., Molec. Phys. 17, 417 (1969).

8.

FREISER M.J., Phys. Rev. Leit. 24, 1041 (1970).

9.

BARRALL E.M., PORTER R. S. and JOHNSON

J. W.

and VISINTAINER

J.J.,

Phys. Re\. Leit. 23, 1421 (1969). SO/Id

State Commuri. 8, 707 (1970).

J. F., J. Phys. Chem.

71, 895 (1967).

10.

DEMUS D., Z. Naturf. 22A, 285 (1967).

11.

FRYBURG G. C. and GELER1NTER E., J. Chem. Phys. 52, 3378 (1970).

12.

BALZARINI D., private communication.

Vol. 8, No. 19

STUDIES IN A NEMATIC LIQUID CRYSTAL Le temps de relaxation spin-réseau T des protons d’un cristal liquide dans la phase nématique a été mesur~en fonction de la tempêrature et de la fréquence. Cette phase est stable entre la temperature ambiante et la temperature de 85°C. On a établi qu’un changement du mécanisme dominant de relaxation ~ lieu a 69 °C.

On peut rapprocher ce rCsultat de celui obtenu par la resonance paramagn~tiquepour le cas de l’acetylacetonate de vanadyl dissous dans le mime cristal liquide, oü l’on trouve qu’ã cette temp~rature l’emp?chement du mouvement moI~culairedevient soudainement trés fort. Ces résultats sont discut~sen tenant compte de la possibilite proposée récemment, d’une transition de la symétrie uniaxe ~ Ia

symétrie biaxe dans la phase nématique.

1581