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Proton spin-lattice relaxation in the solid, nematic and isotropic phases of EBBA
Volume
24, number 2
15 January 1974
CHEMICAL PHYSICS LETTERS
PROTON SPIN-LATTICE RELAXATION IN THE SOLID, NEMATIC AND ISOTROPIC PHASES OF EBBA SD. GOREN, C. KORN Deportment of Pil),sics, University of the Negev,
Beer Shevo. Israel
and S.B. MARKS and R. POTASHNIK Department of Chemistry, University of the Negev. Beer Sheva, Israel Received
22 October
1973
The proton spin-lattice relaxation time has been measured in the solid, nematic and isotropic phases of EBBA [N-~~thoxybenzylidene)9_butylanilinel. TWO IIIinima Were found in the te.nperature dependence of T, in the solid phase. In !he nematic and isotropic phase, the relaxation mechanism can be c!wracterized by activation energies of 5.9 kul/mole and 8.8 k&/mole, respectively.
1. Introduction
p-butylaniline] . EBBA is obtained from MBBA by adding a CH2 group to the methoxy end group of
The question of the mechanism of proton spin-lattice relaxation in the nematic phase of liquid crystals has lately come under close scrutiny. The main relaxation mechanisms attributed to this phase are those of the fluctuation of the order parameter [l] and the fluctuation of the dipolar spin interactions caused by molecular diffusion [2] _ By measuring T, as a function of frequency Vilfan et al. [3] have shown, in their definitive paper, that the dominant relaxation mechanism for PAA (p-azoxyanisole), is the fluctuation of the order parameter while MBBA [N-@methoxybenzylidene)-p-butylaniline] exhibits a frequency dependence characteristic of relaxation by molecular diffusion. Furthermore, it has been shown that by adding CH, groups to the end group of PAA, T1 is significantly lowered [4] _Vilfan et al. [3] argued that by lengthening the molecule an additional relaxation mechtism is introduced and its behavior is similar to that of MBBA. In order to investigate the effect of an addition of an &d groiip to MBBA, we have measured the temperature dependence of Ti in EBBA [N-@-eth~xybknzylid~ne>-
MBBA.
2. Experimental A sample of EBBA obtained from Eastman Kodak Co. was recrystallized twice from ethanol. The sample was degassed under vacuum and sealed off. T1 was measured on a Bruker variable frequency pulse spectrometer at 24 MHz over a temperature range from 130°K to 390°K using the ISO”-90” zeroing technique. The temperature range included the solid, nematic and isotropic phases.
3. Results
.
Fig. I ygives T1 a~ a function of temperature for the solid, &matic and isotropic phases. A double minimum Can be discdrried in the solid phase which we as&n to the reorientation relaxation dtie to the E3 :and,Mi ..
.:,
Volume
CHEhfICAL
24, number 2
PHYSICS LETTERS
15 January 1974
\ j
NEMATIC
ISOTROPIC
PHASE
PHASE
120
140
160
180
200
220
240
Fig. 1_ A plot of TX versus T for the solid, nematic and isotropic cooling. Solid lines are drawn through the experimenral points.
groups. Fig. 1 shows the usual [5] levelling off of Tl as the nematic phase is approached. A sharp change in Tl is observed at 305°K when the sample is heated and at 291 OK when it is cooled, corresponding to the transition to the nematic phase upon heating and formation of a supercooled nematic phase upon cooling_ T1 rises monotonically with the temperature indicating the strong temperature dependence of this parameter. As usual [S] T, drops somewhat at the nematic-isotropic transition and increases with the temperature in the isotropic phase.
4. Discussion In fig. 2 we have plotted the logarithm of the relaxation rate as a function of the reciprocal temperature. Vilfan et al. [3] have shown from their frequency dependence measurements that the relaxation m&hanism in the nematic phase of MBBA is the diffusion controlled intermolecular dipole interactions. This can be written as PI
._....
260
280
300
i
,
320
:
1
340
I
360
1
380 400 - T(%)
phases of EBBA. The circles indicate heating and the squares
where A, B and Care constants and D, the diffusion coefficient, contains the temperature dependence. If the temperature is sufficientIy high and v sufficiently low, the 4 power term can be neglected and In Ti’ versus T-1 should yield a straight line in accordance with the Arrhenius type dependence of the diffusion coefficient D. Estimates from the results of Vilfan et al. [3] on the frequency dependence of T, for MBBA show that the 3 power term can be neglected and the linear dependence in the low temperature region of the nematic phase of fig. 2 with temperature cornfirms this approximation in OUTcase. The rounding off of In Ti’ near the nematic-isotropic phase transition is attributed to the formation of isotropic regions within the nematic phase. It is clear that an activation energy for diffusion can be deduced from the linear portions of fig. 2. We fmd that the activation energy for the nematic phase is 5.9 + 1 kcal/mole compared to 6 f 1 kcal/mole as estimated for.MBBA from the work of Watson and Johnson [S] 1 Thus, adding CH, to MBBA has very little effect
.. .:
:..;. .:
_,
. .._ :
‘.....
._-.
.:
,.-:
”
.:,:‘.
..-
15 January 1974
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
10
9
l -Cooling
a
.-Heating
7 6 5 4 103 -_(rei’l fl
3
I
i 2.6
I
2.7
Ii
2.8
I
,
I
II
,
I
I
I
I
2.Q
3.0
3.1
3.2
3.3
3.4
3.8
3.6
3.7
1
I
3.8 33Q +Ok-l)
Fig. 2. A sernllog plot of 2’1 versus rhe reciprocal temperature for the isotropic and nematic phases of EBBA. The circles indicate heating and the squares cooling. Solid lines are drawn through the experimental points. on the relaxation
behavior. A significant increase in the activation energy was found in going from the nematic to the isotropic phase, the activation energy for the latter being 8.8 + 1 kcal/mole. The result for MBBA [5] is only 7 kcal/mole [S] _The increase of the activation energy during the nematic-isotropic transition is consistent with the drop in Tr when going from the nematic to isotropic phase because of the slower diffusion in the latter phase. This has also been pointed out [S] for the case of MBBA. The relaxation mechanisms for the nematic phase of liquid crystals can be classified as follows. vilfan et al. [3] have shown from the frequency dependence of T1 that the relaxation mechanism in PAA molecules is due to the fluctuation of the order parameter. This mechanism has been shown to be almost independent of temperature [6] and has been confirmed experimentally for PAA [4,5,7,8] _The frequency dependence of T, for MBBA shows [3] that its relaxation mechanism depends on diffusion, thus also explaining
the expected large temperature dependence [5,9] of T1 for this molecule. Adding end chains to PAA reduces T1 considerably [4] and it has been implied by Vilfan et al. [3J that this can be attributed to a mechanism similar to that found for MBBA. This is, however, inconsistent with the essentially temperature independent Tr found in these liquid crystals [41. Thus, the exact reason for the decrease of Tt in the homologs of PAA has yet to be clarified. We see from this study that the addition of an end chain to MBBA has very little influ-
ence on the relaxation behavior of the cIysta1 in the nematic phase. It is possible that the reason that the diffusion mechanism dominates in MBBA while order fluctuation is the corrtrohing factor in PAA is due to the different temperature ranges for the nematic phases. In general the nematic phases for PAA and its homologs occui at higher temperatures than the MBBA homologs and at 1 these temperatures the diffusion is too fast to effec- ..
tively relax the nuclei.
.. 251
VoIume 24, number 2
CHEMICAL PHYSICS LElTERs
References [II C.C Sung. Chem. Phys. Letters IO (1971) 35. [21 J-F- Harmon and B.N. Muller, Phys. Rev. 182 (I 969) 400. I31 M. Vilfan, R. Blinc and J-W_ Doane, Solid State Commun. 1I (1972) 1073. 141 CR. Dybowski, B.A. Smith and C.G. Wade, J. Phys. Chem. 75 (1971) 3834. [S] CL. Watkins and C.S. Johnson Jr., J. Phys. Chem. 7.5 (1971) 2452.
c
15 Jaauary 1974
161 J-W. Deane and D-L. Johnson, Chem. Phys. Letters 6 (197? 291. [71 R-Y. Dong, W.F. Forbes and h1.M. Antar, J. Chem. Phys. 5.5 (1971) 145. [8 ] R. Btinc, D-L. Hogenboom, D-E. O’ReilIy and EM Peterson, Phys. Rev. Letters 23 (1969) 969. 191 R.Y. Dong, W.F. Forbes and M.M. Pintar, Mol. Cryst. Liquid Cryst. 16 (1972) 213.
24, number 2
15 January 1974
CHEMICAL PHYSICS LETTERS
PROTON SPIN-LATTICE RELAXATION IN THE SOLID, NEMATIC AND ISOTROPIC PHASES OF EBBA SD. GOREN, C. KORN Deportment of Pil),sics, University of the Negev,
Beer Shevo. Israel
and S.B. MARKS and R. POTASHNIK Department of Chemistry, University of the Negev. Beer Sheva, Israel Received
22 October
1973
The proton spin-lattice relaxation time has been measured in the solid, nematic and isotropic phases of EBBA [N-~~thoxybenzylidene)9_butylanilinel. TWO IIIinima Were found in the te.nperature dependence of T, in the solid phase. In !he nematic and isotropic phase, the relaxation mechanism can be c!wracterized by activation energies of 5.9 kul/mole and 8.8 k&/mole, respectively.
1. Introduction
p-butylaniline] . EBBA is obtained from MBBA by adding a CH2 group to the methoxy end group of
The question of the mechanism of proton spin-lattice relaxation in the nematic phase of liquid crystals has lately come under close scrutiny. The main relaxation mechanisms attributed to this phase are those of the fluctuation of the order parameter [l] and the fluctuation of the dipolar spin interactions caused by molecular diffusion [2] _ By measuring T, as a function of frequency Vilfan et al. [3] have shown, in their definitive paper, that the dominant relaxation mechanism for PAA (p-azoxyanisole), is the fluctuation of the order parameter while MBBA [N-@methoxybenzylidene)-p-butylaniline] exhibits a frequency dependence characteristic of relaxation by molecular diffusion. Furthermore, it has been shown that by adding CH, groups to the end group of PAA, T1 is significantly lowered [4] _Vilfan et al. [3] argued that by lengthening the molecule an additional relaxation mechtism is introduced and its behavior is similar to that of MBBA. In order to investigate the effect of an addition of an &d groiip to MBBA, we have measured the temperature dependence of Ti in EBBA [N-@-eth~xybknzylid~ne>-
MBBA.
2. Experimental A sample of EBBA obtained from Eastman Kodak Co. was recrystallized twice from ethanol. The sample was degassed under vacuum and sealed off. T1 was measured on a Bruker variable frequency pulse spectrometer at 24 MHz over a temperature range from 130°K to 390°K using the ISO”-90” zeroing technique. The temperature range included the solid, nematic and isotropic phases.
3. Results
.
Fig. I ygives T1 a~ a function of temperature for the solid, &matic and isotropic phases. A double minimum Can be discdrried in the solid phase which we as&n to the reorientation relaxation dtie to the E3 :and,Mi ..
.:,
Volume
CHEhfICAL
24, number 2
PHYSICS LETTERS
15 January 1974
\ j
NEMATIC
ISOTROPIC
PHASE
PHASE
120
140
160
180
200
220
240
Fig. 1_ A plot of TX versus T for the solid, nematic and isotropic cooling. Solid lines are drawn through the experimenral points.
groups. Fig. 1 shows the usual [5] levelling off of Tl as the nematic phase is approached. A sharp change in Tl is observed at 305°K when the sample is heated and at 291 OK when it is cooled, corresponding to the transition to the nematic phase upon heating and formation of a supercooled nematic phase upon cooling_ T1 rises monotonically with the temperature indicating the strong temperature dependence of this parameter. As usual [S] T, drops somewhat at the nematic-isotropic transition and increases with the temperature in the isotropic phase.
4. Discussion In fig. 2 we have plotted the logarithm of the relaxation rate as a function of the reciprocal temperature. Vilfan et al. [3] have shown from their frequency dependence measurements that the relaxation m&hanism in the nematic phase of MBBA is the diffusion controlled intermolecular dipole interactions. This can be written as PI
._....
260
280
300
i
,
320
:
1
340
I
360
1
380 400 - T(%)
phases of EBBA. The circles indicate heating and the squares
where A, B and Care constants and D, the diffusion coefficient, contains the temperature dependence. If the temperature is sufficientIy high and v sufficiently low, the 4 power term can be neglected and In Ti’ versus T-1 should yield a straight line in accordance with the Arrhenius type dependence of the diffusion coefficient D. Estimates from the results of Vilfan et al. [3] on the frequency dependence of T, for MBBA show that the 3 power term can be neglected and the linear dependence in the low temperature region of the nematic phase of fig. 2 with temperature cornfirms this approximation in OUTcase. The rounding off of In Ti’ near the nematic-isotropic phase transition is attributed to the formation of isotropic regions within the nematic phase. It is clear that an activation energy for diffusion can be deduced from the linear portions of fig. 2. We fmd that the activation energy for the nematic phase is 5.9 + 1 kcal/mole compared to 6 f 1 kcal/mole as estimated for.MBBA from the work of Watson and Johnson [S] 1 Thus, adding CH, to MBBA has very little effect
.. .:
:..;. .:
_,
. .._ :
‘.....
._-.
.:
,.-:
”
.:,:‘.
..-
15 January 1974
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
10
9
l -Cooling
a
.-Heating
7 6 5 4 103 -_(rei’l fl
3
I
i 2.6
I
2.7
Ii
2.8
I
,
I
II
,
I
I
I
I
2.Q
3.0
3.1
3.2
3.3
3.4
3.8
3.6
3.7
1
I
3.8 33Q +Ok-l)
Fig. 2. A sernllog plot of 2’1 versus rhe reciprocal temperature for the isotropic and nematic phases of EBBA. The circles indicate heating and the squares cooling. Solid lines are drawn through the experimental points. on the relaxation
behavior. A significant increase in the activation energy was found in going from the nematic to the isotropic phase, the activation energy for the latter being 8.8 + 1 kcal/mole. The result for MBBA [5] is only 7 kcal/mole [S] _The increase of the activation energy during the nematic-isotropic transition is consistent with the drop in Tr when going from the nematic to isotropic phase because of the slower diffusion in the latter phase. This has also been pointed out [S] for the case of MBBA. The relaxation mechanisms for the nematic phase of liquid crystals can be classified as follows. vilfan et al. [3] have shown from the frequency dependence of T1 that the relaxation mechanism in PAA molecules is due to the fluctuation of the order parameter. This mechanism has been shown to be almost independent of temperature [6] and has been confirmed experimentally for PAA [4,5,7,8] _The frequency dependence of T, for MBBA shows [3] that its relaxation mechanism depends on diffusion, thus also explaining
the expected large temperature dependence [5,9] of T1 for this molecule. Adding end chains to PAA reduces T1 considerably [4] and it has been implied by Vilfan et al. [3J that this can be attributed to a mechanism similar to that found for MBBA. This is, however, inconsistent with the essentially temperature independent Tr found in these liquid crystals [41. Thus, the exact reason for the decrease of Tt in the homologs of PAA has yet to be clarified. We see from this study that the addition of an end chain to MBBA has very little influ-
ence on the relaxation behavior of the cIysta1 in the nematic phase. It is possible that the reason that the diffusion mechanism dominates in MBBA while order fluctuation is the corrtrohing factor in PAA is due to the different temperature ranges for the nematic phases. In general the nematic phases for PAA and its homologs occui at higher temperatures than the MBBA homologs and at 1 these temperatures the diffusion is too fast to effec- ..
tively relax the nuclei.
.. 251
VoIume 24, number 2
CHEMICAL PHYSICS LElTERs
References [II C.C Sung. Chem. Phys. Letters IO (1971) 35. [21 J-F- Harmon and B.N. Muller, Phys. Rev. 182 (I 969) 400. I31 M. Vilfan, R. Blinc and J-W_ Doane, Solid State Commun. 1I (1972) 1073. 141 CR. Dybowski, B.A. Smith and C.G. Wade, J. Phys. Chem. 75 (1971) 3834. [S] CL. Watkins and C.S. Johnson Jr., J. Phys. Chem. 7.5 (1971) 2452.
c
15 Jaauary 1974
161 J-W. Deane and D-L. Johnson, Chem. Phys. Letters 6 (197? 291. [71 R-Y. Dong, W.F. Forbes and h1.M. Antar, J. Chem. Phys. 5.5 (1971) 145. [8 ] R. Btinc, D-L. Hogenboom, D-E. O’ReilIy and EM Peterson, Phys. Rev. Letters 23 (1969) 969. 191 R.Y. Dong, W.F. Forbes and M.M. Pintar, Mol. Cryst. Liquid Cryst. 16 (1972) 213.