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A possible polar smectic A—Non-polar smectic A transition line in a binary system?
Volume 72A, number 1
PHYSICS LETTERS
11 June 1979
A POSSIBLE POLAR SMECTIC A—NON-POLAR SMECTIC A TRANSITION LINE IN A BINARY SYSTEM? G. SIGAUD, F. HARDOUIN and M.F. ACHARD Centre de Recherche Paul Pascal, 33405 Talence, France Received 13 March 1979
We show the thermal evolution of the orientational order near nematic—smectic A or smectic A—smectic A transitions which we recently revealed in binary mixtures. We compare the experimental behavior to theoretical predictions of a possible smectic A phase with polarized layers.
The following asymmetric dibenzoate: 4-n pentylphenyl-4’-(4”-cyanobenzoyloxy) benzoate (subsequently called “dibenzoate” for short) exhibits a fairly strong first order N—SA transition. However, the reduced temperature transition TNAITNI = 0.77 is smaller than Mc Millan’s tricritical limit (TNA/TNI 0.87) below which the transition is expected to be second order [1], and we have recently also confirmed that the nematic ordering is nearly saturated at the transition [2]. Meyer and Lubensky [3] concluded that that in this case the only coupling between nematic and smectic ordering cannot explain the first-order thermodynamic character of the N—SA transition, although the primordial importance of this coupling seemed to be strongly supported by numerous experimental data on pure compounds or binary systems [4—8]. Furthermore, by mixing TBBA (terephtal-bisbutyl aniline) in “dibenzoate” we determined [2] with a polarizing microscope two branches of N—SA transitions (fig. 1): a line of N—SAl transitions in good agreement with de Gennes’ and Mc Millan’s theories [1,9] and a line of N—SA2 transitions along which the transitional entropy is not well correlated with the magnitude of TNA/TNI (table 1). On the other hand, it is impossible to detect optically the third line of fig. 1 (dashed line) because the textures of the two smectic mesophases called SAl and SA2, examined by thermal microscopy, are quite the same so that by the contact method there is perfect iso24
T (‘C)
140
Nematic
( 120
At
~
•‘ S.’.’
‘\
Smectic A2
,~)0
12
mote ¾ TB8A Fig. 1. Dibenzoate—TBBA isobaric phase diagram. Transition line detected by thermal microscopy and DSC signal. - - - Transition line solely detected by DSC signal.
morphism with the smectic A phase of TBBA. In fact, we reveal this line of SA1—SA2 transitions only by means of differential scanning calorimetry (Dupont
Volume 72A, number 1
PHYSICS LETTERS
11 June 1979
Table 1 Reduced N—S transition temperature (Mc Milan’s parameter) and entropy of the transitions N—SAl or SAl —SA2 for various mixtures. Compound
TNA/TNI
“Dibenzoate” 5.3 m% TBBA +10.5 m% TBBA +13.4m%TBBA +14.5 m%TBBA +17m%TBBA
0.77 0.75 0.74 0.75 0.76 0.77
+
~SNSA
2 1K1) (meal mole 470 520 390
or SA1—SA2 transitions. As in the pure “dibenzoate” (fig. 2a), figs. 2b and 2c indicate a very weak discontinuity of the nematic ordering at the N—SA2 transi_____________________
1.~
_____________________
CGS g’) 1.2
~
1.4
1,1
~‘~-“-
1,3
1,0
i~o
~
i~ th
i~o
i~o 1,4 1.3
1.1
12 _________________
tho
i~o
_________________
i4i
th
i~ü
1~O
1,5 1,1
1,5 14
~
1.~ 1~0
130
140
~SSA1SA2 (mcal mo1e~K1)
<5 <5
65 35 15
<5
990): at the transition a small heat signal is clearly evident due to the existence of an entropy discontinuity at the transition (table 1). Fig. 2 shows for different mixtures the temperature dependence of the orientational order (via magnetic anisotropy determination) in the vicinity of the N—SM
~x 1O~ (emu
~SNsAl (meal mo1e~K~)
120
~
.~
Fig. 2. Thermal behavior of the diamagnetic anisotropy near the N—SM or SA1—SA2 transitions, for various mixtures.
tion (relative to the fairly large transition entropy values: table 1). Beyond 12 m% TBBA, the orientational order is continuous (figs. 2d, 2e, 2!) but it undergoes two abrupt changes in the slope which confirms the existence of two transitions: the first at the temperature TNA 1 observed by microscopy and for which by DSC we have not been able to estimate the latent heat (L <4 mcal g~)but only a simple discontinuity of the specific heat. The second change in the slope corresponds to the effect on the orientational order parameter of a modification of the smectic A because it arises at the temperature TSAI_SA2 determined by calorimetric measurements. This latter result is explained by Prost [10] who considers a Landau theory with saturated nematic order. His theory yields a topological description of a phase diagram and leads to a calculation of the therma! dependence of the entropy discontinuity along the transition lines which are quite consistent with all our experimental data, as previously discussed [10]. Starting from the Meyer—Lubensky free energy [3], the main feature of this phenomenological model is to de5A1 —SA fine the 2 transition in terms of condensation of the fundamental Fourier component of the mass density after the first harmonic. Nevertheless there is not yet experimental evidence of such a dramatic change. In addition, Prost suggests a possible antiferroelectric arrangement of the layers in the lower temperature smectic A phase. By another way, Photinos and Saupe [11] have predicted, in a mean field calculation based model including the apolar contribution inonthea simple intermolecular interaction, transformation with decreasing temperature from a non-polar smectic A phase to a smectic A phase with polar layers. A 25
Volume 72A,
number 1
PHYSICS LETTERS
comparison may also be attempted with their model. In particular, they note that the polar—non-polar smectic A transition entropies are small and they show the tendency to decrease with decreasing nematic range (to be compared to the lower part of table 1). Moreover, their numerical results emphasize that for a given temperature value, if a non-polar smectic A— nematic transition is a second-order one, on the contrary, a polar smectic A—nematic transition should appear as first order because of the better stability of the layers due to the polarity. We note such an enhancement of the smectic thermal stability from SAl to SA2 (see table I and fig. 1). More generally, they assume that the polar contribution enhances the onentational order (in apparent agreement with our binary system, fig. 2). Initially Photinos and Saupe’s arguments seemed to be supported by Arora et al.’s observations [12] on some members of the homologous series of 4-n-alkoxybenzilidene-4’-aminopropiophenones. Nevertheless, our own observations of a slight transient striped texture at the smectic—smectic phase change for two members of their series, namely, the pentyloxy and the hexyloxy, are fairly in favour of a SA—SB transition,
26
11 June 1979
Finally, there is not enough experimental knowledge to test such a speculation invoking a polarization of the layer. In the same way, we point out that we have no evidence to exclude other models which may also give an account of our results and which we have not mentioned here. References [1] W.L. Mc Millan, Phys. Rev. A4 (1971) 1238. [2] G. Sigaud, F. Hardouin, M.F. Achard and H. Gasparoux, J. de Phys. (Paris), to be published. [3] R.B. Meyer and T.C. Lubensky, Phys. Rev. A14 (1976)
2307.
[4] J.W. Doane, R.S. Parker, B. Cvikl, D.L. Johnson and D.L. Fishel, Phys. Rev. Lett. 28 (1972). [5] F. Hardouin, M.F. Aehard and H. Gasparoux, C.R. Acad. Sci. 277C (1973) 551.
[6] D. W.H. Commun. 13(1973)1521. [7] Syde andJeu, M. Solid Ptak, State J. de Phys. (Paris) 35 (1974) 517. [8] M.F. Achard, F. Hardouin, G. Sigaud and H. Gasparoux, J. Chem. Phys. 65 (1976) 1387. [9] P.G. de Gennes, Mo!. Cryst. Liquid Cryst. 21(1973)49. [101 J. Prost, I. de Phys. (Paris), to be published. [11] P.J. Photinos and A. Saupe, Phys. Rev. Al 3 (1976) 1926. [121 L. Arora, T.R. Taylor and J.L. Fergason, Liquid Crystals and ordered fluids (Plenum, New York, 1970) p.321.
PHYSICS LETTERS
11 June 1979
A POSSIBLE POLAR SMECTIC A—NON-POLAR SMECTIC A TRANSITION LINE IN A BINARY SYSTEM? G. SIGAUD, F. HARDOUIN and M.F. ACHARD Centre de Recherche Paul Pascal, 33405 Talence, France Received 13 March 1979
We show the thermal evolution of the orientational order near nematic—smectic A or smectic A—smectic A transitions which we recently revealed in binary mixtures. We compare the experimental behavior to theoretical predictions of a possible smectic A phase with polarized layers.
The following asymmetric dibenzoate: 4-n pentylphenyl-4’-(4”-cyanobenzoyloxy) benzoate (subsequently called “dibenzoate” for short) exhibits a fairly strong first order N—SA transition. However, the reduced temperature transition TNAITNI = 0.77 is smaller than Mc Millan’s tricritical limit (TNA/TNI 0.87) below which the transition is expected to be second order [1], and we have recently also confirmed that the nematic ordering is nearly saturated at the transition [2]. Meyer and Lubensky [3] concluded that that in this case the only coupling between nematic and smectic ordering cannot explain the first-order thermodynamic character of the N—SA transition, although the primordial importance of this coupling seemed to be strongly supported by numerous experimental data on pure compounds or binary systems [4—8]. Furthermore, by mixing TBBA (terephtal-bisbutyl aniline) in “dibenzoate” we determined [2] with a polarizing microscope two branches of N—SA transitions (fig. 1): a line of N—SAl transitions in good agreement with de Gennes’ and Mc Millan’s theories [1,9] and a line of N—SA2 transitions along which the transitional entropy is not well correlated with the magnitude of TNA/TNI (table 1). On the other hand, it is impossible to detect optically the third line of fig. 1 (dashed line) because the textures of the two smectic mesophases called SAl and SA2, examined by thermal microscopy, are quite the same so that by the contact method there is perfect iso24
T (‘C)
140
Nematic
( 120
At
~
•‘ S.’.’
‘\
Smectic A2
,~)0
12
mote ¾ TB8A Fig. 1. Dibenzoate—TBBA isobaric phase diagram. Transition line detected by thermal microscopy and DSC signal. - - - Transition line solely detected by DSC signal.
morphism with the smectic A phase of TBBA. In fact, we reveal this line of SA1—SA2 transitions only by means of differential scanning calorimetry (Dupont
Volume 72A, number 1
PHYSICS LETTERS
11 June 1979
Table 1 Reduced N—S transition temperature (Mc Milan’s parameter) and entropy of the transitions N—SAl or SAl —SA2 for various mixtures. Compound
TNA/TNI
“Dibenzoate” 5.3 m% TBBA +10.5 m% TBBA +13.4m%TBBA +14.5 m%TBBA +17m%TBBA
0.77 0.75 0.74 0.75 0.76 0.77
+
~SNSA
2 1K1) (meal mole 470 520 390
or SA1—SA2 transitions. As in the pure “dibenzoate” (fig. 2a), figs. 2b and 2c indicate a very weak discontinuity of the nematic ordering at the N—SA2 transi_____________________
1.~
_____________________
CGS g’) 1.2
~
1.4
1,1
~‘~-“-
1,3
1,0
i~o
~
i~ th
i~o
i~o 1,4 1.3
1.1
12 _________________
tho
i~o
_________________
i4i
th
i~ü
1~O
1,5 1,1
1,5 14
~
1.~ 1~0
130
140
~SSA1SA2 (mcal mo1e~K1)
<5 <5
65 35 15
<5
990): at the transition a small heat signal is clearly evident due to the existence of an entropy discontinuity at the transition (table 1). Fig. 2 shows for different mixtures the temperature dependence of the orientational order (via magnetic anisotropy determination) in the vicinity of the N—SM
~x 1O~ (emu
~SNsAl (meal mo1e~K~)
120
~
.~
Fig. 2. Thermal behavior of the diamagnetic anisotropy near the N—SM or SA1—SA2 transitions, for various mixtures.
tion (relative to the fairly large transition entropy values: table 1). Beyond 12 m% TBBA, the orientational order is continuous (figs. 2d, 2e, 2!) but it undergoes two abrupt changes in the slope which confirms the existence of two transitions: the first at the temperature TNA 1 observed by microscopy and for which by DSC we have not been able to estimate the latent heat (L <4 mcal g~)but only a simple discontinuity of the specific heat. The second change in the slope corresponds to the effect on the orientational order parameter of a modification of the smectic A because it arises at the temperature TSAI_SA2 determined by calorimetric measurements. This latter result is explained by Prost [10] who considers a Landau theory with saturated nematic order. His theory yields a topological description of a phase diagram and leads to a calculation of the therma! dependence of the entropy discontinuity along the transition lines which are quite consistent with all our experimental data, as previously discussed [10]. Starting from the Meyer—Lubensky free energy [3], the main feature of this phenomenological model is to de5A1 —SA fine the 2 transition in terms of condensation of the fundamental Fourier component of the mass density after the first harmonic. Nevertheless there is not yet experimental evidence of such a dramatic change. In addition, Prost suggests a possible antiferroelectric arrangement of the layers in the lower temperature smectic A phase. By another way, Photinos and Saupe [11] have predicted, in a mean field calculation based model including the apolar contribution inonthea simple intermolecular interaction, transformation with decreasing temperature from a non-polar smectic A phase to a smectic A phase with polar layers. A 25
Volume 72A,
number 1
PHYSICS LETTERS
comparison may also be attempted with their model. In particular, they note that the polar—non-polar smectic A transition entropies are small and they show the tendency to decrease with decreasing nematic range (to be compared to the lower part of table 1). Moreover, their numerical results emphasize that for a given temperature value, if a non-polar smectic A— nematic transition is a second-order one, on the contrary, a polar smectic A—nematic transition should appear as first order because of the better stability of the layers due to the polarity. We note such an enhancement of the smectic thermal stability from SAl to SA2 (see table I and fig. 1). More generally, they assume that the polar contribution enhances the onentational order (in apparent agreement with our binary system, fig. 2). Initially Photinos and Saupe’s arguments seemed to be supported by Arora et al.’s observations [12] on some members of the homologous series of 4-n-alkoxybenzilidene-4’-aminopropiophenones. Nevertheless, our own observations of a slight transient striped texture at the smectic—smectic phase change for two members of their series, namely, the pentyloxy and the hexyloxy, are fairly in favour of a SA—SB transition,
26
11 June 1979
Finally, there is not enough experimental knowledge to test such a speculation invoking a polarization of the layer. In the same way, we point out that we have no evidence to exclude other models which may also give an account of our results and which we have not mentioned here. References [1] W.L. Mc Millan, Phys. Rev. A4 (1971) 1238. [2] G. Sigaud, F. Hardouin, M.F. Achard and H. Gasparoux, J. de Phys. (Paris), to be published. [3] R.B. Meyer and T.C. Lubensky, Phys. Rev. A14 (1976)
2307.
[4] J.W. Doane, R.S. Parker, B. Cvikl, D.L. Johnson and D.L. Fishel, Phys. Rev. Lett. 28 (1972). [5] F. Hardouin, M.F. Aehard and H. Gasparoux, C.R. Acad. Sci. 277C (1973) 551.
[6] D. W.H. Commun. 13(1973)1521. [7] Syde andJeu, M. Solid Ptak, State J. de Phys. (Paris) 35 (1974) 517. [8] M.F. Achard, F. Hardouin, G. Sigaud and H. Gasparoux, J. Chem. Phys. 65 (1976) 1387. [9] P.G. de Gennes, Mo!. Cryst. Liquid Cryst. 21(1973)49. [101 J. Prost, I. de Phys. (Paris), to be published. [11] P.J. Photinos and A. Saupe, Phys. Rev. Al 3 (1976) 1926. [121 L. Arora, T.R. Taylor and J.L. Fergason, Liquid Crystals and ordered fluids (Plenum, New York, 1970) p.321.