STUDY BY CALORIMETRIC AND MAGNETIC MEASUREMENTS OF PHASE TRANSITIONS IN LIQUID CRYSTALS

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STUDY BY CALORIMETRIC AND MAGNETIC MEASUREMENTS OF PHASE TRANSITIONS IN

LIQUID CRYSTALS

F. Hardouin, H. Gasparoux, P. Delhaes

To cite this version:

F. Hardouin, H. Gasparoux, P. Delhaes. STUDY BY CALORIMETRIC AND MAGNETIC MEA- SUREMENTS OF PHASE TRANSITIONS IN LIQUID CRYSTALS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-127-C1-131. �10.1051/jphyscol:1975124�. �jpa-00215901�

Classification Physics Abstracts

7.130

STUDY BY CALORIMETRIC AND MAGNETIC MEASUREMENTS OF PHASE TRANSITIONS IN LIQUID CRYSTALS

F. HARDOUIN, H. GASPAROUX and P. DELHAES

1. Introduction. — The phenomenological [1] and increase of the temperature, AT. This allows us to microscopic theories [2, 3] established for the different determine the absolute heat capacity C^exp = AQ/AT phase-transitions in liquid crystals presuppose the which can be only identified with the true specific heat existence of second order N -> SA and SA -» Sc transi- far from a transition.

2.1.2 A semi-adiabatic method. — A constant heat- For the N - > SA transition, some recent works [4,5,6, m g p o w e r js applied to the insulated sample. The 7, 8] enable us to appreciate the validity of these variations in temperature are recorded versus time. In a theories, but in the nematic phase, the study of the stationary state, the recorded observed slope curve is in pretransitional effects cannot determine unambi- inverse ratio to the specific heat. Thanks to this method guously the order of this transition. w e c a n increase the number of experimental points near

The determination of the order of the phase transi- a specific heat discontinuity,

tion is currently of great theoretical and experimental „ „ ^m . . .

interest. We thus measured two physical parameters, ²'² T H E M AGNETIC ANISOTROPY A^X. - A^X is a the specific heat and the magnetic anisotropy, in order macroscopic parameter m direct ratio to the onenta- to describe these transitions by the evolution of these t , o n a l o r d e r V^m^r of the medium,

properties, near a phase transition. — In a nematic phase, one can deduce the magnetic anisotropy from a magnetic susceptibility measurement 2. Techniques. — 2.1 THE SPECIFIC HEAT Ce x p.— obtained by Faraday's classical method [10,11].

We used two methods of measurement, both resorting _ u & s m e c t i c h a s C j t w Q m e t h o d s a r e a y a i l a b l e . to the use of an apparatus adapted to the study of _ ,, , „ „. * * - • , , ^{i t}. j

,. . , , r„f • so called static measurement Faraday s method,

• the direct measurement method of Ax, realized 2.1.1 The adiabaticme thod. — A determined heat thanks to a rotating magnetic field. This dynamic pulse AQ is applied to a sample thermally insulated method is usually called the Krishnaris flip-angle from the external medium. We measure the ensuing method [12].

JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 3, Tome 36, Mars 1975, page Cl-127

Centre de Recherches Paul-Pascal, Université de Bordeaux I Domaine Universitaire, 33405 Talence, France

Résumé. — Dans le but d'analyser les transitions de phases dans les cristaux liquides nous avons suivi l'évolution de deux paramètres physiques, la chaleur spécifique et l'anisotropie diamagnétique, au voisinage de diverses transitions de phase.

La chaleur spécifique CeXp a été mesurée à l'aide d'un appareil adapté à l'étude de liquides ou de substances pulvérulentes.

L'anisotropie diamagnétique A/ fut évaluée grâce à une méthode de Faraday classique ; dans le cas du Smectique A une méthode de détermination directe de l'anisotropie magnétique a en outre été utilisée.

Des transitions Nématique-Smectique A et Smectique A-Smectique C ont été étudiées à travers l'évolution des propriétés de quatre composés. Une discussion est faite autour de la nature de ces différentes transitions.

Abstract. — In order to analyse phase transitions in liquid crystals we measured the evolution of two physical parameters, the specific heat and the diamagnetic anisotropy.

The specific heat Cexp was measured by using an apparatus usable for the study of liquids or powders.

The diamagnetic anisotropy Ax was determined by Faraday's classical method and a direct measurement method available only in the case of the smectic phases.

Nematic-Smectic A and Smectic A-Smectic C transitions were investigated through measurements of the evolution of the properties of four compounds.

A discussion is proposed of the nature of the different transitions.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975124

Cl-128 F. HARDOUIN, H. GASPAROUX AND P. DELHAES

3. Results and discuss. - 3 . 1 THE NEMATIC- SMECTIC A TRANSITION. - Recently para-cyanobenzi- lidene-p'-octyloxyaniline, CBOOA, has been one of the most studied materials. But if experimental results [13, 14, 15, 161 and some theoretical predictions are in favour of a second order N + S, transition, never- theless some people do not agree with it [S, 17, 18, 191.

In order to complete this analysis, we studied CBOOA by the two properties previously defined.

Figure 1 shows the temperature dependence of X and AX in the case of CBOOA. We find that, during the cooling from the liquid state, the order parameter increases when the temperature decreases especially when we pass from the nematic state to the smectic. The lack of an order parameter discontinuity, that we can detect experimentally, is in good agreement with the assumption that this may be a second order transition.

C B O O A

FIG. 1 . - CBOOA - Temperature dependence of the magnetic susceptibility (X) and of the magnetic anisotropy @X).

The temperature dependence of the experimental specific heat is represented on figure 2 [IS]. We notice that, together with the solid-smectic and nematic- isotropic transitions, important pre- and post-transi- tional effects appear. These hide the right value of the specific heat and distinguish these transitions from the first-order classical ones. An enlarged detail of the experimental curve near the N ^-t S, transition is pointed out on figure 3. The specific heat-anomaly revealed by a small peak appears, characteristic of a first order transition. This is in good agreement with experimental results recently obtained [17,20]. Besides, we can affirm that it is not an anomaly similar to a A

transition on liquid helium because our measurement method allows us to estimate a specific heat divergence on a temperature range lower than 0.05 OC. The inte- gration of the specific heat anomaly gives us a value equal to 0.10 cal/g, slightly different from the value of the transitional enthalpy (0.06 cal/g) which P. E. Cladis obtained with an A. E. D. apparatus on an extremely pure sample of CBOOA [S]. These values are higher than those obtained by D. Djurek 1201. This can be explained by the fact that our investi.gation.,,method over-estimates the transitional enthalpy because of the existence of pre- and post-transitional effects.

a*

:I

..

FIG. 2. - CBOOA - Thermal 'variation of the experimental specific heat Ce,,.

C B O O A adiabatic

+

FIG. 3. - CBOOA - Thermal variation of Cexp near the N + SA transition.

CALORIMETRIC AND MAGNETIC MEASUREMENTS IN MESOPHASES Cl-129

Consequently we think that the assumption of a first order N -* S, transition, presenting a very small transition enthalpy, gives the best account of our results.

This corroborates the latest theoretical predictions [21]

based on the analogy between superconducting + nor- mal metal and N ^-* SA transitions. In effect, the presence of an additional term in the expression of Landau's free energy suggests that all these transitions are of first order.

It is interesting to point out that we detect a small discontinuity of AX in the case of the p-butoxybenzi- lidene-p'-octylaniline (BBOA), which presents a N ^-* S, transition with a 0.20 cal/g latent heat [22]

slightly higher than the one measured for the CBOOA.

This corroborates that the transition is of first order.

This compound has an orthogonal smectic B phase.

The orientational order increases at the S, -+ S, transition. Besides, these results are in agreement with the AX direct measurement made in smectic phase on these uniaxial mediums (Fig. 4, 5).

-X,,X 107 (cm" c c s x g - ' j A X ~ 1 0 ' (em" c ~ s n g - '

6 5 7 - 0 - 5 1 5

static measurement ( 7 ' S j p++*-

r I

FIG. 6. - 705 ^- Temperature dependence of the magnetic susceptibility ( X ) and of the magnetic anisotropy (Ax).

B B O A

FIG. 7. - 705 ^- Thermal variation of AX measured by the rotating magnetic field method in the smectic phases.

FIG. 4. - BBOA - Temperature dependence of the magnetic susceptibility ( X ) and of the magnetic anisotropy (Ax).

A X X 107 [m" c c s X g - l ) B B O A dynam~c measurement ( T " \

RG. 8. - 704 - Temperature dependence of the magnetic susceptibility ( X ) and of the magnetic anisotropy (Ax).

FIG. 5. - BBOA - Thermal variation of AX measured by the rotating magnetic field method in the smectic phases.

3 . 2 THE SMECTIC A-SMECTIC C TRANSITION. - We studied the two following compounds : the heptyloxy- benzilidene-p'-pentylaniline (705) and the heptyloxy- benzilidene-p'-butylaniline (704) (Fig. 6,7,8, 9).

Their magnetic anisotropy, obtained by any pre- viously mentioned methods, varies with no detectable discontinuity near the transition SA -+ SC realized under a magnetic field.

-x,,,Y 10' [emu CCS xg-'1

"I

v*-

stalrc measurement [ T o \ ] i

i5

A x x i 0 7 [ e m u c c s x g - l ] 7 - 0 - 4

dynsm,c measurement ( T ' \ )

RG. 9. - 704 - Thermal variation of Ax measured by the rotating magnetic field method in the smectic phases.

Cl-130 F. HARDOUIN, H. GASPAROUX AND P. DELHAES

These results could presuppose (like in the TBBA case) that the S, + S, transitions of both compounds are of second order [23, 24, 251. Nevertheless, for the 705, the curve of the experimental specific heat (C,,, versus to temperature : Fig. 10) discloses a small peak attributed to a latent heat estimated by integration at 0.12 cal/g. This result is in perfect agreement with previously made A. E. D. measurements [22]. So, by analogy with the N + S, transition, we can conclude that the S, -, S, transitions of the 704 and 705 compounds are of first order with a small transitional enthalpy. The specific heat discontinuity seems negli- gible but the proximity of the S, ^-t S, transition may alter its evaluation.

The figures 6, 7, 8, 9, 10 call for some extra remarks :

- The results obtained from one hand by a Fara- day's method, from the other hand by a AX direct measurement, present the same relative evolution versus to temperature. One can explain the differences observed in the absolute values simply because these two techniques are references methods ; besides, diffe- rences of strength and above all of homogeneity of the magnetic field (Faraday's method involves a gradient) presuppose that the method of direct measurement leads to a best effective orientation of the molecules and therefore to better results.

- At the S, ^-t S, transition of the 705 (Fig. 6, 7) the magnetic anisotropy increases suddenly ; one can

explain it by a molecular reorganisation in the planes, according to an hexagonal structure with a strong coupling between the orientational and transitional orders. The order parameter-discontinuity, the specific heat anomaly (Fig. 7,lO) and the important transitional enthalpy found, irrefutably point out that this transi- tion is of first order.

-

- The different behaviour of the magnetic aniso- tropy, at the S, ^-t S, transition of the 704 compound (Fig. 8,9) expressed by an important diminution of the order parameter, can be explained by the passage to a tilted smectic B (smectic H) phase. In effect, we cannot organize, in a magnetic field, a biaxial medium (S,) like a uniaxial one (orthogonal S,).

FIG. 10. - 705 - Thermal variation of the experimental specific heat Gem.

CALORIMETRIC AND MAGNETIC MEASUREMENTS I N MESOPHASES Cl-131

In conclusion it is important to notice that the nature uniaxial medium to a biaxial one, allowed us to keep (slightly of first order or quasi-second order according large monocrystalline domains in the smectic C to the vocabulary selected) of the S, + S, transition, phase when the transition is realized under a magne- that is to say the continuous evolution from an tic field.

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