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Static and viscoelastic properties of the various smectic phases of N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline
(50.6) determined by dilatometry and acoustic methods
Y. Thiriet, J.A. Schulz, P. Martinoty, D. Guillon
To cite this version:
Y. Thiriet, J.A. Schulz, P. Martinoty, D. Guillon. Static and viscoelastic properties of the various smectic phases of N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6) determined by dilatometry and acoustic methods. Journal de Physique, 1984, 45 (2), pp.323-329. �10.1051/jphys:01984004502032300�.
�jpa-00209759�
Static and viscoelastic properties of the various smectic phases of N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6)
determined by dilatometry and acoustic methods.
Y. Thiriet, J. A. Schulz, P. Martinoty
Laboratoire d’Acoustique Moléculaire (*), Université Louis Pasteur, 4, rue Blaise Pascal,
67070 Strasbourg Cedex, France
and D. Guillon
C.N.R.S., Centre de Recherches sur les Macromolécules, 6, rue Boussingault, 67083 Strasbourg Cedex, France
(Reçu le 19 juillet 1983, accepti le 20 octobre 1983)
Résumé. - Le comportement des différentes phases smectiques du N-(4-n-pentyloxy-benzylidene)-4-n-hexyla-
niline (50.6) a été étudié au moyen d’une technique dilatométrique. Les résultats obtenus montrent que les tran- sitions N-A, C-B, B-F et F-G sont du premier ordre, et que la transition A-C est du deuxième ordre. Les effets
d’hystérésis observés aux transitions B-F et F-G suggèrent que les dislocations et les défauts jouent un rôle impor-
tant à ces deux transitions. Des mesures calorimétriques complémentaires montrent l’existence d’une phase nou-
velle qui n’apparait que pour les échantillons les plus purs et qui est située entre les phases B et F.
La réponse des différentes phases à une contrainte de cisaillement parallèle aux couches a également été étudiée.
Les résultats montrent que toutes les phases, qu’elles soient solides ou liquides, présentent une réponse visco- élastique, et que le couplage entre le directeur et le cisaillement est l’un des mécanismes qui contribue à la visco- élasticité de ces phases. Dans la phase F le directeur est en plus couplé au réseau local de cette phase. La phase.
solide-B présente un comportement identique à celui déjà observé dans la phase solide-B du N-(4-n-butyloxy- benzylidène)-4’-n-octylaniline (40.8).
Abstract.
2014High resolution dilatometric measurements of the various smectic phase transitions of N-(4-n- pentyloxybenzylidene)-4-n-hexylaniline (50.6) are reported. It is found that the N-A, C-B, B-F and F-G transitions
are first-order while the A-C transition is second-order. Hysteresis effects at the B-F and F-G transitions are observ- ed and seem to show that dislocations and defects play a significant rôle at these transitions. Calorimetry measu-
rements show the existence of a new phase between the B phase and the F phase.
The dynamic behaviour of the various smectic phases has also been investigated in presence of a shear which is
parallel to the layers. The results demonstrate that all the smectic phases have a viscoelastic behaviour. It is shown that a coupling between long-range orientation order (the director) and the shear stress is one of the mechanisms
contributing to the viscoelasticity of these phases. Comparison between the data obtained in phases C and F suggests that the director is coupled to the local lattice of the latter. The behaviour of the crystalline-B phase is
identical to that observed previously in the crystalline-B phase of N-(4-n-butyloxybenzylidene)-4’-n-octylaniline (40.8).
Classification
Physics Abstracts
61.30E - 62.20D - 62.65
1. Introduction.
In smectic systems the molecules are otganised in equidistant layers. A wide variety of phases exists, variously differentiated by ordering within the layers,
correlations between successive layers, and the orien-
tation of the molecules in relation to the normal to the
layers [1].
There is considerable interest at present in measu-
rements of the mechanical properties of these phases [2-8]. Especially since it is possible to study by these
measurements the elastic and viscoelastic properties
of these phases, as well as the influence of defects on
these properties.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01984004502032300
324
The mechanical properties of materials can be characterized by dynamic stress (shear wave) experi-
ments. The medium may respond to the shear wave by viscous flow (liquid), by elastic deformation (solid)
or by some combination of the two (viscoelastic body).
Shear wave techniques were already used to study
the interlayer rigidity modulus C44 in the crystalline B-phase of the liquid crystal N-(4-n-butyloxybenzy- lidene)-4’-n-octylaniline (40.8). Significant relaxation
effects were observed in the low frequency [5-6]
( 10 -1-5 .103 Hz) as well as in the high frequency range
[7-8] (5-100 MHz), showing the existence of a visco- elastic response. The exact origin of this viscoelastic behaviour is as yet far from being fully understood.
Two types of relaxation mechanisms may contribute to the viscoelasticity :
-
Relaxation associated with defects. It has been
proposed that a continuous distribution of mobile defects with a distribution of relaxation times could
explain the low frequency results [6].
-
Relaxation caused by the coupling of short and
long range orientational ordering with the shear flow.
An example of relaxation of this type is given by the
director viscoelastic relaxation. This mechanism, expected in the MHz range, and evoked in studies in 40.8 [8], has never been clearly observed by mechanical
means.
This study proposes to extend the measurements made in the crystal-B phase of 40.8 to other smectic
phases with a view to revealing the effects of short and
long range order on the viscoelasticity of these phases.
The liquid crystal N-(4-n-pentyloxybenzylidene)-4-n- hexylaniline (50.6) was chosen because of the richness
of its smectic polymorphism, as it is a compound
which has 3 stacked two-dimensional liquid phases (A, C and hexatic F) and 2 crystalline phases (B and G).
Another aspect of current research concerns the
behaviour at phase transitions. This problem has been
stimulated by a detailed study of second-order two- dimensional melting via a dislocation mechanism [9].
Heat capacity [10] and dilatometric [11] measurements
have been performed to determine whether a hexatic-B phase melts continuously into a smectic-A phase. The dilatometry method has been used here to charac- terize the nature of the various phase transitions in 50.6, in particular the transition from the crystalline
to the hexatic phases. Differential scanning calorimetry (DSC) measurements have also been made to com-
plement the dilatometric results.
2. Calorimetric and volumetric studies.
2.1 MATERIAL. - The six different liquid-crystalline phases appear in the sequence N-A-C-B-F-G with
decreasing temperature. The molecules are perpendi-
cular to the layers in the A and B phases while they
are tilted with respect to the layers in the C, F and G phases. These phases have been identified and charac- terized by optical microscopy, the miscibility method
and X-ray techniques [12]. Note that the phase
sequence in 50.6 is unusual since a stacked two- dimensional fluid (F) appears between two crystalline phases (B and G). This particular behaviour is not yet understood
Our sample of 50.6 was synthesized according to the
method described by Smith, Gardlund and Curtis [13]
and purified by successive recrystallizations.
The transition temperatures between the different
phases and the transition widths were determined by dilatometry and confirmed by optical microscopy.
The values reported in table I are higher than those previously reported [12], and are indicative of a high purity sample. It was observed that the B-F transition
was particularly sensitive to small amounts of impurity
and to degradation by moisture.
Table I.
-Summary of* DSC and dilatometry results. The transition temperatures and the transition widths are
obtained by dilatometry (see Figs. 1-3). The new phase transition detected in some of our DSC measurements (see
text) is not mentioned in the table. I is the isotropic liquid, N is the nematic and S the smectic of’ type A, C, B, F or G.
2. 2 CALORiMETRic MEASUREMENTS. - These measu- rements were performed with a Perkin-Elmer DSC 2 apparatus. The rate of temperature variation was
usually 2.5 OC/min. All the transitions listed in table I
were observed on the thermographs. Only a slight
shoulder was detected at the A-C transitions and it
was not possible to determine the enthalpy change at
this transition. In all other cases the enthalpy changes
were measured and their values are reported in table I.
The behaviour at the A-C transition is consistent with the second-order nature of this transition determined
by dilatometry.
In some of our measurements, an exothermic peak
at about 1 K above the B-F transition was observed on
cooling. This peak, which has never been reported in previous DSC studies [12], is so small that the enthalpy change could not be determined The occurrence of this
peak suggests the presence of a new phase, the existence
of which should be very sensitive to the purity of the sample. Very recent high resolution X-ray diffraction data seem to confirm the existence of this new
phase [ 14].
2.3 VOLUMETRIC MEASUREMENTS. - High resolution
volumetric measurements were performed in order to
characterize the nature of the order at the various
phase transitions. A first-order transition is characte- rized by a jump in the specific volume and a second- order one by a change in the thermal expansion
coefficient with no volume jump. Pretransitional effects manifest themselves by a non-linear variation of volume as a function of temperature.
Our volumetric data were taken with the use of a
dilatometer of the Bekkedahl type described in detail elsewhere [15]. The dilatometer was filled with about 1 g of the sample and immersed in a large oil bath,
the temperature of which was regulated to within
0.05 K. The sample inside the dilatometer was degassed
and capped with mercury. With a sample thus prepa-
red, the various transition temperatures were stable
over a very long period (4 months) covering warming
and cooling runs. The variations of the specific volume
were determined from the changes in height of the
mercury column. The mercury level was read with a
cathetometer. Changes in specific volume as small as
~ 10-5 cm3/g were regularly resolved. The measure- ments were made at equilibrium, with each tempe-
rature being kept constant for a long period ( > 30 min.).
Figure 1 shows the variation of the mercury height
as a function of temperature near the B-F and F-G phase transitions. The jump at each transition shows that the transitions are first-order. The transition widths are relatively small; 0.3°C and 0.4 °C at the B-F and G-F transitions respectively. There is no
indication of the anomaly detected by DSC near the
B-F transition.
Results for the A-C, C-B, A-N and N-I transitions
are reported in figures 2 and 3. The data show that the B-C and N-I transitions are first-order, the A-N
transition slightly first-order and the C-A transition
Fig. 1.
-The height of the mercury column in centimeters
as a function of the temperature for the B-F and F-G tran- sitions. The data show that both transitions are first order.
Results in the coexistence regions are not shown. The measurements were made at equilibrium.
Fig. 2.
-As for figure 1 but for the A-C and the C-B tran- sitions. The data show that the B-C transition is strongly
first order while the C-A transition is second order.
second-order. Except for the nematic side of the N-I
transition, the data reveal no indication of pretransi-
tional effects associated with a volume change.
Volumetric runs were made by cooling and heating.
There was no observable thermal hysteresis within our temperature resolution at the I-N, N-A, A-C and C-B transitions. In contrast, a hysteresis effect of about
0.2 °C was found at the B-F transition. A similar but smaller effect was also observed at the G-F transition.
These effects might reflect the first-order character of the transitions. However, the fact that these effects
were not seen at the other first-order transitions suggest that dislocations and defects play an important
role at the B-F and F-G transitions.
326
Fig. 3.
-As for figure 1 but for the I-N and N-A transitions.
A pretransitional effect is observed on the nematic side of the N-I transition.
3. Impedometric study.
3.1 METHOD. - The method used in this study to
characterize the response of the medium to the ultrasonic stress consisted in measuring the real part R
of the mechanical shear impedance as a function of the frequency.
In the case of a perfectly elastic solid, R is independent of the frequency and the shear modulus is R 2/p (where p is the density). Deviations from the ideal elastic behaviour occur when relaxation beha- viour is introduced into the system. When relaxation exists R 2 increases with increasing frequency.
In the case of a liquid the quantity of interest is the ratio R 2/ pnf where f is the frequency. This ratio is
independent of the frequency for a Newtonian fluid
(viscosity q
=R’Ipnj) and varies with frequency
for a viscoelastic fluid In the latter case the liquid has a rigidity modulus, like a solid In such cases mechanical measuring techniques can no longer distinguish
between a solid and a liquid
3.2 TECHNIQUE AND EXPERIMENTAL DETAILS. - R was
measured at 5 and 85 MHz by the shear wave reflection
method already used in the study of 40.8 [8] and
illustrated in figure 4. In this method a pulse of shear
waves propagates in a fused silica bar along the path
indicated in the figure, and is reflected from the test
surface. When the material to be studied is placed on
this surface a change in the amplitude of the reflected
wave takes place. R is determined from the measured value of the reflection coefficient r using the relation
where Z. is the shear mechanical impedance of the
fused silica bar and 0 is the angle of incidence of the shear wave.
Fig. 4.
-Schematic diagram of the acoustic system; the vibration is parallel to the smectic layers; 0 is the angle of
incidence of the shear wave. The refracting angle § is very small (§ - 1°) so that the shear wave (wave vector k) pro- pagates practically along the normal to the layers.
The accuracy of R is improved when r is determined from a signal which has undergone several reflections at the interface. In this study the 15th echo at 5 MHz and the 5th echo at 85 MHz were selected for the determination of r.
Significant comparisons between the results at 5 and 85 MHz have been made possible thanks to an
electronic systerh which enabled simultaneous mea- surements to be taken at these two frequencies during
a single run.
The shear cell was tested by using dibutylphtalate.
Temperature was controlled to within 0.01 OC.
The sample was placed between the reflecting surface
of the bar and a coverglass, heated in the nematic
phase, and orientated in the homeotropic configu-
ration thanks to a silane surface treatment of the
plates. By slowly decreasing the temperature the homeotropic configuration was maintained in the A and B phases. In this configuration the smectic layers
were aligned in a direction parallel to the reflecting
surface of the bar (i.e. a direction parallel to the shear displacement). Orientation of the layers parallel to
the reflecting surface was also obtained in the tilted
phases (C, F and G). However, in these phases the
orientation of the molecules was not homogeneous
and no experimental efforts have succeeded in sup-
pressing the director tilt degeneracy.
It was found that the shear-stress response of the various smectic phases near a phase transition is very sensitive to the purity of the sample [8]. Therefore
the sample was degassed before being placed on the bar, and once on the bar it was maintained in a helium gas atmosphere to prevent chemical degradation.
Without these precautions the data were found to be
much less reproducible.
All measurements were in the linear response regime
since no amplitude-dependent effects were observable.
3. 3 RESULTS AND DISCUSSION.
-The curves which show the variations of R as a function of the tempera-
ture at 5 and 85 MHz are given in figures 5 and 6. The
data were taken with decreasing temperature and the sample alignment was checked by optical observation between crossed polarizers. The data were reprodu-
cible from one run to another. Annealing the sample in
the various phases did not substantially alter the
results. The temperatures at which the jumps in R
arise are consistent with the transition temperatures determined by dilatometry.
The two figures call for the following remarks :
-
In the crystalline phases, B and G, R increases with the frequency, which shows that this is not an
elastic response but a viscoelastic one.
-
The liquid phases (A, C and F) also reveal a
viscoelastic behaviour since the value of R 2/ pnf
is a function of frequency. As a result these phases,
for which c44
=0, have a dynamic elasticity modulus.
The values of R at 5 MHz show that this modulus is of the order of - 106 dyn/cm2 for the A and C
phases, and - 10’ dyn/cm2 for the hexatic phase. The
existence of a large viscoelastic relaxation in the F
phase explains the increase in R at the B to F transition which appears unusual at first since one is going from a crystalline to a stacked-hexatic phase.
Fig. 5.
-The real part R of the mechanical shear impedance
as a function of the temperature in the A, C and B phases of
50.6. The coexistence region of the B and C phases is labelled by two parallel lines //. This region is characterized by a
linear variation of R with the temperature which, is not
shown in the figure. The data show that all the phases are
characterized by a viscoelastic behaviour. A pretransitional
decrease is observed at 5 MHz in the B phase near the B-C
transition temperature. The lines are only guidelines for
ease of reading.
Fig. 6.
-As for figure 5 but for the B, F and G phases. The sign II indicates the transition widths (determined by dilato- metry) and results in these regions are not shown. The data show that all the phases are characterized by a viscoelastic behaviour. The lines are only guidelines for ease of reading.
-
The crystal-B phase is characterized by a low interlayer elasticity coefficient c44 because the value of R at 5 MHz shows that this coefficient is less than 10’ dyn/cm2. In addition, the bend occurring in the
variation of R at 5 MHz near TBC reveals the existence of a pretransitional effect which reflects a progressive
decrease in the interlayer elasticity. This effect does not occur at 85 MHz and is therefore dynamic in origin.
-
Phase C is characterized by values of R which
are greater than those of phase A. This effect, in conjunction with the tilting of the molecules, shows
that there is a coupling between the orientational order (the director) and the shear. This mechanism has several possible implications (see below); in particular, it could be responsible for the pretransi-
tional decrease of R in the vicinity of the B-C transi-
tion, which could therefore be explained by a pretran- sitional change in the dynamic behaviour of the director.
- R increases considerably as it goes from the C phase to the hexatic F phase. Given that the only
difference between the F and C phases resides is the existence of a local lattice in the plane of the layers,
this increase would appear to constitute the first
mechanical evidence of any coupling between the
director and the local lattice in the hexatic phase. The
328
friction caused by the mosaic structure of the layers resulting from the non-uniform orientation of the director contributes also to this increase of R. The mosaic texture and the « director-local lattice » coupl- ing should also result in a larger value of R in phase G
than in phase B, consistent with the observed beha- viour.
-
