PRELIMINARY EXPERIMENTS ON DYNAMIC MECHANICAL PROPERTIES OF CHOLESTERYL MYRISTATE

HAL Id: jpa-00216229

https://hal.archives-ouvertes.fr/jpa-00216229

Submitted on 1 Jan 1975

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PRELIMINARY EXPERIMENTS ON DYNAMIC MECHANICAL PROPERTIES OF CHOLESTERYL

MYRISTATE

T. Asada, Y. Maruhashi, S. Onogi

To cite this version:

T. Asada, Y. Maruhashi, S. Onogi. PRELIMINARY EXPERIMENTS ON DYNAMIC MECHAN-

ICAL PROPERTIES OF CHOLESTERYL MYRISTATE. Journal de Physique Colloques, 1975, 36

(C1), pp.C1-299-C1-303. �10.1051/jphyscol:1975150�. �jpa-00216229�

JOURNAL DE PHYSIQUE

Colloque C1, suppliment au

no 3, Tome 36, Mars 1975, page C1-299

Classification Physics Abstracts 7.130

-

7.210 - 7.220

PRELIMINARY EXPERIMENTS ON DYNAMIC MECHANICAL PROPERTIES OF CHOLESTERYL MYRISTATE

T. ASADA, Y. MARUHASHI a n d S.

ONOGZ

Department of Polymer Chemistry, Kyoto University, Kyoto, Japan

RCsume. - On a etudie les proprietes viscoelastiques dynamiques des phases cholesterique et smectique du myristate de cholesteryl. Pour des amplitudes de deformation faibles, les proprietks viscoelastiques dynamiques sont mesurees i I'aide d'un viscosimetre de Couette, a des frequences angulaires de 0,25

a

2,96 s-1 et dans la gamme de temperature 70-83 Co. Dans la phase cholest6 rique, les courbes d'hysteresis varient avec la frequence et de facon non linkaire pour les frequences Ies plus elcvks. Les parties rklle G' et imaginaire G" du module dependent de la frequence dans la phase cholesterique. Au contraire, dans la phase smectique, le module dynamique absolu

1

G

I

ne varie pas avec la frequence et le cristal smectique a les proprietes d'un solide Clastique. Pour des deformations elevees, on a utilise un viscosimetre a cane pour des frequences angulaires de 0,135

a

5,39 s-1 et pour des temperatures de 90

a

50 oC. Les courbes d'hysteresis, a frequence fixe, varient avec la temperature. Elles sont elliptiques dgns la phase cholesterique, ce qui est caracteristique d'une reponse viscoelastique linkaire ; par contre elles sont trbs distordues dans la phase smectique.

On a remarque que Ies proprietes dynamiques des cristaux liquides dependent beaucoup de leur histoire mkcanique et les cristaux liquides cholesttriques ont un comportement de type liquide a p r b plusieurs cycles de deformations sinusoi'dales. Bien que ces phinomtnes rheotropiques soient trks compliques, des etudes preliminaires montrent qu'ils sont plus apparents dans la phase choles- terique que dans la phase smectique. On a compare les resultats obtenus aprks 10 cycles de defor- mation sinusoi'dale

a

une frequence de 0,27 s-1. L'energie dissipk ED dans la phase cholesterique augmente avec la frequence mais de f a ~ o n un peu differente de la phase smectique. E: a frequence nulle decroit lorsque la temperature augmente pour le smectique, mais decroit lorsque la temp6ra- ture baisse pour le cholesterique.

Abstract. -The dynamic viscoelastic properties of two liquid crystal states of cholesteryl myristate were studied. Cholesteryl myristate was selected as a material for study because it appears both in the form of a cholesteric liquid crystal or a smectic liquid crystal, depending on the tempe- rature. At small strain amplitudes, the dynamic viscoelastic properties were measured by means of a coaxial cylinder rheometer in the angular frequency range 0.25-2.96 s-1 and in the temperature range 70-83 oC. Hysteresis loops for the cholesteric state change with frequency and show a non-linear pattern a t the higher frequencies. Both the storage modulus G' and the loss modulus G" for the cholesteric state change with frequency. On the contrary, the absolute dynamic modulus [ G

1

for the smectic state does not change with frequency and the smectic liquid crystal behaves as an elastic solid. At Large strain amplitudes, the dynamic viscoelastic properties were measured by means of a cone-plate rheometer in the angular frequency range 0.135-5.39 s-1 and in the temperature range 90-50 OC. Hysteresis loops for this material a t a fixed frequency change with temperature. The hysteresis loops for the cholesteric state are elliptical, which is a typical linear viscoelastic response, but for the smectic state they are very much distorted.

It has been pointed out that the dynamic behavior of liquid crystals is strongly affected by their mechanical histories. Even cholesteric liquid crystals show liquid-like behaviors after many cycles of sinusoidal strain. Although such rheotropic phenomena are very complicated ones, preliminary studies indicate that they are more evident in the cholesteric state than in the smectic state. Data were compared after 10 cycles of sinusoidal strain at a frequency of 0.27 s-1. The dissipation energy ED for th: cholesteric state increases with frequency in a somewhat different way from that for the smectic state. EO, at z:ro frzq1:ncy for the smectic state decreases with increasing temperature, whereas E$ for cholesteric state decreases with decreasing temperature.

1.

Introduction. - T h e rheological studies of liquid viscoelastic data a s well a s steady flow data. T h e dyna- crystals reported previously [ I , 2,

61

have mostly been mic viscoelastic properties of two liquid crystal phases limited t o steady flow behavior. Studies of flow pro- of cholesteryl myristate are r e p ~ r t e d . Cholesteryl perties alone cannot tell the full story

of

the rheological myristate was selected a s a mate$

I

fo

-

study because properties because the rheological respanse of liquid it show; b ~ t h

a

c h ~ l e s t e r i c phase a n d a smectic phase, crystals is very unique.

To

understand the rheological dep:nding

o

¹ th: t3mperature.

properties in detail, it is necessary t o obtain dynamic i t is well known that the cholesteryl myristate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975150

C1-300 T. ASADA. Y. MARUHASHI AND S. ONOGI

changes phase with temperature (See Table I) [3].

When cooling from the isotropic melt to the choles- teric temperature, a sky-blue color appears first and then, sometime later, a white phase appears in the blue phase matrix. Finally, 20-30 min later, the white phase occupies the full matrix. The white phase may be the focal conic cholesteric phase of this material.

We need to take into account such differences in super- structure when we discuss the rheological properties of liquid crystals.

Mesophase state.

Temperature relations for cholesteryl myristate

Temp.

(*)

T OC State

- -

T

²

83.8 Isotropic liquid 78.6 < ^T < ^83.4 Cholesteric state 70.5 < ^{T ,<} 78.1 Smectic state

T ,< 69.0 Solid crystal

(*)

F. P. Price and J. H. Wendorff, J. Phys. Chem.

75 (1971) 2839.

2. Dynamic behaviors at small strain amplitudes. -

First, viscoelastic behavior a t small strain amplitude will be discussed. A coaxial cylinder rheometer [4] was used to measure the dynamic behavior at small strain amplitude. The diameter of the outer cylinder is 12 mm and that of the inner cylinder is

11

mm. The amplitude of vibration is 0.50. The maximum strain amplitude is about 10.4 %.

First, the sample was held at 90 OC for more than 40 min and then it was cooled to the desired cholesteric temperature or smectic temperature a t a cooling rate of 0.2 OC/min. After remaining at least 30 min at those temperatures, dynamic properties were measured and the strain and torque were recorded on a X-Y recorder.

2.1 CHOLESTERIC

PHASE.

- The cholesteric phase studied here was the white phase in which the super- structure is almost a focal conic texture.

0 1

1

-2 -1 0 1 2

LOG W ( s")

Fro. 1.

-

Storage modulus and loss modulus as a function of frequency for the cholesteric state of cholesteryl rnyristate.

Hysteresis loops for the cholesteric phase change with frequency and show a non-linear pattern at the higher frequencies. The storage modulus G' and loss modulus G obtained from the hysteresis loops are plotted against frequency in figure I . Both G' and G"

increase with frequency. The pattern of the hysteresis loops changes from a linear to a non-linear pattern at the inflection point of the curves (shown by the symbol

I+).

2.2 SMECTIC

PHASE. -

The superstructure of the smectic phase studied here is a focal conic texture.

The hysteresis loop for the smectic phase becomes very slender and it is difficult to obtain the area from this figure ; the shape, however, is very close to that of an elastic body. The absolute dynamic modulus I G I

was obtained from the figure and plotted versus fre- quency, as shown in figure 2.

(

G

(

is constant over the full range of frequencies measured. It is evident that the smectic phase behaves as if it were an elastic body as long as the strain is very small.

0

-

2 -1 0 L O G O (5-'1

FIG. 2.

-

Absolute modulus as a function of frequency for the smectic state of cholesteryl myristate.

3. Dynamic behavior at large strain. - A cone- plate rheometer

[ 5 ] was used to measure the dynamic

viscoelastic behavior at large strain amplitudes. The diameter of the cone is 50 mm and the cone angle is 3.530

;

the material of the cone and plate is SUS-27 stainless steel. In this case, the maximum strain amplitude is 28.5 (%). The minimum detectable torque is 210 dyne.cm.

3.1 TEMPERATURE

DEPENDENCE OF THE DYNAMIC

VIsCOELASTIC BEHAVIOR. - First, the sample was held at 90

OC

for more than 40 min and then cooled at a cooling rate of 0.25 OC/min. The dynamic behavior was measured during the cooling process. The results for a particular angular frequency

(w =

0.27 s-') are shown in figure 3, as an example. In the temperature range 90 OC-83 OC (i. e. the isotropic state), no torque was detected. Bzlow 83 OC, a torque was detected.

The hysteresis loops a t 83 oC, 81 oC, 79 OC are the

ones for the cholesteric phase and are elliptical in

shape. It is noteworthy that the torque decreases with

decreasing temperature in the cholesteric temperature

range. Below the cholesteric temperature, the torque

DYNAMIC MECHANICAL PROPERTIES OF LIQUID CRYSTALS C1-301

increases with decreasing temperature. Below 77 OC,

the hysteresis loops become distorted to a rhombic shape, which is typical of non-linear viscoelastic response. Figure 3 shows that the hysteresis loop for

measurement was focal conic. The dynamic behavior for this cholesteric phase changes with the number of repetitions of the sinusoidal strain and shows a marked rheotropic behavior. For example, the results at

o =

0.27 s-' at 81 OC are shown in figure 5. As is

FIG. 5. - Changes in the dynamic hysteresis loops after repeated sinusoidal strain for the cholesteric state. w = 0.27 s-1, 81 OC.

The number of repetitions of the sinusoidal strain is shown below each curve.

FIG. 3.

-

Hysteresis loops for the dynamic viscoelastic behavior of cholesteryl myristate at various temperatures at a fixed fre- quency (w = 0.27 s-1). Vertical axis : strain. Horizontal axis :

stress.

the smectic phase is of the non-linear viscoelastic type and that the torque increases with decreasing tempe- rature. At 57 OC the hysteresis loop becomes a straight line, and the sample behaves an elastic solid. Crystal- lization of the sample probably finishes a t this tempe- rature. The dissipation energy ED was obtained from the hysteresis loops and is plotted versus temperature in figure 4 (0,

0).

FIG. 4. -Temperature variation of the dissipation energy ED per 1 cycle, for cholesteryl myristate. 0 : w = 0.27 (s-I),

: w = 5.39 (s-I), from cooling experiments. () : w + 0 (s-I), from constant temperature experiments.

3.2 CHOLESTER~C

PHASE. -

After holding the sample at 90OC for more than 40 min, the sample was cooled to the desired cholesteric temperature.

After keeping the sample at the temperature more than 30 min, the dynamic viscoelastic properties were measured. Microscopic observation indicated that the superstructure of the sample at the initial state of the

shown in the figure, hysteresis loops for 1-4 repetitions of the sinusoidal strain obviously show a non-linear viscoelastic pattern. With more repetitions of the sinusoidal strain, the torque decreases and the figure becomes elliptical and increasingly more slender in shape. At last, it becomes a vertical straight line, which is typical of liquid behavior. At this temperature and frequency, after about 150 repetitions, the hyste- resis loop becomes essentially the straight line which is characteristic of a usual liquid. T o avoid the complexity from such rheotropy, the frequency dependence of the dynamic behavior was measured only after 10 cycles of sinusoidal strain (at a frequency of 0.27 s-') had been applied

;

under these conditions the locus of the pen apparently returned to the starting point and a complete elliptical figure was obtained. The measurement at every temperature began at the lowest frequency. The dissipation energy ED per 1 cycle was obtained from the hysteresis loop at various frequencies and plotted versus frequency as shown by the closed circles in figure 6 . The curves ED

us.

log

o

for the cholesteric phase are concave upward and ED increases rather rapidly at the higher frequencies.

-

¹ 0 1 LOG a , (S-I)

FIG. 6. - Dissipation energy ED per 1 cycle as a function of frequency. : cholesteric temperature. 0 : smectic temperature.

C1-302 T. ASADA, Y. MARUHASHl A N D S. ONOGI 3 . 3 SMECTIC PHASE.

- After holding the sample at

90 OC for more than 40 min, the sample was cooled to the desired smectic temperature. Then, after keeping the sample at the temperature more than 30 min, the dynamic properties were measured. By microscopic observation, the superstructure of the sample at the initial state of the measurement was seen to be a focal conic texture. The dynamic behavior for the smectic phase is also slightly rheotropic and the measurement of the frequency dependence was done in the same manner as previously described (case 3.2). Hysteresis loops for the smectic phase are rhombic, which is typical of non-linear viscoelastic response. Also, the torque changes with frequency and temperature. The dissipation energy ED per I cycle is obtained from the hysteresis loop and plotted versus frequency, as shown by the open circles in figure 6. The frequency depen- dence of ED for the smectic phase is different from that for the cholesteric phase.

4. Discussion.

--

At small strain amplitude, the dynamic mechanical behavior for both the choles- teric state and smectic state do not show rheotropy

;

the rheological properties of the sample do not change with the previous mechanical history so long as the applied strain amplitude is not larger than about 10 (%). The results shown in figure 1 and figure 2 were obtained using a coaxial cylinder rheo- meter. The results, though not shown here, which were obtained using a cone and plate rheometer (Weissen- berg Rheogoniometer, Sangamo Controls Limited) are essentially the same. The results shown in figure 1 and figure 2 can be said to be a common dynamic mechanical properties of this material at about 10 (%)

dynamic strain amplitude when using a stainless steel vessel. It is interesting that the smectic state behaves as an elastic solid, whereas the cholesteric state behaves as a viscoelastic body. By microscopic obser- vation, it was seen that the superstructure of the sample in the cholesteric state was almost focal conic texture and that the superstructure of the sample in the smectic state was also composed of focal conic texture. Semi-microscopically the bulk textures of the sample suffered no change before and after the dynamic measurements. But, this does not always mean that there were no migrations of focal conic boundaries and so on. To clarify this interesting point, a further investigation with simultaneous microscopic observa- tion is required during the dynamic measurements.

Though the frequency range covered in this dynamic experiment is not wide enough to discuss the visco- elastic properties of the cholesteric state in detail, it is suggested from the G" us. log o curves in figure 1 that the mechanical dispersion seems to be divided at about o

⁼

1 (s-') into two main mechanical disper- sion regions such as longer relaxation time region and shorter relaxation time region. The behavior in the longer relaxaticn time region

[a

<

16-'

(s-')I must represent the motion of large mass such as large grain

of molecular aggregate. The behavior in the shorter relaxation time region

[o

> I (s-I)] must represent the motion of small mass in the molecular aggregate.

At

large strain amplitude, the viscoelastic behavior for the cholesteric phase shows a marked rheo- tropy [3.1]. This phenomenon may be attributed to the change in superstructure or texture of the cholesteric phase. Porter and Johnson [6] reported that the steady flow viscosity of a cholesteric liquid crystal system becomes lower with increasing shear rate. Also, Pochan and March [7] have reported the orientation of helical axis of cholesteric liquid crystal unit with shear. Taking into account those results, it may be considered that the lowering of the viscosity with increasing shear may be attributed to the orientation of molecular aggregates in the direction of flow as pointed out by Porter

et

al. [6]. In other words, the viscosity of flow for the oriented cholesteric state, such as Grandjean textures, is lower, so long as the direction of shear is parallel to the Grandjean plate.

Then, the results obtained here, that the hysteresis loop becomes narrower and narrower with the repetition of sinusoidal strain, may be interpreted by considering the orientation of molecular aggregates in the choles- teric phase so as to lower the friction among them.

It must be pointed out however, that optical observa- tions as well as mechanical measurements are required to interpret the results in more detail.

The behavior of the cholesteric phase becomes more fluid-like after the repetition of numerous sinusoidal strains, and also, the structure seems to change greatly during the first few repetitions. Consequently, we can obtain the dynamic viscoelastic properties for the initial focal conic textures of the cholesteric state only when the number of repetitions is extrapolated to zero. We may obtain the dynamic viscoelastic pro- perties for some other texture, such as so called dynamic focal conic texture [7], from the measure- ment after many repetitions of sinusoidal strain. The behavior of the dissipation energy as a function of frequency or temperature, after 10 repetitions of sinu- soidal strains (Fig. 6), represents only the behavior of a transient state of the texture which is considered to be a somewhat deformed focal conic texture. E:, the extra- polated value of ED at zero frequency, is plotted against temperature in figure 4 as

O.

It is interesting that both ED and E: decrease with decreasing tempera- ture in the cholesteric temperature range. These results may possibly be explained by assuming that the decreasing flow resistance, due to breakdown of the initial superstructure by repetition of sinusoidal strain, overcomes the increasing flow resistance due to the lower temperature.

The hysteresis l ~ o p for the s m ~ c t i c phase does not

change as much with repetition of sinusoidal strain

even at large strain amplitude. For the smectic phase,

the rheological properties are very much affected by

the magnitude of strain. The rheotropy unde: strains

larger than the yield strain is not s~ great.

DYNAMIC MECHANICAL PROPERTIES O F LIQUID CRYSTALS C1-303

References

[l] BROWN, G. H., SHEU, W. G., Chem. Rev. 57 (1957) 1049. [4] HORIO, M., ONOGI, S., OGIHARA, S., J. SOC. Materials Sci.

[2] PORTER, R. S., JOHNSON, J. F., Chap. 5, Rheology ZV, (Japan) 10 (1961) 350.

F. R. Eirich ed. (Academic press, New York) 1967. [5] MANTANI, M., J. SOC. Materials Sci. (Japan) 22 (1973) 424.

[3] PRICE, F. P., WENDORFF, J. H., J. Phys. Chem. 75 (1971) [6] PORTER, R. S., JOHNSON, J. F., J. Appl. Phys. 34 (1963) 55.

2839. [7] POCHAN, J. M., MARSH, D. G., J . Chem. Phys. 57(1972) 1193.