Comparison between the tilted SmCα* and SmCγ* phases of MHPOBC studied by optical techniques

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Comparison between the tilted SmCα* and SmCγ*

phases of MHPOBC studied by optical techniques

John Philip, Jean-René Lalanne, Jean-Paul Marcerou, Gilles Sigaud

To cite this version:

John Philip, Jean-René Lalanne, Jean-Paul Marcerou, Gilles Sigaud. Comparison between the tilted

SmCα* and SmCγ* phases of MHPOBC studied by optical techniques. Journal de Physique II, EDP

Sciences, 1994, 4 (12), pp.2149-2159. �10.1051/jp2:1994253�. �jpa-00248121�

Classification Physic-s Absfi.ac.ts

61.30 78.20E 78.35

Comparison between the tilted Smcj and SmC( phases of MHPOBC studied by optical techniques

John

Philip (*),

Jean-Rend Lalanne, Jean-Paul Marcerou and Gilles

Sigaud

Centre de Recherches Paul Pascal, CNRS, Avenue du docteur Albert Schweitzer, 33600 Pessac, France

(Receii>ed ^J July 1994. ieceii>ed in

final

form 7 september J994,

accepted12

septembei 1994j

Abstract. We present optical investigations in the SmC,$ and

Smcf

phases of _a low molar _mass

chiral liquid crystal,

4-(1-methylheptyloxycarbonylj

phenyl

4'-octyloxy biphenyl-4-carboxylate

[MHPOBC(R)]. The polarizing microscope observations (PMO), optical rotatory power (ORP) and

anisotropic Rayleigh

scattering (ARS)

investigations

show that the smectic

C,)

~tructure

strongly differs from the

SmC( phase.

The ORP in the smectic Cm

phase

has been found _to be very weak (less than o.2

deg/mm), compared

to

ordinary liquids

with strongly chiral molecules and _to

other smectic C* phases. Although pretransitional ORP effects in the

vicinity

of the

SmA =SmC* phase transition have been reported earlier in chiral

liquid

crystals, no such

behaviour iS noticed in this compound at the

SmA=SmC,)

and

SmC,)=SmC*

phase

transitions. The absence of critical behaviour in the ARS _at the SmA ₌ SmC

f phase

transition is _a clear indication of the nonexistence of

« local biaxial _» character of the SmC,) phase. Moreover, the PMO studies also confirm the

«

macroscopically

uniaxial nature _» of all these

phases

except

SmC(

phase, which _was found _to be birefringent. The possibility ^of a divergence in the _« biaxial fluctuations

» at the

SmC/

=

Sm?

phase transition is also discussed.

1. Introduction.

Since the

discovery

of the chiral smectic

phase

in 1975

by Meyer [I],

this field has attracted wide attention among the scientific

community

because of its

technological applications

as

well _as the keen interest in the fundamental

understanding

of these _new

phases.

In

1989,

Chandani et al. discovered _{a new} chiral

compound

MHPOBC which possesses four

tilted smectic

phases [2],

This material has been _a

topic

of intense research

during

the last 4 years, in order _to understand the

properties

and unravel the structure of these smectic

phases [3-23, 39],

As a result, the structures of two of them have been

clearly

identified _as SmC * and SmC

( phases,

The SmC *

phase

is the classical ferroelectric

phase ],

in which the

(*) Associate Professor,

University

of Bordeaux I, Piesent address Laser

Laboratory, Department

of Physics, Indian lnstitute of

Technology,

Madras 600036, India.

@Les

Editions de

Physique

1994

2150

transverse

dipole

moments of the constituent molecules

align perpendicular

to the

tilting plane,

which leads to a

macroscopic polarization

within the smectic

layers.

The

SmC( phase (so

called

«antiferroelectric»)

is found to be identical to the earlier discovered smectic O*

phase,

^{in which the tilt in}

adjacent layers

alternates in directions

symmetrical

with respect

to the

layer

normal («

herringbone

_»

structure) [24-26].

One should note that the racemic

mixtures

[18, 25],

_or achiral

compounds [26]

exhibit the SmO _or

SmC~ phase,

which is

miscible in any

proportion

with its SmO* _or

Smct analogue.

This proves that _a

possible

in-

plane polarization,

even if it has been observed when

building

thin films

layer by layer [25],

does _not

play

_a fundamental role in this _structure. in summary, the

SmC( phase

^is

essentially

a

bilayered, tilted,

non-ferroelectric and biaxial

phase.

On the other hand, the _{exact structures} of the

remaining Smci

and

SmC( phases

are not yet well understood.

The

SmC( phase (so

called _« ferrielectric

»)

is from several aspects intermediate between the SmC * and

SmC( phases.

In MHPOBC

[20],

it is

separated

from them

by

two

weakly

first

order transitions. It exhibits _a finite

permanent polarization,

weaker than the SmC * one, which

reacts in

characteristically peculiar

_ways to external electric fields

showing multistability

in

thin

planar samples [8]

_or induced

biaxiality

in thick

homeotropic samples [12].

Different

models have been

proposed

in order to describe the

SmC( phase

^from a mixture of _an

essentially monolayered

SmC ^*

(tilt angle

0, azimuthal

angle

~fi) and _a

bilayered SmC( phase (0,

=

0~,

_~fi~ = ~fi, + ar in successive

layers).

One model

[9]

deals with different tilt

angles 0,

in

adjacent layers,

however this is not

supported by X-ray scattering

results

[10].

Others

[2, 27]

introduce different azimuthal

angles (4l,

~, ~ ~fi, and ~fi, _~, ~

4l,

₊

ar),

which do _not

seem to account for the

conoscopic

observations under electric fields

[12].

The _most

reasonable

models,

_at the time of

writing,

are the

multi-layer

_ones

lo,

~, ⁼

0,

_~fi,

~j ⁼

~fi, or ~fi,

~ ⁼ ~fi, +

ar)

which may

possibly

be described

by

a devil's staircase

[18].

As far _as the

Smci phase

is

concerned,

the exact structure is not known at this stage.

According

to an earlier report

[I

I

],

the

Smcj phase

could be considered _to be _a biaxial helical smectic A

phase.

From the

pretransitional

^effect observed in the circular dichroism studies I1,

14],

it has been concluded that the

Smcj

and SmA

phases

_possess ^helical structures.

However,

due _to the lack of direct

experimental evidence,

it may be premature ^{to make such}

hasty

conclusions.

There have been _numerous articles in recent years on various

properties

of these

phases,

with _{some recent} reports

contradictory

to the

previous

ones. We have _no intention to

point

_out

all those

discrepancies

in this paper, but _to report _on a consistent

optical study

of these

phases.

In order _to obtain reliable

results, long

term thermal

stabilization,

step

by

_step thermal variation with very small increments and

good optical quality

of the

samples

are

prerequisites.

One

major problem

however is that

obtaining optical quality homeotropic

and

planar samples

is

especially

difficult with this

compound.

Moreover, these chiral smectic

phases

are observed at

high

temperatures

(above

115

°C). Probably,

this

explains

the limited number of

optical

studies in this

compound.

The

questions

which _we wanted to address from these

investigations

were the

following

_:

I)

do the

SmC,$

and

SmC( phases

exhibit similar

optical properties

?

2)

Do

they

_{possess any}

biaxial character ?

3)

Do the SmA ₌

SmC,)

and

SmC(

=

SmC( phase

transitions show any

critical behaviour in the ORP _or ARS ?

2.

Experimental.

The

experimental

_set-up for the

optical _rotatory

_power

(ORP)

and

anisotropic Rayleigh scattering (ARS)

measurements are shown in

figure

I.

Homeotropically aligned samples

100 _~Lm thick have been

prepared by

the combination of

coating

with

octadecyl-triethoxy

IEEE Bus

FM

IEEE Bus

PC RCU

L--~

MM

PTA BUS

~---~ D

O

OCU R

A.D REF

I-L=@ _'---,~

L AT

~~

P HW

~

A SF

S

Fig.

I.

Experimental

_set-up for

Anisotropic Rayleigh

Scattering (ARS) and

Optical

Rotatory Power (ORP) measurements. L He-Ne Laser (Uniphase model 1135P. A

= 632.8 nm, P

= mW) ^AT

motorized _attenuator BS beam

splitter

P, A polarizers HW motorized halfwave

plate

S

sample

oven SF

: spatial filter (diameter ₌ 7 mm) M mirrors _; AD annular

diaphragm

(intemal

diaineter

=

4.19 _mm, extemal diameter

= 4.85 mm) R

rotating

shutter O _:

objective (Soligor,

f ₌ 105 cm) D diffusers IF _: interference filter (Oriel, A 632,8 _nm, AA

=

0,5 nm) PM

photomultiplier

(Hamamat-

su R6495) PCD photon

counting

device (Hamamatsu C1050) MM multimeter

(Philips

PM2534) FM _: frequency meter (Philips PM6665) RCU _: rotation control unit PC IBM PS8550 OCU _{: oven}

control unit,

silane and

alignment

in a

magnetic

field of about I

T, by cooling

down

slowly

from the

isotropic phase,

Careful

coating

of the surfactant _on the

plates

and

capillary filling

of

samples

between the

plates

result in

well-aligned, optical quality samples.

The temperature ^{of the}

sample

is maintained within millikelvin

long

term

stability by using

_a

triple-walled specially designed

_oven with _two

heating

stages, ^controlled

by

_a computer.

Step by

_step

experiments

can

then be run with steps as small _as 5 mK.The thermal

integrity

of the

sample

was checked

before each measurement. A He-Ne laser beam of

wavelength

632.8 _nm, with _a very low

power

(about

10-6 W within the

sample)

is used. After

spatial filtering,

the beam

intensity

is stabilized and the

input polarization

is chosen

(X axis) by using

a

polarizer

P. The beam passes

through

the

homeotropically aligned sample placed

inside the _oven.

Here,

the molecules _are oriented normal _to the surface of the

glass plates,

^with the director

along

the Z axis. The _oven is mounted in such _a way that the

sample position

_can be

adjusted

to have its

optical

axis

parallel

to the _{wave vector} of the incident laser beam. The

analyzer

A is in crossed

position

with

respect to the

first,

with the output

polarization along

^{the Y axis.} At each stabilized temperature, ^{the rotation of the}

incoming

beam

polarization,

due _to the

optical

_{rotatory power}

of the

sample,

_was

compensated by rotating

the halfwave

plate

in the

appropriate

direction.

When the ORP of the

sample

is best

compensated by rotating

^the half-wave

plate,

^the

intensity

2152

of the

light

transmitted

through

the

analyser

^will have a minimum value and the ORP of the

sample

will be twice the

angle (a

). The half-wave

plate

was mounted _{on a} microcontrolled rotation stage, ^which was interfaced with _a computer. ^One step ^{of the rotation of the half-wave}

plate

represents ^0.01

degree

of _a. The half _wave

plate

_was rotated in the

right

direction, where

the

intensity

has _a minimum

value,

to find the

approximate

minimum

intensity. Then,

with

respect

to this

minimum,

_a

parabola

was

plotted by taking

five _{or more} measurements on each side of the

rough

minimum.

The annular

diaphragm

allows measurement of the

XY-component

of the ARS

intensity

at a

small well defined

angle.

The

scattering

_wavevector in this

experiment

is about 4.15 _x 10~ cm~ ~. The scattered

intensity

is measured

by using

_a

photon counting

unit and _a

frequency

meter. In order _{to correct} the ARS

signal

^for both the fluctuations of the

incoming

^{He-Ne laser}

beam

intensity

and the

photon counting

unit, the laser beam is deviated

by using

_a beam

splitter (BS).

A

rotating cylinder (R), placed

in front of the

photon counting

unit, allows the

signal

and reference measurements

separately.

The rotation of the

cylinder

is controlled

by

_a rotation control unit

(RCU),

interfaced _to the PC

by

a PIA bus. At each

temperature,

^the

ORP,

ARS and the reference helium-neon laser

intensity

are recorded. The whole

experiment

is _run

by

the

computer.

The full details of the

experimental

set-up will be

published

in a

forthcoming

paper

[28].

3. Results and discussion.

3_ j pMO.

Figure

2 shows the PMO of the five different

phases,

^under crossed

polarizers,

in

a 100 _~Lm thick

homeotropic sample.

SmA,

Smcj,

SmC* and

SmC( phases

_are

optically

uniaxial without any apparent texture. But _we have _never obtained _a

good alignment

in

SmC( phase,

^{for which} a texture

always

_appears

leading

to a residual

in-plane birefringence.

a) b)

c) d) e)

Fig. 2. Optical texture of MHPOBC(R) observed under _a polarizing

micrmcope.

in _a 100 _~m thick

homeotropic ~ample,

with crossed

polarizers.

(a) SmA _; (b)

SmC,)

(c) SmC* (low temperature side) _; (d)

SnIC)

and (e)

SmC(.

When

compared

to

conoscopic

observations

[12],

_our results _are

fully compatible

in the first four

phases.

However, the

SmC( phase

_appears to be uniaxial in conoscopy, hence _we have to

assume that the residual

birefringence

cancels _out when

integrated

over the illuminated _area.

Our PMO

experimental

observations show that

SmC( phase

_appears

by

domain nucleation _at both first order transitions from SmC * and the

SmC( phases.

This could be _an indication that the

SmC( phase

can be described

by

_a

multilayer

model

[18]

in which the

continuity

of the

alignment

between the

neighbouring

domains is

probably

difficult _{to ensure.} It may also be reminiscent of what has been observed

previously [33, 36]

in thin

planar samples,

I-e- the coexistence of the SmC * and

SmC( phases

in pure

compounds

_over a finite temperature range.

Obviously,

_as

proved by

DSC

experiments

in bulk

samples,

^this coexistence is

strongly

forbidden from the

thermodynamics point

of view. If this is the _case, the observed

inhomogeneities

_may be induced _at the surfaces

by peculiar anchoring.

3.2 ORP MEASUREMENTS. In the smectic C *

phase,

the molecular axis is

uniformly

rotated

around the _z

axis,

with _a fixed tilt

angle

_Ho. The azimuthal

angle

^is ~fi(z ) ₌ (2 «IL ) _z, where L is the helical

pitch

and _z the coordinate

along

the

layer

normal. The orientation of the molecular

axis is then described

by

the unit _vector

[29]

n(z)

=

(sin 0~

cos ~fi, sin

0~

^sin ~fi, cos

o~). (1)

The components of the dielectric tensor _e

(x)

_are functions of Ho and ^~fi

[291.

The

periodicity

of the dielectric _tensor in SmC *

phase

is L

=

A~/n,

^where

Ao

^is the selective reflection

peak wavelength

and _n is the _mean

in-plane

refractive index. The

optical

rotatory power p for _a beam

propagating

normal to the

layers

is

given by [301

~ ~ ~

A

(A~

~ A~)

~

~~~~4 ~~~

~~~

where

(n~~~

_n~,~ ^{is the}

birefringence

_seen

by

the beam which _can be

approximated [29, 301 by

An _x

sin~

Ho if An is the

anisotropy

of the refractive index in the

corresponding

SmA

phase

_{; A} is the

wavelength

of the

probe

beam. Let _us

point

_out that the above

equation

is valid

only (1/2)

AnL _w A and

pL

« _ar. Both these conditions _are fulfilled in _{our case.} We have assumed that

equation (2)

is valid in all the

well-aligned phases

of _our

study,

and that

bo(T)

and

A~(T)

are the

only

temperature

dependent

parameters. ^{Here the variations of} An and _n with temperature are very small.

Equation (2)

will be used to

interpret

our

experimental

results _on ORP. The variation of the ORP _{as a} function of

temperature

^{in the} SmA and

Smcf phases

is shown in

figure

3. Two main observations from this

figure

_{are :}

(I)

the

ORP,

in the _two

phases

SmA and

Smcf,

is less than 0.2

deg/mm.

^This means that it is smaller than that of

ordinary liquids

with

strongly

chiral molecules. For

example,

in

liquid

camphor,

the ORP is around 0.5

deg/mm.

The

X-ray [101

and refractive index

[121

measure- ments in this

compound

show that the

Smcj phase

is

tilted,

with _a low tilt

angle (about

7° _at the low temperature

edge).

From the

pretransitional

effect in the circular dichroism

signal

I1,

141,

it has been concluded that this

Smcj phase

has _a helical

pitch,

with _a value around

250 _nm.

By setting

_Ao

=

250 _nm in

equation (2),

with the

anisotropy

in the refractive index Art

=

0.192

[281

and

o~

w 7

degrees,

we obtained the ORP

p m 3 _x 10~ ^~

deg. ^mm~'.

Such _a small value of

ofitical

rotation _cannot be

experimentally

detected in 100 _~Lm thick cells. One must also notice the sudden

jump

in ORP _at the

Smcj

# SmC ^*

phase

transition, whereas the tilt

angle

varies

continuously

in these _two

phases. Obviously,

there is _a finite

jump

in the

pitch

of the helix _at the

phase

transition. The

pitch,

if _{non zero,} is

changing

from _a value lower than 250 _nm in the

Smcf phase,

to about 450 _{nm at} the

high

temperature

edge (121.3 °C)

of the

0.4

#f

,i

~ 0.2

fi S

P

_oi .

fl

^~O'

_

i

^o

-0.1

120,5 121,5

emperature(°C)

3.3 ARS MEASUREMENTS. ARS

mainly

reveals the _«

in-plane

_»

(XOY

shown in

Fig. I)

orientational fluctuations. These fluctuations

generate Rayleigh scattering

of _a

light

_wave with

a wavevector q,.

They

_are related to the fluctuations of the dielectric tensor F,~, ^{associated with} the

sample according

to the

following

relation

Ixy(q~)

m

Fxy(qs)(~) ¹³⁾

where

Fxy(q,)

^{is the Fourier transform of}

pxy(r), Ixy

is the XY component of the scattered

intensity

and q~ the

scattering

_wave vector.

Recently,

we have undertaken both the theoretical and

experimental

studies _on the XY

(Fig. I)

_components of the ARS in SmA

phases,

in the

vicinity

of the transition to Nematic

(N)

or SmC

[371.

We showed that

Iyx(q~) scattering intensity

consists of _two

contributions,

which _can be written _as follows

IYX(q,)~ F)((jYX(q~)(~)

⁺

((AF((~(qs)(~) (4)

where _F~

= Fjj

(;)

_F~

(r)

denotes the

anisotropy

of the dielectric _constant of the

sample,

the

symbols

^I and I represent ^the components

parallel

and

perpendicular

to the

layer

normal,

respectively. jxy(q,)

_{appears as an}

integral involving

orientational variables

linking

both the

fluctuating

director of the

phase

and the normal _to the

layer,

in the

laboratory

frame.

AF)(I(q,)

introduces the local biaxial character of the

phase. Then,

the first _term

Fj( jxy(q,) ~),

^is

directly

linked _to the orientational fluctuations of the director of the

phase,

while the second _one

AF)(I(q,)(~)

^is a term

introducing

the _« local _» biaxial

properties

of the

sample. Earlier,

we have also shown that in _« classical _» SmA

phases, ii)

the first _term is

largely

dominant

(absence

of local biaxial

character)

and

(it)

it has _a flat thermal behaviour

down _to

lO~~

_{Kelvin close to the SmA = SmC}

phase transition,

in agreement ^{with the}

theoretical

predictions.

We have also shown that the ARS

investigations

could be _a well-

adapted

tool for the

study

of the biaxial

properties

of chiral smectic

phases

of MHPOBC. In this _case, the second term of

equation (4)

is

expected

to evidence the

diverging

orientational

fluctuations in the

vicinity

of the

phase

transition between _two

locally

uniaxial and biaxial

phases,

whereas the first _term shows _a flat

background

ⁱⁿ the

region

_very close _to the

phase

transition.

Equation (4)

_can be reduced to

IYXIq~) ~10

+

II

_~~ ~

15)

where

lo

and

I,

_are the

background

and critical

amplitudes, respectively,

t ₌

(Tm T)/Tm

_y

is the critical exponent ^{of the}

susceptibility

and _T~* the second order critical temperature associated with the transition

occurring

at T~.

Figure

5 shows the variation of the ARS

intensity

_{as a} function of temperature ^{in the}

SmA, Smcj

and at the

high

temperature

edge

of the SmC*

phases,

where the

large

value of ARS

intensity

observed is due to selective reflection

[29, 301.

In the latter _case, ARS is due to a

continuous variation of the transmitted

light

wave

(linear polarization

of the

incoming

beam becomes

circularly polarized

at the selective reflection

temperature)

and this will _not be discussed here. Two

important

observations from these studies _{are as} follows.

(I)

The

scattering intensity

is very small in the SmA and

Smcj phases.

No

pretransitional

effect is noticed _at the SmA _to

Smcj phase transition,

which is

reported

to be _a second-order transition

[201.

If the

Smcj phase

is _a biaxial

phase,

_one would expect a biaxial critical

contribution in the

vicinity

of the

phase

transition. In _our

experiment,

_no such

divergent scattering intensity

could be noticed. This is _a

strong

^indication of the nonexistence of such _a

« local biaxial _» character of the

Smci phase,

in contradiction _to earlier reports

[8,

11,

391.

It

JOURNAL DE

3

~ (2.5

a

c

w g

c

m _. « c

# 1,5

fl

~o ~

'

l

120.5

~SOO

(400

I

a 300

I

_smc~*

@f200

(

loo

o

°'

l18.7 l18.75 l18,8 l18,85 l18.9 l18.95

Temperature(°C)

Fig, 7. The variation of ARS

intensity

at the

Smcf

= SmC? phase ^transition _in MHPOBC(R). The circles represent the

experimental

data and the solid line is obtained by the expression1= 42 ₊ 0.01 _t- ^' where _t (T~* T)/T~* with T~* 118.945 °C.

Approaching

the

SmC(

_to

SmC( phase

transition from the low temperature ^side causes a neat and

regular

increase

(at

least

by

_a factor of

10)

of the ARS

intensity,

followed

by

_an

intense

peak

_at the transition (at temperature

T~).

^We can

reasonably

_assume that the observed

regular

increase of XY

scattering

is due _{to a} biaxial-uniaxial

phase

transition

likely

_{to occur} at an effective temperature Tm ^{located somewhere above} T~.

So,

two

qualitative explanations

can

then be assumed

(I)

the effective temperature Tm ^{is far above the observed transition}

temperature

T~ and the intense observed

peak

could be due to the

scattering by

the boundaries between

SmC(

and

SmC( phases

at the

transition,

similar but much stronger to what

happens

at the

Smcj

# SmC *

phase

transition

(it)

the effective temperature Tm ^is very close _to the observed transition temperature

T~, ^{In this} case, the whole observation could be taken into _account

by

a

single divergence

according

to

equation (5), Figure

7 may support ^this

argument.

^{The solid continuous line} represents ^{the variation of} the

right

hand side of

equation (5)

with

lo

=42 and

Ii

~

0,01 and _T~*

= I18,945 °C. Here, the critical exponent _y ^{is chosen} as

(according

_{to mean-}

field

theory),

Of _course, this work cannot allow _a definitive choice between the two

explanations.

However, let _us

point

out that similar results have been very

recently

obtained from ARS

investigations

at the

Smci

=

SmC( phase

transition of _a related

compound

where both

phases

_are found _to be well

aligned,

in _a 100 _~Lm thick

homeotropic sample,

4.

Summary

and conclusions.

The

Smcf phase

_{appears as a}

macroscopically

uniaxial

phase,

^which cannot be

distinguished

from the SmA

phase

^neither

by

PMO _nor

by

ORP, There is _no evidence for _a uniaxial-biaxial

phase

transition between the SmA and

Smcj,

which would have been revealed

by

_a

divergence

of the ARS

signal.

At the

Smcf

# SmC*

phase transition,

_we measure a finite

jump

of the

ORP,

that confirms the concomitant

jump

of the helical

pitch [281.

The

SmC( phase,

under the _same

conditions,

has _never shown _a

perfect

^uniaxial

alignment, although

it

reversibly

transforms _to

well-aligned

SmC* _or

SmC( phases

_on

changing

the

temperature, ^{This lack of}

optical alignment

forbids any

quantitative

utilization of ORP and ARS data in this

phase.

On the other

hand,

the

divergence

of the

signal

_on the low

temperature

side of the

SmC(

=

SmC( phase

^transition reveals the

approach

of _a biaxial = uniaxial

2158 JOURNAL DE

«virtual _» transition

iemperature _Tm, confirming

the biaxial character of the SmO* and

SmC( phases [251.

As _a

conclusion,

let _us

point

out that the

optical techniques reported

here are

well-adapted

to the characterization of biaxial = uniaxial

phase

transitions.

So,

the natural

development

of this work consists in the

investigations

in related

compounds,

of

Smci

=

Smci

_or SmA #

SmC~ phase

transitions which have been

recently

shown _to exist in both chiral and non-chiral related

compounds.

Acknowledgments.

We _are

grateful

to Prof. Atsuo Fukuda for his constant

help during

this work. We also thank Dr. H. T.

Nguyen,

^Chisso

Corporation

and Showa Shell

Sekiyu

K. K. for

providing

_us with the

samples.

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