X-ray investigations and magnetic field effect on a nematic phase of disc-like molecules
Domain: Physics
The X-ray diffraction patterns of powder samples of the fluid mesophase of the hexaalkoxybenzoates of triphenylene which is a disc-like molecule are similar to the diffraction patterns of non orientated nematic phase of rod-like molecules. Furthermore, we give evidence for a uniaxial character of this anisotropic medium with a negative diamagnetic anisotropy, the flat molecules are orientated parallel each other by a rotating magnetic field. In addition, the X-ray patterns of orientated samples of the hexyloxy compound revealed pretransitional effects in this nematic phase of disc-like molecules (ND). This short-range translational order, sort of skewed cybotactic groups originates from the existence of a low temperature columnar phase with an original tilted packing here described.
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Publié le : 28/06/2012
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Type de la publication : Rapports et thèses
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Savoirs > Science de la nature
Source : Journal de Physique
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147
X-ray
investigations
and
magnetic
field
effect
on
a
nematic
phase
of
disc-like
molecules
A.
M.
Levelut,
F.
Hardouin*
Laboratoire
de
Physique
des
Solides,
Université
de
Paris-Sud,
91405
Orsay,
France
H.
Gasparoux,
C.
Destrade
and
Nguyen
Huu
Tinh
Centre
de
Recherche
Paul-Pascal,
Université
de
Bordeaux
I ,
33405
Talence,
France
(Reçu
le
14
avril 1980,
révisé
le
Il
septembre,
accepté
le
15
septembre
1980)
Résumé.
2014
Les
diagrammes
de
diffraction
des
rayons
X
d’échantillons
non
orientés
de
la
mésophase
fluide
de
molécules
en
plateau,
les
hexaalkoxybenzoates
de
triphénylène,
sont
identiques
à
la
figure
de
diffraction
d’une
phase
nématique
non
orientée
de
molécules
en
bâtonnets.
De
plus,
nous
montrons
le
caractère
uniaxe
de
ce
milieu
anisotrope
qui
possède
une
anisotropie
diamagnétique
négative,
car
les
molécules
plates
s’orientent
parallèlement
les
unes
aux
autres
sous
l’action
d’un
champ
magnétique
tournant.
Enfin,
les
diagrammes
de
diffraction
d’échan-
tillons
orientés
du
dérivé
hexahexyloxybenzoate
montrent
l’existence
d’un
ordre
prétransitionnel
analogue
à
celui
qui
est
observé
dans
la
phase
nématique
et
au
voisinage
de
la
transition
nématique
SmC.
Ici,
cet
effet
prétransi-
tionnel
est
relié
à
l’existence
d’une
phase
de
basse
température
que
nous
décrivons
et
où
les
molécules
sont
empilées
en
colonnes,
leur
plan
moyen
étant
oblique
par
rapport
à
l’axe
de
la
colonne.
Abstract.
2 0 1 4
The
X-ray
diffraction
patterns
of
powder
samples
of
the
fluid
mesophase
of
the
hexaalkoxybenzoates
of triphenylene
which
is
a
disc-like
molecule
are
similar
to
the
diffraction
patterns
of
non
orientated
nematic
phase
of
rod-like
molecules.
Furthermore,
we
give
evidence
for
a
uniaxial
character
of
this
anisotropic
medium
with
a
negative
diamagnetic
anisotropy,
the
flat
molecules
are
orientated
parallel
each
other
by
a
rotating
magnetic
field.
In
addition,
the
X-ray
patterns
of
orientated
samples
of
the
hexyloxy
compound
revealed
pretransitional
effects
in
this
nematic
phase
of
disc-like
molecules
(ND).
This
short-range
translational
order,
sort
of
skewed
cybotactic
groups
originates
from
the
existence
of
a
low
temperature
columnar
phase
with
an
original
tilted
packing
here
described.
J.
Physique
42
(1981)
147-152
JANVIER
1981,
Classification
-
Physics
Abstracts
61.30
-
64.70E
1.
Introduction.
-
Recently
a
new
mesogenic
series
[1]
of
disc-like
molecules
has
been
discovered
which
exhibits
a
highly
fluid
mesophase,
the
texture
of
which
is
similar
to
the
classical
nematic
one’.
Moreover,
it
can
be
compared
to
the
textures
seen i
the
carbonaceous
mesophase
which
is
a
mesomorphi
state
of
a
complex
mixture
containing
polyaromatic
molecules
with
some
aliphatic
parts
[2].
Therefore,
the
new
phase
of
disc-like
molecules
could
be
a
pure
component
simple
model
for
the
carbonaceous
mesot-
phase.
We
have
performed
X-ray
and
magnetic
investi-
gations
in
order
to
obtain
some
information
on
the
structure
and
the
symmetry
properties
of
this
new
fluid
anisotropic
phase.
2.
Experimental
technics.
- We
have
studied
two
derivatives
of
the
hexaalkoxybenzoates
of
tripheny-
lene
(Fig. 1)
with
alkyl
chains
of six
and
eleven
carbons.
Each
compound
exhibits
two
mesophases :
the
higb
temperature
phase
is
fluid
(with
similar
textures)
for
both
compounds.
The
low
temperature
phase
is
much
more
viscous.
The
texture
of
the
C6
derivative
is
a
mosaic
one
[3]
which
is
observed
for
the
first
time
with
disc-like
compounds
while
the
texture
of
the
C11
derivative
is
similar
to
the
texture
of
the
D1
phases
of
the
alkanoates
of
triphenylene
[4,
5].
X-ray
investigations
have
been
made
on
powder
samples
with
a
Guinier
camera
using
the
CuKà
radiation.
At
the
same
time,
samples
held
in
a
Lin-
deman
glass
capillary
have
been
studied
within a
3
kG
magnetic
field
and
illuminated
with
a
point-
focusing
X-ray
beam
CuK«
perpendicular
to
the
magnetic
field.
*
Permanent
address :
Centre
de
Recherche
Paul-Pascal,
Univer-
sité
de
Bordeaux
1,
33405
Talence,
France.
Article published online by
EDP Sciences
and available at
http://dx.doi.org/10.1051/jphys:01981004201014700
148
Fig.
1.
-
Chemica.l
formula
of
the
hexaalkoxybenzoates
of
tri-
phenylene.
To
determine
to
what
extent
an
extemal
magnetie
field
is
apt
to
induce
a
single
domain
(in
the
bulk)
of the
highly
fluid
mesophase,
we
have
also
performed
magnetic
measurements.
First,
by
means
of
the
classi-
cal
Faraday
method
[6]
(the
magnetic
field
has a
vertical
gradient)
the
magnetic
susceptibilities
arç
measured.
In
addition,
we
have
undertaken
some
experiments
with
a
homogeneous
rotating
magnetic
field
of about
11
kG
[7].
This
latter
technique
is
necesr
sary
to
define
the
sign
of
the
diamagnetic
anisotropy.
3.
Results.
- 3.1
X-RAY
DIFFRACTION
PATTERNS
OF
THE
FLUID
MESOPHASE. -
X-ray
diffraction
pat-
terns
of
samples
held
in
a
capillary
tube
outside
of
the
magnetic
field
in
the
fluid
mesophase
(Fig.
2)
aré
very
similar
to
the
diffraction
patterns
of the
isotrope
liquid
phase
of
the
C11
derivative
or
other
disc-like
mesogenic
compounds
(we
have
not
attempted
to
reach
the
isotropic
phase
for
the
C6
compound
T
& # x 3 E ;
274°C).
We
have
two
broad
diffraction
rings,
Fig.
2.
-
X-ray
diffraction
pattern
of
the
fluid
mesophase
of
a
non
orientated
sample
(C6
derivative).
the
inner
ring
being
of
high
intensity
compared
to
the
outer
ring.
The
outer
ring
lies
at
while
the
inner
is
related
to
the
molecular
size
and
corresponds
to
d
=
20.5
A
for
the
C6
and
d
=
26.5 À
for
the
Cl,.
This
is
very
similar
to
the
diffraction
by
a
non
orientated
nematic
phase.
Nevertheless
a
striking
différence
lies
in
the
relative
intensity
of the
two
rings
:
the
inner
is
of
higher
intensity
in
the
case
of
this
flui4
mesophase
of
disc-like
molecules
while
the
outer
is
more
intense
for
nematics
of
rod-like
molecules.
Moreover,
the
3
kG
magnetic
field
has
no
effect
on
the
fluid
mesophase
of
the
Cl 1
derivative.
Going
down
from
the
isotropic
phase
up
to
the
columnar
phase
appears,
we
observed
only
a
slight
narrowing
of
the
inner
ring
for
X-ray
patterns
in
the
fluid
phase
domain.
In
contrast,
in
the
case
of
the
C6
derivative
the
magnetic
field
orientates
the
sample
(Fig.
3).
We
have
to
notice
that
it
is
necessary
to
heat
at
least
20
°C
above
the
transition
temperature
from
Fig.
3.
-
X-ray
difïraction
pattern
of
the
fluid
mesophase
of
an
orientated
sample
of
hexyloxybenzoate
of
triphenylene
(the
arrow
indicates
the
direction
of
the
director).
a)
3
hours -
220 °C ;
b)
1
hour -
205 °C.
149
the
columnar
phase
in
order
to
have
a
good
alignment
of the
sample.
Thus,
the
viscosity
of the
mesophase
has
likely
an
influence
upon
the
sample
alignment
obtained
by
a
rather
weak
magnetic
field
(-
3 kG).
The
best
orientation
effects
are
obtained
after
a
rotation
of
7c/2
of
the
capillary
tube
around
its
axis
which
is
perpendicular
to
the
magnetic
field.
The
same
process
of
orientation
has
been
successful
for
the
nematic
lyotropic
mesophase
of
type
II
[8]
which
is
supposed
to
have
identical
symmetry
and
magnetic
properties
as
the
disc-like
nematic
phase
(see
3.4).
Further
rotation
of
the
sample
does
not
induce 4
change
in
the
diffraction
pattern.
We
develop
further
the
effect
of
a
rotating
magnetic
field
on
this
phase
(see
3.4).
By
changing
the
angle
between
the
X-ray
beam
and
the
magnetic
field
we
have
established
that
the
diffrac..
tion
pattern
of the
C6
fluid
phase
(Fig.
3)
has
an
infmite
rotational
symmetry
axis
parallel
to
the
axis
of
the
capillary
tube.
One
can
see
two
broad
discs
at
while
the
inner
ring
is
split
into
four
broad
spots.
The
structure
of
the
inner
ring
is
similar
to
the
dif-
fraction
patterns
of
a
nematic
phase
of
elongated
molecules
with
skewed
cybotactic
groups
[9]
howevet
the
angular
extension
of
the
four
spots
on
the
innef
ring
is
greater
in
the
disc-like
compound
(these
diffuse
spots
are
poorly
visible
on
the
over-exposed
patterns
of
the
figure
3).
The
presence
of
such
spots
led
us
tô
study
the
columnar
phase
in
order
to
check
if they
are
indicative
of
a
pretransitional
local
order.
Neverthe-
less,
the
two
outer
discs
lying
at u -
1 A -1
are
Y
g
d
.
characteristic
of a
preferred
orientation
of the
molecu.
lar
lateral
substituents,
these
elongated
substituents
lying
in
the
plane
of
the
applied
magnetic
field
(plane
perpendicular
to
the
axis
of
the
capillary
tube).
3.2
X-RAY
DIFFRACTION
PATTERNS
OF
THE
COLUM-
NAR
PHASES.
-
Non
orientated
sample
of the
columnar
phase
has
been
obtained
from
the
Cl 1
derivative.
The
powder
pattern
of
this
mesophase
is
similar
to
the
powder
pattern
of
the
Dl
phase
of
alkanoates
derivatives
of
triphenylene
[10].
Thereforé,
this
phase
is
likely
constituted
by
columns
of
molecules
in
a
rectangular
two
dimensional
lattice
(space
group
Pgg)
the
lattice
parameters
are a
=
51.8
Á,
b
=
32.6
À
(table
I)
and
the
specific
area
AIM
=
2.58
x 10’
cm2
g- 1
(A
is
the
area
of
a
column
and
M
the
molecular
mass)
and
this
specific
area
is
similar
to
that
of
the
dode-
canate
of
triphenylene
in
the
Dl
phase
[10].
We
recall
that
in
this
phase
the
molecules
are
stacked
in
column$
with
their
plane
perpendicular
to
the
column
axis,
the
columns
forming
a
herring-bone
pseudo-hexagonal
array.
In
the
case
of
the
C6
derivative
the
orientation
obtained
in
the
fluid
phase
largely
remains
in
thé
Table
I.
-
Reticular
distances
observed
on
powder
patterns
of
the
columnar
phases.
columnar
phase
by
slow
cooling.
Although
thèse
samples
are
not
at
all
single
domains,
there
is
gene-
rally
one
principal
domain
in
the
X-ray
beam.
Indeed,
rotating
the
sample,
the
magnetic
field
of
the
X-ray
device
is
unable
to
reorient
the
molecules
in
this
very
viscous
phase.
Then
it
is
possible
to
obtain
various
X-ray
patterns
corresponding
to
successive
rotations
of
the
capillary
tube
around
its
axis
and
describing
the
successive
orientations
of a
same
principal
domain.
One
example
of such
an
X-ray
pattern
is
given
figure
4a
for
which
we
can
analyse
qualitatively
the
structure.
Fig.
4a.
-
X-ray
diffraction
pattern
of
the
tilted
columnar
phase
of
the
hexyloxybenzoate
of
triphenylene.
The
intense
110
reflections
are
pointed
out
by
arrows.
The
other
spots
are
at
least
one
order
of
magnitude
less
intense
and
originate
from
other
domains.
Two
sets
of
two
broad
reflections
are
observed
at
The
first
one
indicates
a
liquid-like
order
of
the
substituents
and
the
second,
which
is
only
visible
for
the
columnar
phase,
is
characteristic
of
a
linear
stacking
order
of
the
cores
of
the
molecules.
This
is
clearly
understood
if
one
considers
the
diffraction
pattern
of
a
liquid-like
line
of
points :
this
would
be
constituted
of
two
diffuse
planes
perpen-
dicular
to
the
column
axis ;
the
distance
between
these
two
planes
is
related
to
the
mean
distance
between
the
points
on
the
line.
Now,
if
we
have
disc-like
molecules
instead
of
the
points,
the
intensity
in
the
150
diffuse
plane
is
modulated
by
the
molecular
structure
factor
which
in
this
case
has
a
cylindrical
symmetry.
Consequently
the
intensity
scattered
by
one
column
is
localized
into
two
diffuse
discs,
the
axis
joining
thé
centres
of
the
discs
being
perpendicular
to
the
mole-
cular
plane.
The
distance
between
the
two
discs
gives
the
mean
distance
between
two
molecules
in
a
column.
At
last,
me
diameter
of the
diffuse
disc
is
related
to
the
size
of the
disc-like
scattering
entity
and
in
this
manner
we
can
see
in
our
case
that
the
central
part
of
the
molecules
alone
contributes
to
such
kind
of
broad
reflections.
In
addition,
the
location
of
the
1 A-
4.5
spots
indicates
that
the
lateral
groups
are
more
or
less
in
the
same
plane
as
the
molecular
cores.
Near
the
centre
of
the
pattern
(Fig.
4a)
two
intense
équivalent
points
are
seen
on
reciprocal
rows
going
through
thç
origin.
These
reflections
originate
from
the
two
dimensional
lattice
formed
by
the
arrangement
of
the
columns.
The
line
joining
these
two
spots
gives
thé
direction
of
a
plane
perpendicular
to
the
column
axig.
Therefore,
remembering
what
was
described
for
thé
diffuse
discs,
we
see
on
figure
4
that
the
plane
of
thë
Fig.
4b.
-
Projections
of the
reciprocal
space
on
the
(001)
and
(110)
planes :
1 :
equatorial
section
ofthe
Ewald
sphère ;
2 :
section
of the
Ewald
sphere
at
the
level
1 o A-1;
3 :
localization
of
the
scattered
intensity
by
the
molecular
cores.
molecules
is
not
perpendicular
to
the
column
axis
and
we
conclude
for
the
first
time
in
favour
of
a
tilted
columnar
phase.
Unfortunately
we
are
unable
tg
deduce
the
reciprocal
lattice
of
column
arrays
from
such
samples
since
they
are
too
badly
oriented
for
this
purpose.
Powder
patterns
enable
us
to
assign a
centred
rectangular
lattice
for
the
column
array
;
the
lattice
parameters
are,
in
a
plane
perpendicular
to
thé
column
axis
(Fig.
5) a
=
30.7
À,
b
=
28.4
À
(table
I).
The
symmetry
properties
of
this
phase
can
be
described
by
the
three
dimensional
monoclinic
space
group
Cm
or
C2/m
(depending
on
the
molecular
conformation
and
on
an
eventual
orientational
disor-
der)
in
which
we
suppress
the
periodic
translational
order
along
the
c
direction.
We
have
also
to
point
out
that
as
there
is
no
periodicity
along
c,
the
column
Fig.
5.
-
Schematic
representation
of
the
lattice
of
the
tilted
columnar
phase
of the
hexyloxybenzoate
of triphenylene.
lattice
is
described
in
a
plane
perpendicular
to
the
column
axis
that
is
to
say
perpendicular
to
the
c
direction.
We
assume
also
that
the
shorter
axis
b
is
perpendicular
to
the
mirror
plane
because
this
assumption
ensures
a
better
packing
of
the
columns.
Thus
it
is
not
the
conventional
description
for
a
monoclinic
cell.
_
Figure
4b
shows
the
projection
of
the
reciprocal
space
on
the
a*
b*
plane
and
its
intersection
with
the
Ewald
sphere
when
the
X-ray
beam
is
parallel
to
this
plane
and
perpendicular
to
the
110 )
direction.
The
X-ray
pattern
of
figure
4a
corresponds
to
an
orientation
which
differs
from
the
scheme
4b
by
a
rotation
( N
30°)
of
the
sample
around
the
110 >
row.
From
the
figure
4a,
one
can
estimate
the
angle
ac
between
the
core
plane
and
the
100
direction :
this
angle
a
is
of
the
order
of
550
(Fig.
5).
The
specific
area
of
a
molecule
can
be
deduced
from
the
area
per
column :
since
there
are
two
columns
per
rectangular
lattice.
The
area
of
a
molecule
is
taken
in
a
plane
parallel
to
the
molecular
core
Amal
=
435.9 Â2
;
and
from
the
cos
55°
mass
of
one
molecule
2.94
x
10-21
g
we
deduce
a
specific
area
of
2.58
x
10+’
cm2
g-1,
therefore
we
obtain
the
same
value
for
the
C11
and
C6
compounds.
151
3. 3
PRETRANSITIONAL
LOCAL
ORDER
IN
THE
FLUID
MESOPHASE.
-
From
the
structure
of
this
phase
we
are
now
able
to
understand
the
diffraction
pattern
of
the
C6
nematic
phase.
The
four
diffuse
spots
in
the
inner
ring
are
at
the
same
distance
as
the
110
reflections
of
the
columnar
phase.
In
the
nematic
phase
small
domains
with
a
columnar
order
are
present :
as
the
domains
are
small
we
only
observe
the
first
order
of
reflection
and
moreover
it
is
broad.
In
fact,
the
four
spots
correspond
to
the
intersection
of
two
circles
with
the
photographic
plate ;
these
circles
are
perpen-
dicular
to
the
capillary
axis
and
we
can
assert
that
all
the
discs
are
parallel,
the
order
of
the
centres
of
mass
is
liquid-like
whereas
the
column
directions
of
the
different
domains
are
on
a
cone
of
420
of
half
appex.
The
axis
perpendicular
to
the
molecular
plane
is
perpendicular
to
the
plane
defmed
by
the
magnetic
field
and
the
rotation
of
the
capillary
and
we
can
measure
the
apparent
diameter
of the
molecule
looking
at
the
first
intensity
maxima
in
the
direction
of
the
magnetic
field
(c.-- 23
Â).
3.4
MAGNETIC
MEASUREMENTS.
-
The
magnetic
experiments
were
performed
on
different
samples
of
the
same
compounds
using
two
different
apparatus.
The
Faraday
device
is
equipped
with
a
magnet
provid-
ing
fields
up
to
15
kG.
Moreover
the
rotating
magnet
gives
a
maximum
field
of
11
kG.
First,
in
order
to
check
the
influence
of the
magnetic
field
upon
the
degree
of
orientation
of
the
nematic
phase
we
have
studied
the
diamagnetic
susceptibility
of
these
compounds.
The
thermal
evolutions
of
this
parameter
x
(measured
in
the
direction
of the
magnetic
field,
during
slow
cooling
from
the
isotropic
phase)
corroborate
the
existence
of a
spontaneous
orientating
effect
of
the
applied
field
at
the
isotropic-fluid
meso,
phase
transition
for
both
C6
and
C11
compounds
(i.e.
a
sharp
decrease of
the
absolute
value
of
the
diamagnetic
susceptibility,
figure
6).
In
addition,
we
have
to
note
that,
contrarily
to
the
columnar
phases
of triphenylene
derivatives
investigated
elsewhere
[ 11],
this
effect
as
a
function
of
the
magnetic
field
value
is
saturated
for
H >,
5
kG.
Furthermore,
the
study
of the
behaviour
of a
sample
hung
by
a
quartz
wire
in
a
homogeneous
rotating
magnetic
field
of
about
11
kG
[7]
shows
that
the
fluid
mesophase
is
isotropic
in
the
rotation
plane
of
the
magnetic
field :
no
magnetic
torque
occurs
and
the
sample
remains
in
its
initial
position.
Thus,
a
single
domain
of
this
fluid
mesophase
is
obtained
with
a
rotating
magnetic
field :
this
medium
is
magnetically
uniaxial
with
the
preferred
axis
perpendicular
to
the
magnetic
field
rotation
plane.
For
an
uniaxial
system,
the
magnetic
anisotropy
is
defmed
by
where
X~
I l
is
connected
to
the
magnetic
susceptibility
measured
in
a
direction
parallel
to
the
main
axis
and
Fig.
6.
-
Discontinuity
of
the
magnetic
susceptibility
at
the
iso-
tropic-fluid
mesophase
transition.
xi
to
the
susceptibility
measured
perpendicularly
to
this
axis.
In
the
case
of
this
fluid
mesophase
xl
is
the
diama-
gnetic
susceptibility
in
the
magnetic
field
direction
thus
| X~ |
1
>
1 Xl.
1
(x
~
and X,
0)
and
the
diama-
gnetic
anisotropy
in
this
mesophase
is
hence
negative
: .
We
can
note
the
strong
tendency
for
this
mesophase
to
give
extinction
between
crossed
polarizers :
this
also
suggested
an
optically
uniaxial
mesophase.
Moreover,
the
upper
X-ray
study
has
pointed
out
that
the
aromatic
cores
of
the
molecules
are
parallel
to
the
rotation
plane
of
the
magnetic
field.
Thus
the
pre-
ferred
axis
(i.e.
the
director)
is
perpendicular
to
the
molecular
core
in
good
agreement
with
the
disc-like
molecule
symmetry
and
with
the
chemical
structure
of the
molecule
(Fig. 1)
(the
diamagnetism
essentially
depends
on
the
aromatic
part
and
thus
the
largest
magnetic
susceptibility
in
absolute
value
is
likely
per-
pendicular
to
the
polycondensed
rings).
In
order
to
determine
the
magnetic
anisotropy
of
this
phase,
we
have
to
notice
first
that
it
is
not
sure
to
obtain
a
single
domain
in
a
static
magnetic
field.
Indeed,
if A x
is
negative,
the
director
can
be
orientated
not
only
in
one
direction
but
can
point
everywhere
in
the
plane
perpendicular
to
the
magnetic
field
(as
it
is
the
case
for
the
nematic
of
rod-like
molecules
of
the
alkyl
bicyclohexyl
carbonitriles
[12]
and
for
the
lyotropic
mesophase
of
type
II
[8]).
152
Nevertheless,
in
the
limit
of
our
experimental
accuracy,
the
value
of
the
susceptibility
along
the
magnetic
field
direction
is
unchanged
after
turning
the
sample
around
a
vertical
axis
in
order
to
improve
alignment.
Therefore,
the
diamagnetic
susceptibility
measured
in
the
direction
of
the
static
magnetic
field
by
the
Faraday
method
seems
to
correspond
to
the
susceptibility
perpendicular
to
the
director
(i.e.
/J.
If
we
assume,
like
for
rod-like
nematogenic
com-
pounds,
that
the
average
magnetic
susceptibility
does
not
depend
on
the
temperature
and
on
the
state
of the
material
we
can
write :
and
then :
Thus,
as
for
a
classical
nematic
[6],
the
magnetic
susceptibility
measurement
in
the
direction
of the
held,
respectively
in
the
isotropic
phase
and
in
the
fluid
mesophase,
lead
to
determine
the
magnetic
anisotropy
of
this
latter
phase.
In
fact,
very
small
amounts
of
ferromagnetic
impurities
prevent
from
obtaining
thç
intrinsic
thermal
evolution
of
the
disc-like
compound
anisotropy
although
they
do
not
disturb
the
discon-
tinuity
at
the
transition.
Thus
near
the
isotropic
phase
we
find :
We
can
explain
the
different
values
of
OxM
between
the
C6
and
the
C11
compounds -
first
considering
that
larger
the
aromatic
over
aliphatic
parts
ratio
is,
larger
the
molecular
anisotropy
is
-
secondly,
may
be
the
degree
of
orientational
order
at
the
isotropic
phase -
fluid
mesophase
transition
has
to
be
taken
in
account.
This
difference
in
the
magnetic
anisotropies
associated
to
a
higher
viscosity
of
the
Cl,
could
explain
that
we
cannot
get
orientated
X-ray
patterns
(under
3
kG
magnetic
field)
with
this
compound
contrarily
to
what
is
observed
for
the
C6
compound.
4.
Conclusion.
- To
conclude,
our
X-ray
results
agree
with
the
first
microscopic
observations
of
the
fluid
mesophase
of
disc-like
molecules.
Indeed,
this
mesophase
can
be
considered
as
a
nematic
phase
of
disc-like
molecules
(ND
for
short)
because
there
is
no
translational
order
and
the
flat
molecules
are
more
or
less
parallel
each
other
(from
the
X-ray
patterns
it
is
difficult
to
precise
the
molecular
fluctuations).
In
addition,
we
give
evidence
for
an
uniaxial
cha-
racter
of
this
medium
with
a
negative
diamagnetic
anisotropy,
this
fluid
mesophase
is
spontaneously
orientated
in
the
bulk
by
a
rotating
magnetic
field.
As
for
rod-like
mesogenic
compounds,
the
X-ray
patterns
of
orientated
samples
have
revealed
pre-
transitional
effects
in
the
ND
phase.
Also,
as
for
series
of
rod-like
materials,
the
low
temperature
ordered
phase
which
succeeds
to
the
nematic
phase
can
exhibit
different
structures
according
to
the
length
of
the
substituents.
Thus,
the
possibilities
of
investigations
supplied
by
the
nematic
phase
of
disc-like
molecules
seem
to
be
as
large
as
the
expected
ones
from
the
classical
nematic
phase
a
long
time
ago.
References
[1]
NGUYEN
HUU
TINH,
DESTRADE,
C.,
GASPAROUX,
H.,
Phys.
Lett.
72A
(1979)
251.
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GASPAROUX,
H.,
DESTRADE,
C.,
FUG,
G.,
Mol.
Cryst.
Liq.
Cryst.
(1980),
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DESTRADE,
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M.
C.,
GASPAROUX,
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