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Deuteron magnetic resonance comparison of mesogen molecules with octyl and octyloxy end-chains

B. Deloche, J.C. Harvolin

To cite this version:

B. Deloche, J.C. Harvolin. Deuteron magnetic resonance comparison of mesogen molecules with octyl and octyloxy end-chains. Journal de Physique, 1976, 37 (12), pp.1497-1504.

�10.1051/jphys:0197600370120149700�. �jpa-00208553�

DEUTERON MAGNETIC RESONANCE COMPARISON

OF MESOGEN MOLECULES WITH OCTYL

AND OCTYLOXY END-CHAINS (*)

B. DELOCHE and J. CHARVOLIN

Laboratoire de

Physique

^{des Solides}

(**),

Université

Paris-Sud,

⁹¹⁴⁰⁵

Orsay,

^France

(Reçu

^le

7 juillet 1976,

^{révisé le 19 aofit}

1976, accepti

^{le 23 aofit}

1976

Résumé. ²⁰¹⁴ L’ordre moléculaire dans les cristaux liquides thermotropes ^{BOBOA et} CBOOA,

deutérés respectivement ^sur leur chaîne octyle ^et octyloxy, ^est étudié par des expériences de résonance

magnétique nucléaire des deutérons (DMR). ^{Dans les deux} cas, à cause des différents effets de moyenne ^tout le long ^{de la} chaîne, les groupes méthyles ^et méthylènes ^sont parfaitement ^{résolus sur}

les spectres ^DMR. Bien que les deux composés aient des chaînes de longueurs similaires les doublets

quadrupolaires

des groupes

méthyles présentent

des variations en température ^très différentes dans les

phases

^{nématique et smectique A. Ceci ne} vient pas d’un comportement

dynamique

différent dû à la présence de l’atome d’oxygène ^{sur la chaîne}

octyloxy

^mais provient d’un effet de

parité

géomé- trique ^{relié à} l’orientation moyenne de l’axe méthyle par rapport à l’axe moléculaire

long.

^Cette interprétation résulte d’une analyse ^{en deux} étapes ^des mouvements moléculaires et des effets de moyennes correspondant : déformations internes et rotation d’ensemble de la molécule, puis ^fluc-

tuations collectives des axes moléculaires

longs.

^{Dans ces} conditions la différence observée dans l’évolution des

splittings

peut ^être comprise ainsi : la DMR du groupe

méthyle

de la chaîne octyle

est dominée par la dynamique intramoléculaire alors _que celle du groupe méthyle ^{de la chaîne}

octyloxy

et de tous les groupes méthylènes ^est contrôlée par la

dynamique

collective. Enfin, ^{nos résultats} expérimentaux ^sont confrontés avec une théorie de champ moyen concernant l’ordre des chaînes terminales dans les systèmes thermotropes.

Abstract. ²⁰¹⁴ The presence of molecular order in thermotropic

liquid

crystals ^{BOBOA and CBOOA}

with perdeuterated

octyl

^and

octyloxy

^end-chain

respectively

^is investigated through ^deuteron

magnetic

^resonance experiments (DMR). ^{In both cases} methylene ^and methyl groups ^are

perfectly

resolved in the DMR spectra because of different motional averaging

along

the entire chain. Although

both compounds have similar chain lengths, ^the quadrupolar

splittings

^{of their}

methyl

^deuterons

exhibit very different temperature dependences ^{in both} the smectic A and nematic

phases.

^{This does}

not come from different

dynamical

behaviour due to the presence of the oxygen ^{atom on} the octyloxy chain, but arises from a

geometrical

parity ^{effect related to the} average orientation of the

methyl

^axis

with

respect to

^the long molecular axis. This interpretation results from an analysis of the molecular motions, ^{and their}

corresponding

averaging effects, ^{in two} stages : first, internal deformations and overall rotation of the molecule, second collective orientational fluctuations of the long ^molecular

axes. Under these conditions the observed discrepancy in the variations in splitting ^can be understood

as follows : the DMR of the

methyl

group of the

octyl

chain is dominated

by

the intramolecular

dynamics ^{whereas that of the}

methyl

group of the octyloxy chain and of all the

methylene

^{groups is}

controlled by the collective

dynamics.

Finally, ^our experimental ^{results are}

compared

^{with a mean}

field theory concerning the end-chain ordering ⁱⁿ thermotropic systems.

Classification Physics ^Abstracts

7.130 ^- 8.660

1. Introduction. ^- The usual

picture

^{of most meso-}

gen molecules consists of an

elongated rigid

^aromatic

core with more or less flexible

alkyl

^or

alkoxy

^end-

chains. The different chemical affinities and mechanical

properties

of these aromatic

and paraffinic

parts

certainly plays

^a role in the occurrence of a definite

mesophase.

^{It is}

known,

^for

instance,

that the exis- tence of smectic

phases

is related to the _presence and

length

of end-chains

[1]

^and that their isomerism could intervene in the

thermodynamics

of the nematic-

isotropic

transition

[2]. Obviously, microscopic

^studies

(*) This work has been supported by ^{D.R.M.E., contract}

no 73/573. ^Part of this work has been presented ^{at the 2nd} Specialized Colloque, Ampere, Budapest, Hungary (August, 25-29, 1975).

(**) Laboratoire associe au C.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197600370120149700

1498

of

liquid crystals

^are

required

^{on a} molecular scale so

that relations between

phase

^structure and molecular behavior can be established. ^As shown

recently,

nuclear

magnetic

^{resonance of}

selectively

^deuterated

molecules

(DMR)

^{can be used to} obtain information about relative orientations and

dynamical

^behaviour

of different _groups of the molecule

[3]. Indeed,

the DMR spectra ^{can be}

analysed

^{in terms of the}

orientational order

parameter

^{of the C-D bonds of} interest with respect ^{to the}

applied magnetic

^{field i.e.}

the

liquid crystal

director in all the uniaxial

phases

studied here. But the exact

meaning

^{of this} parameter in terms of the

physical properties

^{of the}

liquid crystal

cannot be ascertained without further

investigations.

The need for so careful a

study

^{can be found}

by looking

at our

previous

results for the deuterated

butyl

^chain

of the

terephthalidene-di-p-butylaniline (TBBA) [3] :

in

spite

^{of the}

important

structural differences between the nematic and smectic

phases

^the

change

^{in the}

orientational order

along

the entire chain is almost invariant

through

^{the whole}

mesophase

range. The

surprising tendency

^{for the} second and the third

methylene

^{deuterons to have the same} orientational order with

decreasing

temperature ^{allows to} suspect ^a weak

sensitivity

of the DMR method to certain

changes

of conformational states. In

fact,

the average direction of the C-C bond between the

methylenes

under consideration _may be assumed to be

nearly parallel

^{to the}

postulated long

molecular rotational axis.

Hence,

^{in this} case, isomerism around this bond is not discernable from the overall molecular rotation and cannot affect the orientational order further.

Such a

pair clustering

is also visible for the deuterated

octyl

chain of the

butyloxybenzilidene-p-octylaniline (BOBOA)

^{and was}

suggested by

^{us to be the result of}

alternating

orientations of the C-C bonds

bearing

the

methylene

groups with respect ^{to the assumed}

long

molecular axis.

To test this

assumption

^{we then}

compared

^{the well}

identified DMR spectra ^{of the}

methyl

ends of the

octyl (C8)

^and

octyloxy (O-C8)

chains of

liquid crystals (BOBOA)

^and

p-cyanobenzilidene-p-octyl- oxylaniline (CBOOA).

^The

corresponding

chain skele- tons have

respectively eight

^{and nine} segments ^{and an}

important change

^{in the} average orientation of the

methyl

symmetry ^{axis with} respect ^{to the}

long

^axis

may be

expected

^{from such a}

change

ⁱⁿ

parity.

^This

study

will also

provide

^{us with the}

opportunity

^to

compare the behaviour of

alkyl

^and

alkoxy

^end-chains

of similar

lengths

^{and to} reconsider the theoretical results of the self-consistent molecular field calcula- tions for

alkoxy

^chains

[2].

2.

Experimental.

^{- 2.1 SAMPLES. - The}

octyl

^and

octyloxy

^{chains are studied on two} Schiff s base derivatives. The molecules and their

respective phases

are the

following :

(i)

p -

Butyloxybenzilidene -

p -

dl, - octyl -

^{2.6 -}

d2 - aniline,

^or

BOBOA - d 19 :

This molecule was

synthesized

from deuterated

octyl-

aniline

[3].

(ii) p-cyanobenzilidene-p-d17-octyloxyaniline,

^or

CBOOA-d17 :

(iii) p-cyanobenzilidene-p-a,

^a’

d2-octyloxyaniline,

or

CBOOA-d2 :

The last two molecules were

synthesized

^{from a}

condensation of

cyanobenzaldehyde

and deuterated

octyloxyaniline.

^The

dl7-octyloxyaniline

^{and the}

a,

a’-d2-octyloxyaniline

^were obtained from commer-

cial

n-octyl-d17-iodide (Merck, Sharp

^and

Dohme)

and a,

a’-d2-octanol respectively, following

^refe-

rence

[4].

^{In both cases} the deuterations were controlled

by high

resolution NMR and were estimated to be better than 98

%.

^{Then the}

samples

^were

outgassed

and sealed under vacuum in

glass ampoules.

The temperature ^{of the}

sample,

controlled

by

heated air

flow,

^was

regulated

^{to 0.15 °C and thermal}

gradients

^{on the}

sample

^{were of the same order.}

2.2 NMR. - The orientation of a C-D bond in an

external

magnetic

^field

Ho

^and its modulation

through

molecular motions are accessible

by

^a

study

^{of the}

magnetic

^resonance spectrum of the deuteron. We recall

that,

^{due to} the tensorial

coupling

of the deuteron

quadrupolar

^moment with the electric field

gradient

of the

charge

distribution of the C-D

bond,

^{the DMR}

line is

split

^{into a}

symmetrical

^doublet structure. The characteristic

splitting

^is

given by

^the

following

expres- sion in

magnetic

field units :

where the static

quadrupolar coupling

^constant

e2

QQIPD

^{is about} 260 gauss for deuterons on

phenyl

or

methylene

^sites

[5] ;

^{0 is the}

angle

between the C-D bond of interest and the direction of the

applied magnetic

^field. When molecular motions take

place

such as the C-D bond reorientates with

frequencies larger

^{than the} characteristic

frequencies

^{of the}

static interactions

( N 170 kHz)

^{a time} average of

(3 ^cos’ 0 - 1)

^must be taken in

(1).

^{Then the}

quadru- polar splitting

appears

proportional

^to the orien- tational order parameter of the C-D bond with respect

to the direction of the

magnetic

^field

Ho.

In case of uniaxial reorientation around a symmetry axis

making

^an

angle

^{a with the}

applied magnetic

field and an

angle

y with the C-D bond

(as

illustrated in

figure

^{4 in a}

particular case),

^{the total}

averaging

over azimuthal

angles

^{leads to the}

following simplified expression

^{of the}

spherical

harmonic addition theo-

rem :

In uniaxial

liquid crystalline phases (nematic,

^smec-

tic A and B

phases)

^the

optical

^{axis is} the obvious choice for an axis of uniaxial symmetry. ^{In biaxial}

phases (smectic

^{C and H}

phases)

the existence of a

molecular axis of uniaxial symmetry,

corresponding

to the overall rotation of the molecule around a

long axis,

^{was shown}

by studying

^the

change

^{of the NMR}

spectra

^{when the}

sample

^was rotated with

respect

^to the

magnetic

^field

[3, 6].

^It is reasonable to assume the existence of both axes in uniaxial

phases, although

the

analysis

^{of the DMR} spectra and rotation

patterns

does not enable us to

distinguish

^between

them;

^{in the} latter case the average orientation of the molecular axis and the direction of the

macroscopic

symmetry axis are the same. The need for a distinction between these two axes will _appear below where we

interpret

the DMR spectra ^{and their} temperature

dependence.

The 13 MHz

(or

²⁰

kgauss)

^DMR spectra ^were obtained with a

pulsed spectrometer

of the classical type. ^{The free}

precession

^was

integrated

^{with a boxcar}

integrator

^{while the}

magnetic

^{field was}

swept through

resonance; thus the recorded

signal

^was the Fourier transform of the free induction

decay (i.e.

^the

absorp-

tion

spectrum)

^{if the}

phasing

of the detection was

appropriate.

2.3 RESULTS. ^- DMR spectra ^of

BOBOA-dlg

^and

CBOOA-dl7

ⁱⁿ their nematic

phase

^{are shown in}

figure

^{1. The first}

striking

feature is that in both cases

they

exhibit well resolved structures.

Except

^{for the}

phenyl

deuterons which

present

^a

quadruplet

^struc-

ture

(due

^{to an} additional

dipolar coupling

^between

the

phenyl

deuterons and their

neighbouring

pro- tons

[3])

^{all the}

^remaining

^doublet structures corres-

pond

^to

methylene

^or

methyl

deuterons. The different values of the observed

splittings

^AH arise from diffe- rent motional

averaging

^{of the}

quadrupolar couplings

of the C-D bonds

along

the entire chain.

In both spectra ^the

methyl

groups ^are

easily

^iden-

tified from their relative line

intensities;

^{as for the}

a-CD2

^{of the}

chain,

^the

assignment using

^the

CBOOA-d2

is obvious in one case whereas it is based

on arguments ^of

splitting

and linewidth in the other

case

[3].

^{All the}

remaining

^{lines are due to the inter-}

FIG. 1. ^- DMR spectra obtained with two selectively ^deuterated liquid crystals in their nematic phases. Only ^{half the} spectra, ^which

are symmetrical ^{around zero} magnetic field, ^are represented. ^Each

line was identified assuming increased motional averaging ^{from the} phenyl ^{to the end} methyl group. The associated number labels the

corresponding ^chain segment. (Other possible assignments ^are

discussed in the text.)

mediate

methylenes ;

^at present ^a definitive

assignment

is not easy but as

previously suggested [3]

^{a reasonable}

assumption

^{is to}

postulate

^{an increased motional}

averaging,

^{i.e. a} monotonic decrease of the

quadru- polar splitting,

^with

increasing separation

^{from the}

phenyl

group.

Obviously,

other identifications _may be

proposed, but,

in any case, ^an arrangement ^{of the} intermediate

methylenes

ⁱⁿ

pairs

^is apparent ^{from the} observed DMR spectra.

Indeed, following

^the argu- ments

presented

^{in the}

introduction,

^{deuterons of}

similar

splittings

and linewidths in

figure

¹

might correspond

^to

neighbouring methylenes

^{whose C-C}

bond is

nearly parallel

^to the assumed

long

^molecular

axis.

3.

Methylene

^and

methyl ordering.

^{- 3 .1 BOBOA.}

- A detailed

study

^{of the}

phenyl

^and

methylene splittings

^versus

temperature

is shown in

figure

^2.

In any ^case the

splittings

^{exhibit a} temperature

depen-

dence of a sort that is

commonly

^{observed in most of}

the

previous

^NMR

experiments

^on

liquid crystals,

^i.e.

a continuous decrease with

increasing

temperature except ^{at the}

phase

transitions where small disconti- nuities

(some %)

^are observed and are related to the

thermodynamics

^{nature of the}

phase

transitions.

Surprisingly,

^as observed in

figure 3,

^{a drastic}

change

^{occurs in the}

splitting

behaviour when we

consider the end-chain

methyl

group : the

methyl splitting

goes

through

^{zero when the} temperature ^is increased in the nematic

phase.

^{Since it is} unreasonable to assume that the motions of the

methyl

groups become

isotropic

^{at a}

particular temperature

^{in the} nematic _range, such unusual behaviour

suggests

^that

the process which controls the

change

of the orienta- tional

order (

³

^cos2

^{0 -}

1 )

^{is not the same in this}

case as in the case of the

methylene

^or

phenyl

groups.

We shall look for such _processes

by

^{a two} stage

analysis

of molecular motions in uniaxial

phases :

(i)

molecular deformations and overall molecular

1500

FIG. 2. ^- Variations in the half-splittings ^{of the} methylene ^and phenyl ^{deuterons as a function of} temperature ⁱⁿ the different

mesophases ^of BOBOA-d19. Only ^the half-splittings ^{of the center}

of gravity ^{of the} phenyl component ^are reported.

FIG. 3. - Variations in the half-splittings ^{of the} methyl ^deuterons

of the octyl ^{chain of} BOBOA-d19 for the different mesophases.

When the splitting ^{is zero the} half-height linewidth is 200 mG. For

comparison, the full-line curve, drawn from data in figure 2, ^{after a} change ^{of scale on} the vertical axis, shows the trend of the last

methylene splitting ^variation.

rotation are taken into account

[7]

^{and lead to micro-}

scopic

^uniaxial symmetry ^{around a}

long

^molecular

axis; (ii)

collective orientational fluctuations of these

long

^{axes lead to}

macroscopic

^uniaxial symmetry.

Therefore,

if these molecular motions are assumed to be

uncoupled

^{the two terms} in the formula

(2)

^{can be}

averaged separately.

^This

assumption

^{of different}

time scales for the motions under examination is

certainly

valid when the orientational fluctuation modes of

long wavelengths

^are considered but _may be drastic for those of shortest

wavelengths.

^{Then the}

splitting expression

^becomes

proportional

^{to the}

following expression :

when the overall rotation of the molecule is faster than the internal

deformations,

^{or :}

in the

opposite

^case; in both formulae

(see Fig. 4)

y defines either the orientation of a

methylenic

^C-D

bond or that of the

methyl

^{C-C bond}

[8]

^with respect

to the

long

molecular axis.

FIG. 4. ^- Definition of the angles occurring ^{in the} splitting expres- sion (3) ^or (3). ^Here only ^the methyl ^{deuterons are} considered and _y is the angle ^{of the} long molecular axis with the symmetry axis of the

methyl group, i.e. the last C-C bond of the studied chain.

Hence,

^{the measured DMR}

splittings

appear ^as

products

^{of two terms related to the}

dynamics

^{of the}

liquid crystal

^{as follows :}

(i)

^{the first term} is related to the collective

dynamics

of the

molecules;

^{it is} the usual order parameter ^S which describes the orientational fluctuations of the

long

^{molecular axes relative to}

Ho ;

(ii)

the second term is related to the intramolecular

dynamics;

^it is the order parameter

describing

^the

motion of the C-D bond of interest relative to the

long

molecular axis.

Consequently,

if internal motions take

place

ⁱⁿ

such a way that the

angle

y remains constant or varies around 0° or 900

(i.e.

far from the

region

^of

important slope

^{on the curve}

(3 ^cos’

y -

1))

the variations in

splitting

^are

mostly

^dominated

by

^{the first term : thus}

the

temperature dependence

^{must be}

quite

^similar

to that of the order

parameter S

^{as it can}

easily

^{be seen}

in

figure

^{2 for the}

phenyl

^and

methylene

^deuterons.

On the contrary, ^if y varies around 45° where the

slope

of the curve

(3 ^cos’

y -

1)

^is

maximum,

the variations in

splitting

may be dominated

by

the second or the intramolecular term and _may therefore exhibit the unusual behaviour shown in

figure

^{3. In}

particular,

the observed

collapse

^{of the}

methyl

doublets in the nematic

phase

suggests ^that

either

³

^cos’

y -

I >

goes

through

^zero

or y >

goes

through

^the

magic angle (i.e.

^54°

5)

^{at this} temperature ^from

(3)

^or

(3’).

In any case, the

methyl splitting

appears ^to be extre-

mely

^{sensitive to}

changes

ⁱⁿ

shape

of the molecules.

In

particular,

the break in

slope

^{as well as the} strong

discontinuity (about

¹⁰⁰

%)

^{observed at} the nematic- smectic A and smectic A-smectic B transitions _may arise from

changes

^{in the} relative average

orien-

tation of the

methyl

^and

long

^{axes caused}

by

^the

change

in molecular conformations due to the for- mation of smectic structures. From the data in

figure

^{3 we} may be able to deduce the

dependence

^of y

on the

temperature

^{and the}

corresponding change

ⁱⁿ

the whole

mesophase.

Since the absolute value of the

slope

^{related to the} variations in the

splittings

^decreases

on

going through

^{the zero}

point

^with

increasing

^tem-

perature, ^we deduce that _y fluctuates around

increasing

mean values as we pass from the smectic to nematic

phases. Moreover,

^{if we} consider that the order

parameter

S = B 3 cos22 rx - 1)

is

equal respectively

^{to 0.8 at} the smectic B-smectic A transition and to 0.4 at the

nematic-isotropic

^transi-

tion,

^we find from the

expression (3)

^{that the mean}

value of _y increases

by

^{about 3°}

through

^{all the}

mesophase

range.

To

support

^this

interpretation,

^a

significant

^test

would be the addition of an extra atom to the

octyl

chain so as to

modify

^the average orientation of the

methyl

axis relative to the rotational axis of the molecule.

3.2 CBOOA. -

Specifically,

^{to test} this idea we

have studied the

CBOOA-d17

^{where the}

octyloxy

chain under examination is identical to the

previous

one

except

^{for the} presence of an oxygen ^{atom at} the

beginning

of the chain. Such a

change

^{in the}

parity

^of

the segment number of the

octyl

chain should

yield

^a

direction of the

methyl

axis close to that of the

long

molecular axis. Under these conditions we may

predict

^{that the}

corresponding methyl splittings

^should

be much

larger

and exhibit a very different tempera-

ture

dependence

^from that of the

octyl chain, regardless

of the case

(3)

^or

(3) quoted

^above.

As

expected, figure

¹ shows that the

splitting

^of

the

methyl

^{deuterons of such an}

octyloxychain

^is

much

larger

^{than in the}

previous

^case.

Furthermore,

the temperature

dependence

^{of the}

methyl splitting

appears from

figure

^{5 to be}

quite

^{similar to} that of the

methylene

groups, with no

cancellation;

^{so it is}

mainly

controlled

by

the collective

dynamics S,

^as

opposed

^to

the case of

octyl

chains. Such a

geometrical

^even-odd

effect can also

explain

^{that the}

methyl splitting

^{of a}

butyl [3]

^or

propyloxy [9]

chain is observed to be much smaller than that

of a pentyl [10]

^or

ethyloxy

^one

[9].

FIG. 5. ^- Variations in the half-splittings ^{of the} methylene ^and methyl ^{deuterons as a} function of temperature through ^{the meso-} phase range of CBOOA-dl7. ^The representative symbols ^{of the} methylene groups between the first and the last one of the chain are

the same as in figure ^2.

The manifestation of such a

parity effect,

^{which is}

related to the _average orientation of the

methyl axis,

is _very

prominent

in the DMR

spectra

^{but could also}

occur in the PMR

spectra.

^For

example,

^{such an}

effect could contribute to the

alternating

^behaviour

of the

dipolar

^echo

decay recently

observed in nematic

phases

of successive members in an

homologous

^series

C3

^to

C10 [11].

4.

Comparison

^{of an}

alkyl

^and

alkoxy

^{chain. - The}

substitution of the first carbon atom of an

alkyl

^chain

1502

by

^an oxygen ^atom

weakly

^{alters the}

geometrical characteristics, length

^and

angle,

^{of the covalent}

bond

[12] ;

^{on the other}

^hand,

^{such a} substitution _may

modify

^the

possible

isomerizations of the first

methy-

lene _groups and therefore the

dynamical

^behaviour

of the chain. From our results it is then

tempting

^to

compare the decrease of the order

along

^an

alkyl

^and

alkoxy

chain of similar

length.

^The

changes

ⁱⁿ

quadru- polar splittings along

^{the entire chain are}

given

ⁱⁿ

figure

^{6 for the case} in which both systems ^{are in}

thermodynamics

^states which have

nearly

^{the same}

values of the order parameter ^{S. In both} cases, the relative decrease of the

methylene

^{order with} respect

to the symmetry ^{axis of the}

sample

^are

qualitatively

similar.

However,

^a

comparison

of the chain order with respect ^{to the}

long

molecular axis in each system

(i.e.

^{the term}

dependent

^on y in the

splitting expression)

would be more

interesting;

but in order to be

signi-

ficant such a

comparison

^must be done for identical values of the orientational order parameter ^S.

Moreover,

the chains should also be

compared

^when

the

packing

conditions for each system ^{in the}

liquid crystal phases

^{are similar.}

Segment number

FIG. 6. ^- Variations in the quadrupolar half-splittings ^{of the} methylene ^and methyl groups ^{as a} function of position along ^an octyl ^and octyloxy chain. The two chains are compared ^{in states}

which correspond ^to nearly ^{the same} value of the order parameter ^S.

As shown

previously,

^a

precise

determination of S is not so evident

since,

^{in both} cases, this

requires

^the

knowledge

^{of the} average molecular geometry ^around the

long

rotational axis.

Therefore,

^{we are forced} to compare the two systems ⁱⁿ

thermodynamics

^states

which do not

correspond necessarily

^{to the same order}

parameter

S ;

however this

only

introduces a constant ratio between the numerical values of the two curves

in

figure

^6.

The

packing

conditions are more

important

^because

the

mobility

^{of a chain is}

strongly dependent

^{on its}

average free lateral _space, which is

mainly

^determined

by

^the

proximity

^of

neighbouring

^{molecules as inferred}

by

^our

experiments

ⁱⁿ

lyotropic liquid crystals [13].

In summary, the ^two chains

apparently

^{exhibit a}

similar

dynamical behaviour,

but this conclusion is

only qualitative.

^A

quantitative study

^{of the effect of}

the _oxygen atom on the chain order can

only

^{be done}

with chains of the same

length

^{attached to identical}

aromatic cores.

5.

Comparison

^with

theory.

^{- The}

decay

^{of order}

measured

along

the entire

alkoxy

chain of the CBOOA

can be

compared directly

^with theoretical results obtained

by Marcelja

^on

alkoxy

chains. This author has used a self-consistent molecular field

approxi-

mation in which the statistical

averages

³

^cos’ 0 2013 1 )>

are calculated

by

^{summation over} chain confor- mations

[2].

^The

experimental

and theoretical results

are

compared

ⁱⁿ

figure

⁷ and there is a reasonable agreement ^{for the mean} values of the orientational

order (

³

^cos2

^{0 -}

1 >

^{and their mean rate of}

decay

in the central part of the chain. Some

discrepancies however,

^{such as the}

alternating

^{behaviour and the}

slopes

of decrease in orientational order ^at the

methyl

and

phenyloxy extremities,

appear and must be considered in detail. The first

discrepancy might

^be

easily

^removed

by interchanging

^the

experimental

data for the fourth and fifth segment

(i.e.

^{the third and}

the fourth

carbon)

^as

suggested

in section 2.3. The second _one, i.e. the low

experimental

value of the

methyl order,

^{will not be}

analysed

^{because exact}

assignment

^of the last calculated

point (methyl

^or

methylene)

is made uncertain due to the

disagreement

in

figure

⁵ of reference

[2]

between the number of carbon atoms on the calculated curve

(N

⁼

8)

^and

that of the

corresponding caption (N

⁼

9). Lastly

this one on the

phenyloxy side,

where the

sharp

calculated decrease in orientational order between the first two

methylenes

^contrasts with the slower observed _one, can be understood

through

^{a more}

detailed

analysis

^of

Mareelja’s

^model.

FIG. 7. ^- The observed orientational orders ( ^{3 cos2 0 -} 1 > ^{as a} function of position along ^an octyloxy ^{chain in} CBOOA-dl7 ^and

those calculated by Marcelja [2] ^{from a mean field} approximation.

(0 ^{is the} angle between the C-D bond of interest and the direction of the applied magnetic field.)

In the theoretical treatment the

averaging

^effects

due to the orientational fluctuations are introduced

by summing

^over three discrete orientations of the

methylene-oxy-phenyl

group

(C6H4-O-CH2)

^{of the}

molecule which is assumed

rigid.

^{In order to obtain a}

value of the order parameter ^S

equal

^{to 0.429 at the}

nematic-isotropic transition,

three orientations relative to the

magnetic

^{field were}

chosen;

^{these are}

given

ⁱⁿ

polar

coordinates

(0, T)

^{as follows :}

(0, 0), (49, 0)

^and

(49, n). Therefore,

^no

cylindrical

symmetry ^{around a}

long

molecular axis occurs in the absence of orien- tational fluctuations. Next the chain motions are

introduced

through

isomeric rotations around the skeleton C-C

bonds,

^{in the} molecular field. Under these conditions all the motions take

place

^about

only

one

axis,

^{i.e. the}

magnetic

^field

Ho, and,

^{when the} molecular core has the orientations

(49, 0)

^and

(49, n),

the first C-C bond

(and

^all

subsequent

^{odd C-C}

bonds)

is inclined at 49° with respect ^{to the}

field ;

^{then the}

effect of isomerizations or oscillations around this bond is to decrease

strongly

the order of the second

methylene CD2

^{relative to the first one.}

Thus,

^{in this}

model,

^the orientational fluctuations can contribute in

differentiating

between the

methylene

groups and could even introduce an

alternating

behaviour if their

corresponding

^{C-C bonds are too} tilted relative to

Ho.

This is not at all the case in our

interpretation

^{of the}

experimental

^results

through

^{a two} stage

averaging

process;

here,

^once the intramolecular _average around

a

long

^{axis is}

made,

the orientational fluctuations off this axis relative to

Ho

^cannot differentiate between the

methylene

groups any further and this occurs

regardless

of the value of S.

Obviously

^{such a} compa- rison

might

appear ^more

significant

if the value of S at the

nematic-isotropic

transition for the CBOOA

was introduced in the calculation to define the initial orientations of the molecular

rigid

core; however since this value of S

changes only slightly

^{from one}

system ^to

another,

the final result must not be affected

too much.

In

summary, the disagreement

between the

theory

and

the experimental

results does not come

from

the simulation of the orientational fluctuations but arises

mainly

from the absence of an overall rotation of the

methylene

groups around a

long

^{molecular axis.}

6. Conclusion. ^- DMR on two

thermotropic liquid crystals

^with

perdeuterated octyl (C8)

^and

octyl-

oxy

(O-C8)

^{chains has been}

presented.

^{In both}

cases

the different

methylene

^and

methyl

groups ^are

easily

identifiable in the DMR spectra. This demonstrates

once

again

that the end-chains are not in a

rigid

^all-

trans conformation in the

liquid crystalline phases.

Effects due to the _presence of the _oxygen atom on the

octyloxychain

^{are not}

clearly

discernable from a

comparison

^{of the two} systems; ^{but our} conclusions in this case are

necessarily

^{limited because we are not}

sure to compare the two chains in the same

packing

conditions. The direct

comparison

^of

experimental

and

existing

calculated data for the

alkoxy

^{chain in}

uniaxial

phases strongly

^{favours a}

dynamical analysis

in terms of two distinct types of motions.

First,

^fast

intramolecular deformations and most

probably

overall rotation result in an average

cylindrical

symmetry ^{around a}

long

axis for each

molecule;

^then slower orientational collective fluctuations modulate the orientation of this

long

axis relative to the _symme- try axis of the

sample.

This model is used to

explain

the differences observed in the temperature ^variations of the DMR orientational order of end

methyls

^of

chains with even and odd number of segments. ^It appears from these

experiments

^that the orientational order obtained from the NMR results

depends

^not

only

^on the collective

dynamics

^{but also on the intra-}

molecular _one;

hence,

information on collective fluctuations cannot

always

be extracted

straightfor-

wardly

^{from the NMR}

data, particularly

^{when the}

axis of the interaction under examination is

strongly

tilted with

respect

^{to the}

long

molecular axis.

Thus,

whereas the DMR of all the

phenyl

^and

methylene

groups under examination are

mainly

^{sensitive to the}

collective

dynamics,

^{the DMR} of the end

methyl

group of a chain with

eight

segments

strikingly

^illus-

trates the

importance

of the intramolecular

dynamics

for

geometrical

^reasons. Under these conditions the end

methyl

group ^can be used as a very sensitive

probe,

in

particular

^{to detect}

changes

ⁱⁿ the molecular state in relation to the structure of the different

phases.

Acknowledgments.

^{- We are very}

grateful

^to

L. Li6bert and P. Keller for

synthesizing

^the

samples

of deuterated CBOOA.

Note added in

proof.

^- Similar cancellation of the

methyl splitting

^{has been}

recently

^{observed for an}

heptyloxy

^chain

(0-C,) by

J. W. Doane who pro-

posed

^{a different}

interpretation

^{at the Sixth}

Liquid Crystal

Conference

(Kent, August 1976).

References

[1] ^Mc MILLAN, W. L., Phys. ^{Rev. A 4} (1971) ^1238.

DOANE, ^J. W., PARKER, ^R. S., CVIKL, B., JOHNSON, ^{D. L. and} FISHEL, D. L., Phys. ^{Rev. Lett. 28} (1972) ^1694.

[2] MAR010CELJA, S., J. Chem. Phys. ⁶⁰ (1974) ^3599.

[3] DELOCHE, B., CHARVOLIN, J., LIÉBERT, ^{L. and} STRZELECKI, L., J. Physique Colloq. ³⁶ (1975) C1-21.

[4] ^BUU HOÏ, ^N. P., GAUTHIER, ^{M. and DAT} XUANG, N., Bull. Soc.

Chem. (1962) ^2154.

[5] BURNETT, L. J. and MULLER, ^B. H., ^{J. Chem.} Phys. ⁵⁵ (1971)

5829.

BARNES, R. G. and BLOOM, ^J. W., J. Chem. Phys. ⁵⁷ (1972) ^3082.

MILLETT, F. S. and DAILEY, ^B. P., ^{J. Chem.} Phys. ⁵⁶ (1972) ^3249.

[6] Luz, Z., HEWITT, R. ^{C. and} MEIBOOM, S., ^{J. Chem.} Phys. ⁶¹ (1974) ^1758.

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1975.

1504

[7] CHARVOLIN, J., DELOCHE, B., ^J. Physique Colloq. ³⁷ (1976)

C3-69.

[8] ^The last C-C bond of the chain is also the symmetry axis of the

methyl group and its orientation with respect ^{to the} long

molecular axis is relevant here due to the methyl ^free

rotation.

[9] ROWELL, ^J. C., PHILLIPS, ^W. D., MELBY, ^{L. R. and} PANAR, M.,

J. Chem. Phys. ⁴³ (1963) ^3442.

[10] EMSLEY, ^J. W., LINDON, ^{J. C. and} LUCKHURST, ^G. R., ^Mol.

Phys. ³⁰ (1975) ^1913.

[11] BODEN, N., LEVINE, ^Y. K., LIGHTOWLERS, ^{D. and} SQUIRES, ^R. T., Chem. Phys. ^{Lett. 34} (1975) ^63.

[12] ^Molecular structures and dimensions, ^Vol. A1, ^edited by Olga

Kennard D. G. Watson et al., University ^Chemical Laboratory, Cambridge, England.

[13] ^{The disorder} of lipid ^chains depends ^{on the mean area} per polar head at the soap-water interface. MELY, B., CHARVOLIN, ^J.

and KELLER, P., Chem. Phys. Lip ¹⁵ (1975) ^161.