Highly oriented fibres of copper laurate obtained by melt-spinning of its columnar mesophase

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Highly oriented fibres of copper laurate obtained by

melt-spinning of its columnar mesophase

Anne-Marie Giroud-Godquin, Pascale Maldivi, Jean-Claude Marchon, Pierre

Aldebert, Alain Péguy, Daniel Guillon, Antoine Skoulios

To cite this version:

Short Communication

Highly

oriented

fibres of

_copper

laurate

obtained

_by

melt-spinning

of

its

columnar

_mesophase

Anne-Marie

_{Giroud-Godquin(1),}

Pascale

_Maldivi(1),

Jean-Claude

_Marchon(1),

Pierre

Aldebert(2),

Alain

_Péguy(3),

Daniel

_Guillon(4,5)

and Antoine

_{Skoulios(4,5)}

(1)Laboratoires

de

_{Chimie/Chimie}

de

_{Coordination, Département}

de Recherche

Fonda-mentale,

Centre d’Etudes Nucléaires de

Grenoble,

38041

_Grenoble,

France

(2)Service

de

_{Physique/Physicochimie}

_{Moléculaire, Département}

de Recherche

Fonda-mentale,

Centre d’Etudes Nucléaires de

Grenoble,

38041

_Grenoble,

France

(3)Centre

de Recherches sur les Macromolécules

Végétales, CNRS,

BP 68, 38402

Saint-Martin-d’Hères,

France

(4)Groupe

des Matériaux

_Organiques,

Institut Charles Sadron,

CNRS-ULP,

6 rue

Bous-singault,

67083

_Strasbourg,

France

(5)Institut

de

_Physique

et Chimie des Matériaux de

_Strasbourg,

_{CNRS-ULP-EHICS,}

BP 20, 67037

_Strasbourg,

France

(Reçu

le 21 novembre _1988, révisé le 3 _{janvier 1989,}

_accepté

le 5 _janvier

₁₉₈₉₎

Résumé.2014 Le laurate de cuivre _peut être obtenu sous forme de fibres _par extrusion à chaud de sa

mésophase

colonnaire

thermotrope.

L’examen de ces fibres _par

micro-scopie électronique

à

_{balayage et par}

diffraction des _rayons X révèle un très haut

_degré

d’orientation du laurate

_cuivrique

à l’état cristallin et dans la

_mésophase.

Abstract.2014

_Copper

laurate was

_processed

into oriented fibres

_by

melt

_spinning

of its

thermotropic

columnar

_{mesophase ;}

examination

_{by scanning}

electron

_microscopy

and

X-ray

diffraction shows a

_{high degree}

of orientation of the spun fibres in the solid state

as well as in the columnar

mesophase.

Classification

Physics

Abstracts 61.30E - 64.70E

There is considerable current interest in the

_design

of

_{metal-containing}

molecular

materials with

_specific

_properties

such as electrical

conductivity

or

ferromagnetism

[1].

Liquid

crystalline

behaviour would be an added bonus to the desired

_properties,

as it

would

_{possibly provide}

ease of

processing

and

macroscopic

orientation. The search for efficient methods of

_alignment

of nematic

_{liquid crystals}

was

highlighted

by Janning’s

discovery

in 1972 that a _very thin film of silicon monoxide

_deposited

onto a surface

at an

oblique angle

resulted in

uniform,

_permanent,

and

reproducible

orientation of

514

the _mesogen on the surface

[2].

This _process has become _very

_popular

with

_liquid

crystal display manufacturers,

and it is

_widely

used in the

_industry.

The

_alignment

of columnar

_{liquid crystals,}

on the other

_hand,

has been a

_challenging

research area

since the

_discovery

of discotic _mesogens

_[3].

Several

_alignment

_techniques

_including

use of a

magnetic

field

[4],

various surface treatments

_[5],

slow flow

_through

a

capillary

[6],

or

stretching

of the

mesophase

[7]

have resulted in limited

degrees

of

_alignment.

A

process

employing

a

mechanically operated

_pin

which draws a strand from a reservoir cup of the mesogen at a desired

_temperature

in the columnar

phase

has

_recently

been shown to

_{give highly}

oriented fibres

_[8].

In this

_{communication,}

we _report that _upon

melt-spinning,

the columnar

_mesophase

_{of copper}

laurate

_[9]

_{(Scheme I)}

_easily

affords oriented solid

_fibres,

which _upon

_heating

result in columnar

_{liquid-crystalline}

fibres of

high

anisotropic

order.

Scheme 1.

About _{2 g} of

_{microcrystalline, powdered}

_copper laurate

_(dicopper

tetrakis(do-decanoate))

were

_put

inside the

heating cylinder

of a

manually operated rod-spinning

apparatus

(Davenport

Melt Indexer

_3).

The extrusion

_temperature,

120°C,

was 13°C above the transition to the columnar

_mesophase.

The

_spinneret

diameter was 0.3 mm. The

_length

of the _spun fibres was limited to 10-20 cm

by

the _poor mechanical

prop-erties of the solid material.

_Figure

la

_displays

a

scanning

electron

microscope

pho-tograph

of a _copper laurate fibre coated with

platinum-gold by

vacuum

deposition.

Evidence of an

_anisotropic

inner structure is

_{provided by}

the surface

_corrugation,

shrink-age of a melted core sheathed in a clotted

plastic

skin as the fibre cools _away from

the

_spinneret.

The observed

_anisotropic

thermal contraction is

_fully

consistent with the

_previously

_reported

thermal evolution of the intercolumnar distance

_[9],

_nearly

constant value of the

intracolumnar

_stacking

_period

_[10],

and

_sharp

increase in molar volume _upon the transition to the columnar

_mesophase

_[10].

On the other

_hand,

slower

extrusion rates can result in the formation of a central hole when the thicker skin

re-sists

_corrugation,

as shown in

_figure

1 b.

_Finally,

the microfibrillar structure seen in the fibre core

_(Fig.

_la)

_suggests

the presence of columnar microdomains

parallel

to

the

_spinning

direction.

Fig.

1.-

_Scanning

electron

_microscope

_photographs

_{of melt-spun}

_copper laurate fibres.

Aniso-tropic

contraction upon

cooling

is indicated

by

the

corrugated

surface

_(a)

and the central hole

516

Fig.

2.-

_X-ray

diffraction

_diagrams

of the columnar

_mesophase

_{of copper}

laurate at 150°C :

(a)

melt-spun

fibre ;

the fibre axis is

_{approximately}

horizontal and

_{perpendicular}

to the

_{beam ;}

X-ray

diffraction _patterns were taken at various

_temperatures

for _copper laurate fibres

_placed

in 0.5 mm Lindemann

_capillaries,

and

they

were recorded

photograph-ically

with a

_flat-plate

camera

_parallel

to the fibre axis. The beam from a

_rotating

anode

_(Elliott

_GX20)

was

perpendicular

to the

_spinning

direction.

_Figure

2a shows

a

typical

_pattern

obtained at 150°C in the columnar

_{mesophase ;}

the

_{corresponding}

pattern

obtained for an unoriented

_sample

is shown for

_comparison

in

_figure

2b. The localization of diffracted

_intensity

maxima in limited lobes in

_figures

2a indicates that the

_sample

is oriented.

Examination of the

_{small-angle equatorial}

_region

shows

_{sharp Bragg}

reflection

arcs with

reciprocal spacings

in the ratios 1 :

31/2 :

41/2 :

71/2.

The

high degree

of

orientation is

_apparent

from the low extent of

_arcing

of the reflections. The

_pattern

is characteristic of a two-dimensional

hexagonal

_array of columns that are

_parallel

to

the

_spinning

direction. On the other

_hand,

the rather narrow reflection observed at wide

_{Bragg angle}

in the meridional

_{plane corresponds}

to the

_{4.7 A stacking period}

of the binuclear _copper cores

_along

the column axes. A diffuse

_isotropic

band

partially

overlapping

the latter reflection

_corresponds

to the

_stacking

of the disordered

_aliphatic

chains of the

_complex

_[11].

_Finally,

the

_half-height

width of the meridional reflection indicates a coherence

_length

of

_{approximately}

_{120 Â ;}

this

_corresponds

to a stack of about 25 binuclear

_diffracting

_units,

i.e. a

medium-range crystalline

order within the

columns of the

_mesophase.

This result is in

_agreement

with our recent

_study

of the

analogous

copper stearate

complex by

copper K

edge

EXAFS,

which indicates that a

high

degree

of order of the

_polar

cores is retained in the columnar

_mesophase

[12].

The

_X-ray

_pattern

shown in

_figure

3a is that of an "as-made" _copper laurate fibre at room

_temperature.

Numerous reflections are seen in the

equatorial plane ;

these can be shown to

_belong

to two different

_patterns.

one

_pattern

is derived from a two-dimensional

_hexagonal

_array, almost identical to that of the columnar

_mesophase

(Fig.

2a) ;

apparently,

the columnar structure is

_quenched

in the solid

_phase

as the

spun fibre

quickly

cools away from the

spinneret.

The other

pattern

exhibits a series of

_{regularly spaced}

reflections with a

_{period corresponding}

to that of the lamellar

crystalline phase

of _copper laurate

_{[9] ;}

the

_{corresponding}

distribution of diffracted

intensity

about the

_equatorial

_plane

reflects

_preferential

orientation of the

_crystal

sheets

_parallel

to the

_spinning

direction. This is consistent with a

crystal growth

from an oriented

mesophase.

Interestingly, annealing

the "as-made" fiber at 100°C

_(7°C

below the

_phase

_transition)

for 24 h results in

_{disappearance}

of the

_quenched

_hexagonal

phase

and

_improvement

of the

_crystalline

order and orientation of the lamellar solid

phase

(Fig. 3b).

These results demonstrate that the

_{melt-spinning technique}

is an _easy and reli-able method for the

_alignment

of the

_thermotropic

columnar

_mesophase

of a

meso-genic

transition metal

_complex.

The

_degree

of orientation _may not be as

_high

as that obtained

_by

some of the

previously

described

techniques

[7,8],

but a

significant

improvement

of the

_present

method is its

_simplicity

and the ease of obtention of

_large

amounts

_{(grams !)}

of oriented

_samples.

The

_ability

to obtain oriented fibres over

macroscopic

distances is

_likely

to

_{bring significant}

_progress for the

_processing

and

518

Fig.

3.-

_X-ray

diffraction

_diagrams

of _copper laurate fibres at room _{temperature :}

(a)

Acknowledgments.

We wish to thank Mrs.

_Sybil

Marthon for

_taking

the

_scanning

electron

_microscope

photographs.

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