Microstructure of halato telechelic polymers bearing group IVb metal carboxylate end-groups

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Microstructure of halato telechelic polymers bearing

group IVb metal carboxylate end-groups

Jean-René Regnard, Pierre Lagarde, Claudine E. Williams, Gilberto Vlaic,

Pascal Charlier, Robert J. Jérome

To cite this version:

Microstructure of halato telechelic

_{polymers bearing group}

IVb

metal

_{carboxylate end-groups}

Jean-René

_Regnard

_(1,*),

Pierre

_Lagarde

_(1),

Claudine E. Williams

_(1),

Gilberto

Vlaic (2),

Pascal Charlier

₍₃₎

and Robert J. Jérome

₍₃₎

(1)

LURE

_(Laboratoire

_pour l’Utilisation du

Rayonnement Electromagnétique),

CNRS-CEA-MEN,

University

of Paris-Sud, 91405

_Orsay,

France

(2)

Dipartimento

di Scienze Chimiche, Universita di

Cagliari,

09124,

Cagliari, Italy

(3)

Macromolecular

Chemistry

and

_{Organic catalysis Laboratory, University}

of

Liège,

Sart

Tilman, B6, 4000

_{Liège, Belgium}

(Received in

_Februarv

1 st, 1990,

accepted

on

_April

_26,

₁₉₉₀₎

Résumé. 2014 On a

_analysé,

_par

_{spectroscopie}

_EXAFS, la structure locale des

agrégats

d’ionomères

carboxylato-téléchéliques

neutralisés par un excès de zirconium. La

_présence

de liaisons Zr-O-Zr

explique

la

_grande

stabilité du matériau aux solvants

polaires. L’augmentation

du nombre de voisins zirconium avec la

_{quantité d’agent}

de neutralisation

indique

une croissance des

agrégats

avec le taux de neutralisation. On propose un modèle

_compatible

avec les

propriétés mécaniques

de ces matériaux.

Abstract. 2014 The local structure within the ionic aggregates of a

_{carboxylato-telechelic}

ionomer neutralized with an excess of zirconium has been

_{investigated by}

EXAFS spectroscopy. The

presence of non-ionic Zr-O-Zr intramolecular bonds

explains

the great

stability

of the material towards

_{polar compounds.}

The number of Zr

_neighbours

in the aggregates has been found to

increase with the total amount of zirconium in the

sample.

A model consistent with the

unique

mechanical

_properties

of the

resulting

material is

_proposed.

Classification

Physics

Abstracts 61.40K - 81.20S

Introduction.

Ionomers are materials in which ion

_{pairs, randomly spaced along}

a

nonpolar polymeric

backbone,

associate into microdomains

_responsible

for a thermoreversible network

[1-4].

Halato-telechelic

_{polymers (HTP’s)}

form a class of model ionomers since

they

consist of short chains

_selectively

terminated

by

a _{salt group which may} be either a neutralized

_acidic,

a

neutralized basic or a

_quaternized

amino group

_[5-8].

The network

_{junction points}

are now

situated at the chain ends and thus

separated

by

a well-defined characteristic distance.

Both the

_quantitative

fonctionalization of a

_{carboxy-telechelic polymer}

and the

complete

neutralization of the acid

end-groups

are

_key

_parameters

for the contribution of all chain

segments

to the

_load-bearing

_capability

of the material. A

_highly

controlled method for neutralization of these

_{carboxy-telechelic}

_polymer

has been

_{reported using}

stoichiometric

amounts of very reactive alkaline or alkaline-earth metal alcoxides

_[5].

This

_technique

requires

that the alcohol formed as a

_byproduct

is

_completely

eliminated in order to

_displace

the reaction

_equilibrium

and also to avoid the ion

_pair

solvation. The related metal

carboxylates

promote

the formation of

_{highly interacting}

ion

_{pairs [5,}

_7,

_8].

Provided the

chain

_length

is

uniform,

there is

_{complete microphase separation}

between the chain

_segments

and the ionic groups which are

_incorporated

into

_{multiplets [9].}

These domains are

surrounded

_by

a volume from which other ionic domains are

excluded,

and

_arranged

in a

liquid-like

manner in space. In these

materials,

there is no evidence for ionic clusters as

postulated

in the more classical ionomers

_[9].

However the contribution of the ionic

aggregates

as

_physical

cross-links and

fillers,

although important,

is not sufficient to account

for the small-strain tensile moduli of

_{carboxy-telechelic polyisoprenes}

and the modulus

enhancement has been attributed to

_{entanglements}

in the form of

_{interlocking loops}

formed when both ends of a telechelic chain are located in the same

_aggregate

_[10].

It has also been

shown that the ionic microdomains in materials with a

_higher

modulus are more

ordered,

as

shown

_by

Extended

_X-ray

_Absorption

Fine Structure

_{Spectroscopy (EXAFS).}

It is indeed observed that metal a,

«carboxylato polyisoprenes

are able to strain-harden more as the

coordination number of the metal atom increases. Furthermore the

_degree

of local order

strongly

depends

on the neutralization

_pathway,

all other conditions

_being

identical

[11, 12].

So,

when an a,

«carboxylic

acid

polybutadiene (PBD)

is

quantitatively

neutralized

by

Zn in

such a way that no

byproduct

which could solvate the ion

pairs

is

produced,

a

_highly

ordered

structure is observed. A coordination of shell of five Zn atoms at 3.22 A is indeed

_reported

which

_disappears

when the Zn

_carboxylate

ion

_pairs

are _{formed in the presence of} a

_solvating

agent.

An exact

_description

of the conditions of neutralization is therefore

mandatory

for

complete sample

characterization.

On the other

hand,

when telechelics are neutralized with tetravalent transition

metals,

the

resulting

material has

_{unique properties}

related to the different nature of the

metal-carboxylate

bond. Indeed in contrast to alkaline and alkaline-earth

_metals,

elements such as

titanium and zirconium form bonds which are

_mainly

covalent and their

_dipolar

interaction is

noticably

weaker. A

_{cross-linking}

effect is observed

_only

when an excess of alkoxide groups is

used in the neutralization

_steps

and

_{subsequently hydrolyzed}

[13,

14],

an observation which

led to believe that the network is stabilized

_{by carboxylato-metal-oxohydroxide}

groups

[8,

14].

The

_resulting

material contains small

_inorganic

domains and shows a

_great

_stability

towards

_{polar compounds,}

which contrasts with that of alcaline telechelics. Indeed no

structural

_change

is seen

by small-angle-neutron scattering

when Ti-PBD has been immersed

in deuterated water whereas an increase in the domain size

corresponding

to

_hydration

of the

metal

_by

_D20

is observed for Na-PBD

_[15].

The nature of the tetravalent metal also influences the mechanical

_{properties. Dynamic}

mechanical measurements have shown that

_{oxy-Zr-carboxylates}

form

_{bigger although}

less stable

_aggregates

than the Ti

_counterparts

and that a,

lù-carboxylic

acid PBD exhibits a more

complex

viscoelastic behaviour when neutralized

_by

Zr instead of Ti. This has been

tentatively

attributed to the

_{nonequilibrium aggregation}

of the

_{oxymetal carboxylates and/or}

to the occurrence of at least two different

_thermally

activated _{processes in the Zr}

_containing

polymer [14].

The surface

_composition

of the

_ionomer,

as

_{analysed by}

ESCA,

is also

différent : for Ti-PBD the surface

_layer

contains no Ti and is followed

_by

a

_large

concentration

_gradient

towards the bulk whereas the

_{Zr-containing}

domains tend to be

upon the

hydrolysis

of the metal alkoxide groups used in excess

_[16].

EXAFS

_spectroscopy

of an a,

«carboxylic

acid PBD neutralized with an excess of Ti

_{isopropoxide (400}

and 600

_%)

in

the presence of water has also confirmed the existence of Ti-O-Ti intermolecular bonds and has shown that the

_majority

of the Ti atoms are included in dimer and trimer units. Two

different Ti-O bond

_lengths

have thus been measured

_[17].

Since the Zr behaviour contrasts in some

_aspect

with that of

Ti,

it is of interest to

_probe

the local environment around Zr atoms in order to _compare the microstructure within the

aggregates

of

_oxy-Ti

and

_oxy-Zr

carboxylates.

For this purpose, EXAFS

spectroscopy

has been used to

_provide

a

_description

of the short range order around Zr in terms of

_type

and number of

_neighbours,

distances and thermal and static disorder in these distances.

Experimental.

SAMPLE PREPARATION. - The

_{samples investigated}

were obtained

_by

neutralization with

zirconium

_isopropoxide

of a

dicarboxylic

acid

polybutadiene (Hycar

CTNB 2000 X

156),

commercialized

_by

BF

_Goodrich,

of molecular characteristics as follows :

Mn

= 4

600 ;

MW/Mn

=

1.8 ;

functionality

= 2.01 and

cis/trans/vinyl

ratio =

20/65/15.

A five wt % solution

of the

_{carboxy-telechelic polybutadiene}

was

prepared

in toluene

previously

dried

_by

refluxing

over calcium

_hydride.

The

_polymer

was then dried

_by

three successive

_azeotropic

distillations of toluene and an

_appropriate

amount of Zr

_{isopropoxide (0.1}

mol

L-1

1

solution in

_toluene)

was added to the final 5 wt % solution of the

_polymer.

Zr alkoxide was used in the stoichiometric ratio but also in various excess to the

_carboxylic

acid

_{end-groups. Samples}

called

_{ZR 1,}

_ZR2,

ZR4 and ZR6

_correspond

to an

_{alkoxide/acid}

molar ratio of

_1,

_2,

4 and

6,

i.e. to a

_degree

of neutralization of

_100,

_200,

400 et 600

_%,

_{respectively.}

The neutralization reaction was driven to

_{completion by azeotropic}

distillation of the

_isopropanol

formed as a

reaction

_byproduct.

When ca. 20 % of the toluene volume was distilled off under reduced

pressure, it was

_{replaced by}

fresh solvent and a further distillation run was carried out. After three distillation runs, the 20 % of distilled toluene was

_{replaced by}

toluene saturated

by

water in order to

_hydrolyze

the unreacted Zr alkoxide functions.

_Again

three distillation runs were achieved under these

_particular

conditions. Toluene was

_finally

distilled off

_completely

and the

_polymer

was dried to constant

_weight

under vacuum at 65 °C. It was molded at 120 °C

(150 °C

for

_ZR6)

into disks whose thickness was

_adjusted

to

_get

the best

_{signal-to-noise}

ratio in the EXAFS

_{experiment (absorption}

coefficient times thickness on the order of

_2).

THE EXAFS TECHNIQUE, EXPERIMENTAL AND DATA ANALYSIS. - All

_samples

were studied

in air and at room

_temperature

on the EXAFS 1 station at LURE-DCI. The

_storage

ring

was run at 1.85 GeV with

_positron

currents of about 250 mA. For

scanning

the energy range

around the Zr

_K-edge

at

_{17 998 eV,}

a Si 331 channel cut monochromator was used. The EXAFS

_spectra

were recorded in the transmission mode at 2 eV intervals over the energy range 17 800-18 800 eV

_using

two ionization chambers filled with argon. Each data

point

was

collected for 2 seconds and each

_complete

_spectrum

was measured 5 times for the

_highest

Zr-content

_(ZR4

and

ZR6)

and 7 times for the lowest concentrations

_(ZR2

and

_{ZR 1 ).}

The

experimental amplitude

and

_phase

functions for the Zr-O

pair

were obtained from the cubic

perovskite

BaZr03

(dzr-o =

2.095

_Á,

_{N (O )}

=

6).

where _{u o} is the smooth atomic like contribution above the

_edge

and u B the

background

originating

from

_{pre-edge absorption}

processes.

For a

K-edge,

the

_{simple back-scattering theory gives}

the

_relationship

between the EXAFS

signal X (k )

and the structural

_parameters

_{[18] :}

where Ri

is the average distance which

_separates

the

_absorbing

atom from the

_Ni

neighbouring

atoms

_defining

the ith

shell,

with a r.m.s.

_{deviation 0- i ; exp (-}

2 o-2i k2)

is an

effective

_Debye-Waller

factor

_resulting

from the statistical and thermal distortion of the

equilibrium

structure.

_{Fi(k) and ~ i (k)}

are the

_amplitude

of

_{back-scattering}

and the total

phase

shift,

respectively,

characteristic of the selected

_{(absorbing atom)/(back scattering}

shell

i) pair,

and A

_{(k )}

is the

_{photoelectron}

mean free

_path.

Data

_processing

was carried out on a

VÀX

730

_computer

_according

to a standard

procedure [19]

that is

_{only briefly}

summarized hère :

uB is

extrapolated

from the

pre-edge

region

fitted with a Victoreen function

(ILB x

=

CA 3

₊ DA

4)

and then subtracted from the

experimental data. u 0 (k)

is obtained from

a

_{smoothing procedure (200 iterations)}

of the EXAFS oscillations

_starting

50 eV above the

edge.

The slow residual oscillations

_{of X (k)}

are then eliminated

_by

a multi-iterative process

which affects

_only

the very small R-values

(typically R

1

Á)

of the modulus of the Fourier

transform.

Eo

was

_arbitrarily

chosen as the inflection

_point

of the Zr

_K-edge

in the selected model

compound BaZr03 (Eo

= 18 006

eV).

The reduced data for the model

compound

and the

ionomers are shown in

_figure

1. The

_{k3 multiplied}

_spectra

were then Fourier transformed in

the domain

_{[51, 680]}

eV after

_application

of a

_Hanning

_window,

_yielding

the moduli

1 FT

1

shown in

_figure

2. The main feature of these radial distribution functions

_(RDF)

is the

presence of two distinct

_peaks

for each

_ionomer,

attributed to Zr-O and Zr-Zr atom

_pairs,

whose intensities vary

systematically

with Zr-content without much

_change

_{in position}

_[20].

The

_broadning

of the first

_peak

for

_sample

ZR 1 could not be

_{reasonably explained.}

Results and discussion

Provided that the

_Fi(k)

_{and Oi(k)}

functions are

known,

the structural

_parameters

Ni, Ri

and _ai can be determined for a

_{given compound.}

We use a

_{semi-empirical approach :}

the first

_peak

of

_BaZr03

was Fourier filtered and the

_{experimental phase}

function was

extracted on the basis of its well characterized

_{crystallographic}

structure

_(each

Zr is surrounded

_by

6 _{oxygen atoms} at the distance of 2.095

Â).

_{Subsequently,}

the filtered

_signal

was fitted

_{using experimental phase}

and theoretical

_{[21] amplitude}

functions to evaluate the À and a

_parameters

of

_{equation (3) (N being}

fixed at

_{6.0). Finally}

we calculated the

experimental amplitude

not convoluted with the

_Debye-Waller

term, but still convoluted with the mean free

_path

term. The

_experimental

functions were then transferred to the back-transformed first

_peak

of the four

_ionomers,

and the structural

_parameters

of the first shell

were obtained

_by

a least _square

_fitting

of the data in the interval between 70 and 600 eV.

During

the

_fit,

a variation of k was allowed to take into _{account any}

_{approximation}

in chemical

_{transferability}

of

_amplitude

and

_phase

functions. The

_adjusted

wave vector is k’ =

_{( k 2 +}

0.2624 DE

_{0)1/2 .}

The best fits are

_reported

in

_figure

3 and the calculated structural

parameters

are listed in Table I.

The first

_important

conclusion is that it is

_impossible

to fit the data with

_only

one Zr-O

Fig. 1. - Experimental

EXAFS

X (E)

vs. E for the four zirconium-neutralized a, (J)

car-boxypolybutadiene samples

labeled ZRI, ZR2,

ZR4,

ZR6 and for the model

_{compound BaZr03.}

Note

Fig.

2. -

Fig.

3. - Fit

(full line)

of the Fourier filtered first

_{peak (dotted line)}

for the ZR 1 and ZR4 ionomers.

Table 1. - Parameters

_of

the

_first

coordination shell

_split

in 2 subshells noted a and

b for

the

introduced. It is

interpreted

as

_showing

the existence of 2

types

of Zr-O bonds. The total number

of oxygen

atoms is constant for all

_samples

with an _average value of 6.0 ± 0.5 but

_they

are not distributed

_equally

between the 2 subshells : in the inner subshells at 2.10

_À,

the number increases with the

_degree

of neutralization up to 400

_%,

whereas it decreases in the

outer subshell at 2.25

Á.

The

_{samples containing}

a

proportion

of 2 or 3 Zr per chain

(ZR4

and

ZR6)

have almost the same local structure _{of oxygen} atoms.

Attempts

to fit the back-Fourier-transformed second

_{peak (Zr-Zr pairs)}

of the ionomer

RDF with

_amplitude

and

_phase

shifts extracted from

_{tetragonal yttrium-stabilized Zr02 [22]}

as a model

_compound

were unsuccessful. Hence for the second

shell,

a

_comparison

within the

series of

_samples

was carried out and two different

_analyses

of the data were

_performed.

First we used the method

proposed by Sayers et

al.

[23].

If it is assumed that the Zr-Zr distances are

similar in the four

ionomers,

then the ratio between the

_envelopes

A

_{(k )}

of the EXAFS

_signal,

relative to the second shell of two

_{samples i and j}

is :

A

_plot

of In should be linear and

_give

the ratio of the coordination numbers and

the u 2

values. As

an

_example

of this

_{procedure, figure}

4 shows the

_plot

of In

(A41AI)

as a

function of

k 2.

A

satisfactory

linear behaviour was obtained for all combinations and the

results are noted as « Method 1 » in table II. The second method uses one of the

samples

as a

reference

_{compound :}

the

_{experimental phase}

and

_amplitude

for the Zr-Zr atom

_pair

are

calculated

by backtransforming

the second

_peak

of the ZR 1

_sample

Fourier

transform,

arbitrarily setting

N = 1 and R = 3.5

À (a

reasonable value from

Fig. 2).

Then the fit of the

back Fourier transform of the second

_peak

of the other ionomers was achieved with these

functions. An

_example

of these fits is shown in

_figure

5. As

_reported

in table

II,

both methods

give

results in

_good

agreement.

Note that on average, each Zr atom is surrounded

_by

an

Fig.

4. - Ratio of the EXAFS

Fig.

5. - Fit

_{(full line)}

of the Fourier filtered second peak (dotted line) for the ZR6 ionomer.

Table II. - Evolution

_of

the relative coordination

number,

the

_{effective Debye- Waller factor}

and the Zr-Zr distance in the Zr-neutralized HTP

_compared

to

_sample

ZRI.

increasing

number of Zr

_neighbours

in the second shell when the total amount of Zr in the

ionomer

_increases,

but with a

_{higher degree}

of disorder. We believe that this is due to an

increasing

static disorder from ZR 1 to ZR6. Low

_temperature

_{investigations}

are

_planned

to

elucidate this

_{point ; they}

will be

_reported

in a

_forthcoming

_{paper. It is}

_interesting

to remark here that a casual

_glance

at the moduli of the Fourier transforms

_{(Fig. 2)}

would hint the

opposite

conclusion since the

_intensity

of the second

_peak

decreased with

_increasing

ion-content. The conclusion that the average number of Zr

neighbours

increases is

_{supported by}

the

_chemistry

of the

system.

Indeed the

_hydrolysis

of an

increasing

amount of unreacted zirconium alkoxides should lead to a more extended network of Zr-O-Zr bonds as

Conclusions

Three main conclusions can be drawn from this

experimental investigation.

First two

_types

of Zr-O bonds exist in the

_aggregates

of a, w

carboxylic

acid

polybutadiene

neutralized at and above

_{stoichiometry}

with Zr

_isopropoxide

followed

_by

the

_hydrolysis

of the unreacted

alkoxide groups.

Indeed,

two coordination shells have been found around each Zr atom at

2.10 and 2.25 ± 0.02

_À,

that one can

_{reasonably assign}

to Zr-O-Zr and

_{Zr-carboxylate}

(-COOZr ) and/or

Zr-OH

_bonds,

_{respectively.}

This observation is consistent with the

chemistry

involved _{in the neutralization process}

_{(Eqs. (4)}

and

(5)).

It also confirms the results of the ESCA

_analysis

of octanoic acid neutralized with various amounts of Zr alkoxide followed

_{by hydrolysis}

of the unreacted

_functions

_[16].

The

_O1

1 core level

peak,

that is

unimodal when the octanoic acid is neutralized at

_{stoichiometry,}

shows

_clearly

two

components

when an excess of Zr alkoxide is used. The extra

_peak

has been assessed to the Zr-O-Zr bonds

_{(Eq. (5)).}

Note that the same conclusion has been reached when the

carboxy-telechelic PBD was neutralized with an excess of Ti

_isopropoxide

_{in the presence of} water

_{[17] ;}

two O-Ti distances were also measured

_by

EXAFS

_{spectroscopy.}

Second,

when the amount

_{of neutralizing}

_agent

increases from

_{stoichiometry (ZR 1 )}

to a six

fold molar excess

_(ZR6),

the relative

_proportion

of _oxygen atoms in

_{oxy-bridges (first}

oxygen

subshell)

increases whereas the total number of _{oxygen stays} constant. This observation

means

_that,

whatever the

_OR/COOH

molar

_ratio,

each Zr involved in a

carboxylato-oxohydroxide

aggregate

is surrounded

_{by approximately}

the same number of near

neighbour

oxygen atoms, which can be

_expressed

as

The main

_change

in the local structure of the

_end-group

aggregates

as

_{promoted by}

an

increasing departure

from

_{stoichiometry}

has to be found in the relative

proportion

of oxygen

atoms in the two subshells. That the

_population

of oxygen atoms in

_oxy-bridges

(R

= 2.1

À )

increases _at the expense of those

contributing

to

carboxylate and/or

hydroxyde

groups when the excess of

_hydrolyzable

Zr alkoxide

_increases,

means that the

cross-linking

of

PBD extremities

_gets

more efficient.

Finally

this trend is

_{supported by}

results

_concerning

the third shell

_{populated by}

an

increasing

number of Zr atoms when the total amount of Zr in the

_sample

increases

(Table II).

It is thus

_likely

that the size of the

_aggregates

increases.

However,

as the basic unit

grows in

size,

it becomes more disordered as shown

_by

the

_{large Debye-Waller}

factor. It is

difficult at this

_point

to devise a more detailed model of the actual structure.

As

_{already pointed}

out,

qualitatively

similar results are obtained when a,

«carboxylic

acid

PBD is neutralized to 400 and 600 %

_by

Ti

_{isopropoxide [17].}

The total number

of

oxygen

atoms is however smaller

_{(4.4 compared}

to

₆₎

and a dimer of Ti has been

proposed

as the

basic

_entity

whose characteristics are constant _{in the range of the molar} excess used

₍₄₀₀

to

600

_%).

In that case, the coordination number of Ti is close to one

(0.8)

and the short range

order is

_high.

The results

_reported

in this

_study

are consistent with

_previous

viscoelastic measurements,

and

_particularly

with the

_{steady-flow viscosity}

of a 10

_{g . dl -1}

solution

of a,

w-carboxylic

acid

PBD neutralized with Zr alkoxide

_[15].

A viscous solution is observed at the

_{stoichiometry}

of the neutralization reaction.

ultimately

an elastic

_gel

is formed. This

_change

in the solution

_viscosity

can

_only

be accounted

for

_by

a more efficient

_{cross-linking}

of PBD. It is also worth

_recalling

that the bulk a, «

carboxylic

acid PBD neutralized with a fourfold excess of Ti or Zr

alkoxide,

exhibits a

rubber-like

_plateau

in the viscoelastic measurements but the modulus is

_higher

for Zr than for Ti

_{[ 15].}

Thus,

at a constant

_degree

of neutralization

(i.e.

400

_%),

the mean number of chain ends

attached to an

_aggregate

of

_oxohydroxide

metal

_carboxylate

is

_higher

for Zr. This is in

qualitative

agreement with the EXAFS observations which conclude to a

_higher

coordination

number of Zr

_compared

to

_Ti,

all other conditions

_{being kept}

constant.

References

[1]

EISENBERG A. and KING M.,

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₍₁₉₈₁₎

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[3]

LONGWORTH R.,

Developments

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Polymers

1,

Applied

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1983, Ch. 3.

[4]

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BROZE _G., JÉROME R. and TEYSSIÉ

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Macromolecules 15

₍₁₉₈₂₎

920 and 1300 ; 16

₍₁₉₈₃₎

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_{Polym. Phys.}

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₍₁₉₈₃₎

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[7]

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(Plenum,

New

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1985,

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[8]

JÉRÔME R. and BROZE _G., Rubb. Chem. Technol. 58

₍₁₉₈₅₎

223.

[9]

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₍₁₉₈₆₎

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[10]

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[11]

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₍₁₉₈₃₎

L-717.

[12]

VLAIC G., WILLIAMS C. E., JÉRÔME R., TANT M. R. and WILKES G. L.,

Polymer

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₍₁₉₈₈₎

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[13]

BROZE G., JÉRÔME R. and TEYSSIÉ _Ph., J.

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[14]

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₍₁₉₈₅₎

1376.

[15]

JÉRÔME R., TEIXEIRA J. and WILLIAMS C. E.,

unpublished

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[16]

JÉRÔME _R., TEYSSIÉ _Ph., PIREAUX J. J. and VERBIST J. _J.,

_{Appl. Surf.}

Sci. 27

₍₁₉₈₆₎

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[17]

VLAIC _G., WILLIAMS C. E. and JÉRÔME R.,

Polymer

28

₍₁₉₈₇₎

1566 ;

and CHARLIER P., CORRIAS A., JÉRÔME _R., REGNARD J. _R., VLAIC G. and WILLIAMS C. E.,

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Phys.

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[19]

LAGARDE P., Rayonnement

Synchrotron

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Proceedings

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[20]

Note that the features below the first

peak

have not been taken into account since

_they

should have no

_physical

_significance

for a

_light

back-scatterer like oxygen ; the

shouldering

could be due to

a low

_frequency

_component that has not been filtered out of the EXAFS

signal.

The features below and above the second

peak

have been taken as «

_ripples

» due to the limited

k-space

used for the Fourier-Transform.

[21]

TEO B. K. and LEE P. A., J. Am. Chem. Soc. 101

₍₁₉₇₉₎

2815.

[22]

TUILIER M. H., DEXPERT-GHYS J., DEXPERT H. and LAGARDE P., J. Solid State Chem. 69

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