The effect of substituent groups on polymer conformation in good solvent: polyoctene and polydecene

HAL Id: jpa-00212425

https://hal.archives-ouvertes.fr/jpa-00212425

Submitted on 1 Jan 1990

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

The effect of substituent groups on polymer

conformation in good solvent: polyoctene and

polydecene

J.P. Aime, S. Ramakrishnan, R.R. Chance, M. W. Kim

To cite this version:

The effect of substituent groups

on

polymer

conformation

in

good

solvent:

_polyoctene

and

_polydecene

J. P. Aime

_(*),

S.

_{Ramakrishnan,}

R. R. Chance and M. W. Kim

EXXON Research and

_Engineering

Company,

Clinton

_Township,

Route 22 East, Annandale, New

_Jersey

_08801, U.S.A.

(Reçu

le 5 octobre 1989,

accepté

le

_{23 janvier 1990)}

Résumé. 2014 La taille des _groupements latéraux est un

paramètre

_important

pour

comprendre

les corrélations locales des

_polymères

_conjugués

solubles. Nous

_présentons

une _preuve

_{expérimentale}

supplémentaire

de l’influence de l’extension des _groupements latéraux sur la

rigidité

locale par une étude de diffusion de lumière sur les

_polymères

saturés:

_polyoctène

et

_{polydécène.}

Les résultats montrent sans

_ambiguïté

la relation entre la

_{longueur statistique}

et l’extension des substituants. Nous montrons

_également

que la

longueur

statistique

n’est pas le

paramètre adéquat

pour décrire les

propriétés

optiques

observées pour les

polymères conjugués

en solution.

Abstract. _{2014 The influence of the size of the side group is} a

_key

_parameter for

understanding

the local

_rigidity

of soluble

_{conjugated polymers.}

We _report an additional

_experimental

_proof

with a

light scattering study

upon very

simple

saturated

polymers

with different side group extension :

polyoctene

and

_polydecene.

The results show an

_unambiguous

relation between the statistical

length

and the size of the side group. We also show that the

magnitude

of the statistical

_length

is

not the relevant _parameter for

_describing

the

_{optical properties}

of the

_{conjugated polymers}

in solution.

Classification

Physics

Abstracts 61.25H

Introduction.

Conjugated polymers

in solution are now

_widely

studied.

_{Polydiacetylene}

and

_{polythiophene}

with suitable

_{side-groups [1-3]}

are model

_systems

which can be dissolved in common

organic

solvents. These

_systems

exhibit a

_large

amount _{of very}

_{interesting phenomena}

such as

thermochromism, solvatochromism,

and

_large

non-linear

_optical

response.

They

also

_provide

an excellent

_opportunity

for

_studying

the relation between electronic

properties

and conformational disorder in low dimensional materials. This is a

_quite

difficult

task,

and a

_large

amount of

_experimental

and theoretical work has been devoted to this

subject during

the

past

ten _years

_[4].

(*)

Permanent address :

_Groupe

de

Physique

des Solides, Université Paris VII, Tour 23, 2, Place Jussieu, 75251 Paris, France.

In the

_present

paper we will focus our attention on the effect of the side _group size on the overall chain conformation.

_{Conjugated polymers}

can

_only

be dissolved

_{by using large,}

flexible side groups. For soluble

polydiacetylenes,

the

_side-group

is so

_large

that we can

consider it as a lateral short chain

_(see

Tab.

III).

For

example

the series of

polymers

_nBCMU,

PTS12,

or the soluble

polydiacetylene

made

by Plachetta

and Schulz

[5],

have

_{typical lateral}

extensions that lie between 30 and 40 A. For

_alkyl

substituted

_{polythiophenes}

a shorter

substituent,

such as an

_octyl

_group or even a

_butyl

_{group, is}

_enough

for

_obtaining

_polymers

that are soluble in THF or nitrobenzene

_{[3, 6].}

On the other

band,

poly-paraphenylene-vinylene

with an

octyl

substituent is still rather difficult to dissolve.

Several theoretical studies have been devoted to the effect of the conformational disorder

on the

_{conjugation length [7,}

8,

9].

The

conjugation length

is a

_parameter

_describing

the

_pi

electron delocalization

_along

the

_conjugated

backbone structure. This

_length

is related to the average

overlap

between the

_pi

electron orbital. The

_overlap

can be decreased either

_by

curvature fluctuation or torsional fluctuation.

Recently

Rossi et al.

_[9]

have

_given

an

explicit

expression

for the

_{conjugation length}

in terms _{of the average torsional}

_angle

between

monomer units. On the other

hand,

Aime and

Bargain [10]

have shown that the average

torsional

_angle

is a

key

factor for

understanding

the chain conformation in

good

solvents.

There are two

_approaches

for

_describing

the chain conformation of

_{conjugated polymers}

in solution. One is to consider the

_pi

electronic structure as the

_leading

term of the local

_rigidity.

This is the

_approach

taken

_{by Allegra et}

_{al. [11] ;}

the force constant _{used in their paper allows}

them to record

_fairly

well the observed

_{statistical length}

of 3BCMU and PTS12 without

_taking

into account the

_side-group

effect. This work

_suggests

that a

_{conjugated polymer}

will be stiffer than a saturated one,

mainly

because in the former the local correlations between monomer

units will be enhanced

_by

the

_pi

electron

_overlap.

This claim appears rather reasonable but is unable to

_explain

the thermal behavior of the chain conformation of PTS12

_[10],

and the

differences in statistical

_lengths

observed for the same

_type

of

_system

with different side

groups

(see below).

The second

_approach

is to take into account

_explicitly

the

_geometrical

structure of the side group. This

approach gives

a coherent

_{picture allowing explanation}

of the various statistical

lengths using

a

_simple

_argument.

It has been shown that the statistical

_length

is

_strongly

dependent

on the lateral extension of the side group

[10].

On the other hand a recent

calculation

_[12]

made on the

_{polydiacetylene gives}

a barrier

height

for a

_pi/2

rotation between

monomer units

_equal

to 0.6

Kcal/mole,

i.e. an _energy barrier of the order of kT. This latter

result

_supports

the

_{approach suggesting}

that the’local stiffness will be

_{governed by}

the steric hindrance between side groups.

Electronic

_properties

of

_{conjugated polymers}

are _very

_dependent

of the number of monomer units in the

_conjugated

sequence.

Experiments

on short

_oligomers

show

_large

change

of the

_position

of the maximum

_absorption

as a function of the number of monomer

units,

the infinite chain state

_{being approached asymptotically}

in

_{1 /n.}

In contrast for

saturated

_polymers,

there is

_basically

no difference between the

_absorption

_spectrum

of an isolated monomer

_unit,

in solution

_phase

for

_example,

and a monomer unit

belonging

to a macromolecule.

_Independent

of the number of monomer

units,

we do not

_expect

_any

_change

on the electronic excitation

_spectrum

as a function of the conformational disorder.

Using

saturated

_polymers,

we avoid _any local

ordering

of the backbone due to the

_{special properties}

of the

_pi-electron

_systems

of

_{conjugated polymers. Following}

this

_aim,

the

_present

_paper

report

a

_study

of saturated

_polymers

with the same backbone but different

_{side-group lengths.}

The paper is

_organized

as follow : In section 1 we discuss the

_{approximations}

used for

determining

the

_{statistical length}

from the measure of the radius of

_gyration.

In section 2 we

1.

_{Light scattering :}

method and measure of the statistical

_length.

The

_{light scattering intensity}

due to

_{optically isotropic polymer}

coil is

_{governed by}

where

_{P (q )}

is the normalised

_scattering

function. The

_Rayleigh

ratio

_R0

was obtained

where the index T relates to the standard

_(toluene)

and i is the

_{scattering intensity originating}

from the

_density

fluctuation of the

_polymer.

An

_{extrapolation}

at nul concentration was then

made,

allowing

the use of the relation for a coil dimension smaller than the

_wavelength

A

where,

and

With A = 6 328 A and n = 1.3878 for the solvent

_heptane,

the index increment

dn /dc

is 0.145 for

_poly-decene

and 0.134 for

_poly-octene.

The

_{angular dependence}

of the

_scattering

function

_gives

some information about the

shape

of the coil. With the

_knowledge

of the molecular

_weight

and the structure of the monomer

unit,

the radius of

_{gyration provides}

also some information about the internal structure of the coil. For a

_given

total contour

_length

of the

_polymer,

the

_magnitude

of the radius of

_gyration

depends

on the local correlations between monomer units. For a chain with noticeable local

correlation,

the model

_{given by Kratky}

and Porod

_[13]

or that

_by

Frenkel-Landau-Lifchitz

[14]

can be used. The latter one describes the macromolecule as a thin thread with curvature

fluctuation. A characteristic

_length

is obtained

_measuring

the average cosine of the

angle

between two unit vectors

_tangent

to the curve :

where

_{fp is}

the

_{persistence length (half}

of the statistical

_length),

i

the contour

_length

between the two

_{points along}

the curve.

_Using

the relation

_(3),

we obtain the end to end distance and hence the radius of

_gyration

which has been first obtained

_by

Benoit et al.

_[15]

The relation

(4)

_neglects

the excluded volume effect and hence will not be valid for macromolecules with a

_{large degree}

of

_{polymerization}

N. For

_example,

Schaefer et al.

_[16]

give

a criterion about the

_magnitude

of N which allows estimation of a

_limiting

value in a such

way that the self-avoidance becomes

negligible.

This

_happens

in the

_{Flory approximation}

when N N * with N * a

_(Ep/a)3.

This relation

_gives

an

_insight

_{about the way} the

_magnitude

of the

_{persistence length}

can reduce the effect of the

_long

_range

interaction for chain with

finite

_length.

2.

_Experimental

results for saturated

_{polymers poly-octene}

and

_poly-decene.

Sample

preparation.

1-octene and 1-decene were distilled from Na-metal and stored under an inert

_atmosphere.

The

_{polymerization}

was carried out in toluene

_(distilled

over

Na-benzophenone) using TiCI3.AA (Staufer Chemicals)

and

_{Et2AICI (Aldrich)}

as the

_catalyst.

Typically, TiCI3.AA (82.5

mg, 0.534

mmol)

was taken in 42 ml of toluene

_(to

make a 15 wt%

monomer

_solution)

and 645 mg

_(5.34

_{mmol ;}

_{A1 /Ti}

_{= 10)}

of

_Et2AICI

was added to it and

stirred _{for about 5 min. 6 gm}

_(53.4

_{mmol ;}

_monomer/Ti

=

100)

of 1-octene _was then added _to

the

_catalyst

solution. After

_stirring

for about 1

_h,

_{the solution had turned very viscous and the}

polymerization

was terminated

_by

the addition of an excess of

_2-propanol.

The

_precipitated

polymer

was washed

_thoroughly,

redissolved in toluene and

_{reprecipitated}

in

_2-propanol,

and dried in a vacuum oven. Both

_polyoctene

and

polydecene

were obtained as white translucent

materials.

Fractionation. - Both the

_polymers

were fractionated

_{using cyclohexane}

and acetone as the

solvent/non-solvent

mixture. The

_polymers

were dissolved in

_cyclohexane

to make up a 0.2-0.1 wt% solution. A small amount of

_inorganic

residue

_(Ti02)

was removed

_by

_{centrifugation.}

The clear

_polymer

solution was taken in a round bottom flask and acetone was added

dropwise

with

_{vigorous stirring,}

until the solution turned turbid and remained so upon

stirring

for an additional 20 min. The solution was then warmed up a few

_degrees

to

_clarify

the solution and was allowed to stand for about 12 h at room

_temperature.

The

precipitated

polymer

was

_separated

_{by centrifugation}

and the process was

repeated

with the

centrifugate

to

_give

the various fractions.

Sample analysis.

The molecular

_weight

data and the

_{polydispersities}

were determined

_by

GPC

_using

THF as the solvent. A Waters 600E

_delivery

_system

connected to a Waters 410 refractometer was used. A series of three

styragel

columns with pore sizes of

103,

104

and

106 Â

was used to effect the

_separation.

The

_analysis

of the data was

_{performing using}

a

polystyrene

standard calibration curve. The values of

_MW/Mn

thus obtained are

_given

in table I.

The

13C-NM R

_spectra

was done

using

a 1-2 wt% solution of the

_polymer

in

CDCl3 using

a Bruker AM360

_{spectrometer,}

and the

_spectra

are referenced to TMS. The

_spectra

of both the

unfractionated

_polymers

are shown in

_figure

1. The

_peak

at _{40.26 ppm in} both the

_polymers

represents

the backbone

_methylene

carbon. The nature of this

_peak

is _very sensitive to the

stereoregularity

of the

_polymer

and has been used to calculate the

_tacticity

of

_{poly( l-alkene)s}

such as

_{poly(octadecylethylene) [17].}

Isotactic

_{poly( 1-alkene)s}

exhibit a

sharp peak,

while the

atactic

_polymer

exhibits a broad

_multiplet

with

_{peaks corresponding}

to the different mm, mr and rr diad _sequences. The

_spectra

of both

_polyoctene

and

_polydecene,

used in this

investigation,

show a _very

_sharp

and

_{symmetric peak indicating}

a very

high (>

90

%)

isotactic

index. This is also in

_agreement

with

_{previous findings using}

this

_catalyst

system

[18].

This

confirmation of the

_{stereoregularity}

is a

_prerequisite

to the conformational

_analysis

of these

polymers,

as the

_tacticity

may be

expected

to

_change

the solution chain conformation

Table la. - Results on small

angle light

scattering

on

polyoctene.

Table Ib. - Results on small

angle light

scattering

on

polydecene.

Fig.

1. - 13C-NMR spectra of

Results and discussion.

Polyoctene

and

_polydecene

are _very

simple

linear

polymers. They

differ

only by

the number

of

_CH2

units attached at the backbone : 5 and 7 for

_polyoctene

and

_{polydecene respectively.}

The Zimm

_plots

for

_polyoctene

and

_polydecene

are shown in

_figures

2 and 3

_{respectively.}

The linear

_{extrapolations}

of the Zimm

_{diagrams give}

the values for molecular

_weights

and radius of

_gyration

listed in table I. The

_linearity

of the

_extrapolated

function

_{Kc /Ro}

=

f (c)

and the

_positive

values of the second virial coefficient in both cases show that

_heptane

is a

good

solvent for

_polyoctene

and

_polydecene.

Typical

values

_extrapolated

at nul concentration are shown in

figure

3.

Using equation (2),

a linear

regression

is

applied.

For a

_given

contour

_length,

the radii of

_gyration

are much

_larger

for these

_systems

than the ones observed for other saturated

polymers.

Typical

values are

_reported

in table II. If we

want to _{express these values in} term of statistical

_lengths,

we have to be careful about

Fig.

2. - Zimm

_{plot polyoctene M,,}

= 434 000.

polydispersity.

Since the values obtained with

_{light scattering correspond}

to a

_Z-average

on

the radius of

_gyration,

the

_{equation (4)}

becomes

[20]

where

Table II.

_{- Typical}

data

_{for flexible polymers.}

Fig.

4. - Data

extrapolated

at c = 0.

Polydecene Mw

= 1.43 M.

As it is shown in

_figure

5,

for a

_given

radius of

_gyration,

the

_{polydispersity}

can be very

important

and would

_yield

a

_fairly

inaccurate value of b if

_ignored.

The radius of

gyration

of

_polyoctene

and

_polydecene

as a function of the

_degree

of

polymerization

together

with the values

_computed

with the

_equation

₍₅₎

are

_reported

in

figures

6a and 6b. In the

_figures

are also

_given

the lower and

_higher

values which can be

reasonably

used for

_recording

the observed data. This

_provides

an estimation of the

Fig.

5. - Variation of the statistical

length

as a function of

_Mw/Mn,

Mw

= 539 000,

Rg

= 531 A,

Polydecene (22).

increase of the

_magnitude

of the

_{statistical length}

as a function of

_M,,,

_meaning

that within the

range of

MW

used the excluded volume effect remains

_negligible.

A

_{polydispersity equal}

to 1.35 is used for the

_computed

values.

Also,

using

the same

statistical

_lengths,

we

_report

the radius

_{of gyration expected}

with

_MW/Mn

= 1.35 for

samples

having

a

_{large polydispersity (see}

Tab.

I).

Using

the same method as described

_above,

we

_report

_computed

values of b for linear saturated

_{polymer (Tab. II).}

The

_comparison

between the values

_given

in table 1 and table

II,

shows that

_polyoctene

and

_polydecene

are more

_rigid

than the conventional saturated

polymers.

It is

_important

at this

_stage

to

_emphasize

the difference. If we look at the internal

structure of the

_polyoctene

and

_polydecene,

we could

_expect

a more flexible behavior

_keeping

in mind the value obtained with the

_{poly-butylthiophene b}

= 55 A

[6].

From

_figure

_6,

we deduced b = 65 A for

polyoctene

and b = 85 A for

polydecene.

Polydecene

has been

_previously

measured

_by

J. C. W. Chien et al.

_[21]

_{(1) ;}

_they

measure a

radius of

_gyration

of 531 A for a molecular

weight equal

to 539 000. This result leads to a

statistical

_{length larger}

than the one obtained with our own measurements. Even if we

consider an

_improbably

_{large polydispersity}

of 4 or

_5,

their measure

_gives

a value around

100 A

_{(Fig. 5).}

3.

_Comparison

between

_conjugated

and saturated

_polymers.

Since our aim is the

_{understanding}

of the internal conformation of

_{conjugated polymers}

in

solution,

we summarize the

_experimental

values obtained for the

_{polydiacetylenes}

and

polybutylthiophene.

The structure of the monomer unit of the

_{polydiacetylene}

is

RC=CR-C=-C,

where R are different

_{side-groups given}

in table III. The data available are

mainly given

with the measure of the radius of

_gyration,

in that case, as we have done

above,

the statistical

_length

is obtained with the

_{equation (5).}

_{A convenient way is} to

_plot

the

statistical

_length

as a function of the extension of the side group in the solvent. The latter

parameter

is either measured

[6,

22]

or calculated from the chemical structure of the

substituent.

(’)

In their paper

[21],

J. W. Chien and T.

_Ang

used

_MW

= 396 000, due _{to an} inaccurate data

’ Fig.

6. -Variation of the Radius of

Gyration

as a function of

_{M,, using}

the relation

(5)

and

Mw/Mn

= 1.35.

a) Polyoctene ;

Radius of

gyration

for

Mw/Mn

= 1.35

(see Text)

b = 55

(-);

b = 65

(- -) ;

b = 75

(...). b)

Polydecene ;

Radius of

gyration

for

M,,IM.

= 1.35, b = 75

(-) ;

b = 85

(- -) ;

b = 95

(...).

Table III. - Some

_{examples of}

soluble

_{polydiacetylenes}

with

_general

structure

_given

in the

Figure

7 shows

_{unambiguously}

a

relation

between the lateral extension of the side

groùp

and the

_magnitude

of the statistical

length.

Without

_considering

the

_{polymers polyisoprene}

and

_{polybutylthiophene,}

two series can be constituted : the first one with

_{polyisobutylene,}

polystyrene, polyoctene

and

_{polydecene ;}

the second with the

_{polydiacetylenes.}

For each of these two

series,

the backbone is identical which allows us to _{relate any}

_change

of the chain

conformation to the

_change

of the

side-group

structure. The

_figure

7

_suggests

the same

_type

of

behavior for both

_conjugated

and saturated

_polymers.

The

_{conjugation along}

the backbone will have a

relatively

small influence on the local stiffness as

_compared

to the size effect of the

substituent,

provided

the substituent is

_large.

Fig.

7. - Variation of the

statistical length

as a function of the lateral extension.

Light scattering

data :

Saturated

_{polymers (0), Conjugated polymers (D) ;}

Neutron

_scattering

data for

_{conjugated polymers}

(+) ;

Computed

values with

_{equations (6)}

and

_{(7) (-).}

The

_{representation}

used is

_{obviously oversimplified}

since we do not consider the details of

the chemical structure, such as the

_{hydrogen-bonds}

in

_p3BCMU

and

_p4BCMU

_[23].

We can

also

_expect

a different

_types

of interaction between different substituents such as for benzene

rings (polystyrene)

or

_methylene

units

(polyoctene

and

polydecene).

In

fact,

a

_description

at a

microscopic

level of the effect of the

_side-group

on the conformation of macromolecules is very difficult even for the case of close

_{packing [25].}

Another relevant

parameter

is the ratio of the average section of units

belonging

to the

_side-group

versus the monomer unit

_length.

The monomer unit

_length

of

_{vinyl polymers}

is about 2.54

A,

for

_{polythiophene}

it is 3.9 A and

for

_{polydiacetylene}

it is 4.9 A in the trans conformation.

_Figure

7 does not take into account

the differences in the monomer unit

lengths.

We discuss that

_point

below,

still in a _very

simplified

fashion.

The thermal behavior of the chain conformation of PTS 12 does not follow the one

_expected

for a worm-like chain

_[10].

_Up

to now, the best way for

understanding

the absence of

variation of the statistical

_length

with

_temperature

is to introduce a rotation fluctuation

between monomer units. This leads to an increase of the _average effective moment of inertia of the ribbon and therefore to an increase of the statistical

_length.

This result means that the

where E is the elastic

_modulus,

and

with

where

k2 -

1 -

(I2/I1 )2

I1 =

(al a2)/ 12

and

₁₂

=

(a,

a2)/ 12

are the moment of inertia for a

rectangular section, a2

is the lowest dimension of the section

_(a2

= 4

Á),

and a1

is

equal

to the

lateral extension of the side _group.

Equation (7) gives

the _average effective moment of inertia

_goveming

the fluctuation in

curvature of the

_{macromolecule. 0}

is the rotation between

_neighboring

monomer units. We

report

in

_figure

7 some

_computed

values

_{for 0}

_equal

to 18° and E

_equal

to

1010 dyn

cm-

2.

The

general

evolution is

_surprisingly

well

_reproduced.

The

_discrepancy

_{which appears} can be understood in the

_following

way : for a

given

structure of the

side-group,

for

example

methylene

units,

the

_magnitude

of the rotation induced

_by

the

_repulsion

between side group is

dependent

of the monomer unit

_length.

For small monomer unit

_lengths,

the rotation will be

larger.

Since the average effective moment of inertia increases with the increase of the average

rotation,

the

statistical length

will be also

larger.

Therefore,

we can

_expect

a

_larger

rotation for the saturated

_polymers

than for the

_{polydiacetylenes.}

This close

_packing

argument

means that the

_repulsion

between side group will be still more efficient for saturated

polymers

than for substituted

_{polydiacetylenes.}

The

_figure

7

_suggests

also an average torsion between monomer units

(at

least for the different

_{polydiacetylenes)}

identical in

_spite

of the

_{large change}

in the

_magnitude

of the

statistical

_length.

It means that the measured statistical

_length

is not

_directly

related to the ir

electronic distribution or

conjugation length

for the soluble

conjugated polymers.

In other

words the

_{conjugation length}

will be related to the torsion

_[9].

The latter remark is well

supported

with the

_experimental

result

_{given by}

Plachetta and Schulz

_{[5]. They}

observe a

maximum of

_absorption

similar to the other

_{polydiacetylenes}

while the statistical

_length

is about twice

_{larger :}

550 or

660 Â

instead of 300

Á.

We can

anticipate

the

following

feature which appears at first

_{glance paradoxal :}

in the

event where we have a

_planar

_{ribbon structure,} i.e.

_{~ =}

0 in

_{equation (7) (corresponding}

at

an _{average effective} moment of inertia

_having

its lowest

_value),

the

_polymer

will have a

_larger

flexibility,

while the

_{conjugation length}

could be increased because of the

_{disappearance}

of the torsional fluctuations. In that case we will observe a red shift of the maximum

_absorption

together

with a decrease of the statistical

_length.

This leads to the

_opposit

behavior of the one

proposed

for

_explaining

the

_sol-gel

transition

_{of p4BCMU}

with a coil to rod transition

_(see

for

example

first Ref. in

_[4]),

or the one used for

_describing

the relation between conformational

disorder and

_{optical properties}

in term of fluctuation in curvature

_(second

Ref. in

_[7]).

4. Conclusion.

By

changing

the structure _{of the side group} we have shown a

change

of the local

_rigidity

on the conformation of _very

_simple

saturated

_polymers

in

_good

solvent.

_Polyoctene

and

_polydecene

appear rather stiff

by comparison

with other

vinyl

polymers

or even

_{polybutylthiophene.}

The

origin

of the local

_rigidity

can be related to the extension of the

_side-groups

_{and the average}

rotation between monomer units. These

_systems

can exhibit a

persistence

in curvature

_leading

to an helical worm-like chain behavior

_[26]

The

_present

light scattering experiment

does not

allow discrimination between the two

_types

of conformation.

Using

equations (11)

and

₍₁₈₎

[20]

which

_correspond

to the radius of

_gyration

for chain with

_persistence

in curvature, we obtain the same statistical

_length

and need to introduce additional

_parameter

_(the

_average

torsion)

which is unknown.

_Only

small

_angle

neutron

_scattering

will be able to

_provide

more

accurate information about the local conformation.

_Experiments

are

_planed

and will be done in a near future. We can

_expect

also that substituted

polyacetylenes [27]

will show a similar behavior.

_They

will be _very

_good

candidates for

_studying

the relation between the

_conjugation

along

the backbone and its torsion.

Acknowledgments.

It is a

pleasure

to thank Mireille _{Adam for many}

interesting

discussions. One of us

(J.P A)

would like to thank C.N.R.S. for financial

_support.

References

[1]

PATEL G. N.,

Poly. Prepr.

19

₍₁₉₇₈₎

154.

[2]

PATEL G. N., CHANCE R. R. and WITT J. D., J. Chem.

_Phys.

70

₍₁₉₇₉₎

4387.

[3]

ELSENBAUMER R. L., JEN K. Y. and OBOODI R.,

Synth.

Met. 15

₍₁₉₈₆₎

164.

[4]

See for

_{example: Polydiacetylenes,}

D. Bloor and R. R. _{Chance, Eds.,} NATO ASI E 102

_{(1985) ;}

SCHOTT M. and WEGNER G., in Nonlinear

_{optical properties}

of

_organic

molecules and

crystals,

Vol. 2, D. Chemla and J.

_Zyss,

Eds.

(Academic

Press Inc.,

Publishers, Orlando)

1987.

[5]

PLACHETTA C. and SCHULZ R. C., Makromol. Chem.,

Rapid

commun. 3

₍₁₉₈₂₎

815.

[6]

AIME J. P., BARGAIN F., SCHOTT M., ECKHARDT _H., ELSENBAUMER R. L. and MILLER G. G.,

Phys.

Rev. Lett. 62

(1989)

55 ;

[7]

SCHWEIZER K.

_S.,

J. Chem.

_Phys.

85

₍₁₉₈₆₎

_1156;

SOOS Z. and SCHWEIZER K., Chem.

_Phys.

Lett. 139

(1987)

196.

[8]

PINCUS P.

A.,

ROSSI G. and CATES M. _E.,

_Europhys.

Lett. 4

₍₁₉₈₇₎

41.

[9]

ROSSI G., CHANCE R. R., SILBEY R., J. Chem.

_Phys.,

_{in press.}

[10]

AIME J. P. and BARGAIN F.,

Europhys.

Lett. 9

₍₁₉₈₉₎

35.

[11]

ALLEGRA G., BRUCKNER

_S.,

SCHMIDT M. and WEGNER G., Macromolecules 19

₍₁₉₈₆₎

399.

[12]

BREDAS J. L. and HEEGER A. _J., submitted to macromolecules.

[13]

KRATKY O., POROD _G., Rec. Tra. Chim.

_Pays-Bas

68

₍₁₉₄₉₎

1106.

[14]

LANDAU L. D. and LIFSHITZ E. M., Stat.

Phys.,

2nd ed.

_(Pergamon

Press,

London)

1959, p. 478.

[15]

BENOIT H. and DOTY P., J.

_Phys.

Chem. 57

₍₁₉₅₃₎

958.

[16]

SCHAEFER W., JOANNY J. F. and PINCUS P., Macromolecules 13

(1980)

1280.

[17]

SEGRE A. L., ANDRUZZI F., LUPINACCI D. and MAGAGNINI P. L., Macromolecules 14

₍₁₉₈₁₎

1845.

[21]

CHIEN J. C. W. and ANG T., J.

_Polym.

Sci.

_Polym.

Chem. 24

₍₁₉₈₆₎

2217.

[22]

RAWISO M., AIME J. P., FAVE J. _L., SCHOTT M., MULLER M.

_A.,

SCHMIDT M., BAUMGARTL H.

and WEGNER G., J.

_Phys.

France 49

₍₁₉₈₈₎

861.

[23]

AIME J. P., BARGAIN F., FAVE J. _L., RAWISO M. and SCHOTT M., J. Chem.

Phys.

89

₍₁₉₈₈₎

6477.

[24]

WENZ G., MULLER M. A., SCHMIDT M. and WEGNER _G., Macromolecules 17

₍₁₉₈₄₎

837.

[25]

KITAIGORODSKII A. I., Molecular

_Crystal

and Molecules

(Academic

Press, N.

_Y.)

1973.

[26]

KIRSTE R. G., Kolloid-Z.Z.

_Polym.

244

₍₁₉₇₁₎

290.

[27]

MUSADA _T., HIGASHIMURA T., Adv.

_Polymer

Sci. 81

_{(1987) 121.}

[28]

DAVIDSON N. _S., FETTERS L. J., FUNK W. G., HADJICHRISTIDIS N. and GRAESSLEY W. W., Macromolecules 20

₍₁₉₈₇₎

2614.

[29]

MATSUMOTO _T., NISHIOKA N. and FUJITA H., J.

_Polym.

Sci.,

Polym. Phys.

10

₍₁₉₇₂₎

23.

[30]

FUKUDA _M., FUKUTOMI M., KATO Y. and HASHIMOTO T., J.

_Polym.

Sci.,

Polym. Phys.

12

₍₁₉₇₄₎